Virus (Continued)..

Examples of Viruses and the Diseases they cause:

T4 Bacteriophage This transmission electron micrograph shows a T4 bacteriophage, a virus that infects only bacteria (and in this case only Escherichia coli). Phages lack any reproductive machinery and rely on the apparatus of bacteria in order to replicate. They do so by attaching to the cell wall of the bacterium with the spidery tail fibres visible here. The tail is a sheath that contracts to inject the contents of the head, the genetic material (DNA), into its host. Within 25 minutes of infection, the bacterial apparatus successfully commandeered, viral progeny fill the cell. The overcrowded bacterium bursts, releasing approximately 100 new copies of the bacteriophage.

Adenovirus.This icosahedral adenovirus particle measures 75 nm in diameter. It is protected by the capsid (shown here in red)—a "shell" or "coat" consisting of two types of protein. The protein subunits (capsomeres) are visible here as small, circular shapes. The spikes protruding from each vertice on the capsid surface interact with cell receptors and determine the virus's infective properties. At the core of the virus particle its DNA is contained, associated with a third type of protein different from those in the capsid.
This is a false colour photograph of an adenovirus particle seen under an electron microscope.

The Human Immunodeficiency Virus (HIV), which causes acquired immune deficiency syndrome (AIDS), principally attacks T-4 lymphocytes, a vital part of the human immune system. As a result, the body’s ability to resist opportunistic viral, bacterial, fungal, protozoal, and other infection is greatly weakened. Pneumocystis carinii pneumonia is the leading cause of death among people with HIV infection, but the incidence of certain types of cancers such as B-cell lymphomas and Kaposi’s sarcoma is also increased. Neurological complications and dramatic weight loss, or “wasting”, are characteristic of end-stage HIV disease (AIDS). HIV is transmitted sexually, through contact with contaminated blood, tissue, or needles, and from mother to child during birth or breast-feeding. Full-blown symptoms of AIDS may not develop for more than ten years after infection.

Rabies Virus.The rabies virus is usually transmitted to humans by a bite from an infected dog, but the bite of any animal (wild or domestic) is suspect in an area where rabies is present. Symptoms of the disease appear after an incubation period of ten days to one year and include fever, breathing difficulties, muscle spasms, and, in later stages, an irrational fear of water. Death almost invariably occurs within three days to three weeks of the onset of symptoms. For this reason, the emphasis of treatment is on prevention. Dogs may not be brought into the United Kingdom until a lengthy period of quarantine has elapsed, while in the United States, domestic dogs are vaccinated yearly and stray dogs are killed.

Hepatitis B Virus.(HBV) causes inflammation of the liver. The virus is recognizable under magnification by the round, infectious “Dane particles” accompanied by tube-shaped, empty viral envelopes. Manifestations of this condition include jaundice and a flu-like illness, while chronic infection can lead to serious pathologies such as cirrhosis and cancer of the liver.

Virus..

VIRUS

Virus (biology) (Latin, “poison”), any of a number of organic entities consisting simply of genetic material surrounded by a protective coat. The term “virus” was first used in the 1890s to describe agents that caused diseases but were smaller than bacteria. By itself a virus is a lifeless form, but within living cells it can replicate many times and harm its host in the process. There are at least 3,600 types of virus, hundreds of which are known to cause a wide range of diseases in humans, other animals, insects, bacteria, and plants.

The existence of viruses was established in 1892, when Russian scientist Dmitry I. Ivanovsky discovered microscopic particles later known as the tobacco mosaic virus. The name virus was applied to these infectious particles in 1898 by the Dutch botanist Martinus W. Beijerinck. A few years later, viruses were found growing in bacteria; these viruses were dubbed bacteriophages. Then, in 1935, the American biochemist Wendell Meredith Stanley crystallized tobacco mosaic virus and showed that it is composed only of the genetic material called ribonucleic acid (RNA) and a protein covering. In the 1940s development of the electron microscope made visualization of viruses possible for the first time. This was followed by development of high-speed centrifuges used to concentrate and purify viruses. The study of animal viruses reached a major turning point in the 1950s with the development of methods to culture cells that could support virus replication in test tubes. Numerous viruses were subsequently discovered, and in the 1960s and 1970s most were analysed to determine their physical and chemical characteristics.

Characteristics

Viruses are submicroscopic intracellular parasites that consist of either RNA or deoxyribonucleic acid (DNA)—never both—plus a protective coat of protein or of protein combined with lipid or carbohydrate components. The nucleic acid is usually a single molecule, either singly or doubly stranded. Some viruses, however, may have nucleic acid that is segmented into two or more pieces. The protein shell is termed the capsid, and the protein subunits of the capsid are called capsomeres. Together these form the nucleocapsid. Other viruses have an additional envelope that is usually acquired as the nucleocapsid buds from the host cell. The complete virus particle is called the virion. Viruses are obligate intracellular parasites; that is, their replication can take place only in actively metabolizing cells. Outside living cells, viruses exist as inert macromolecules (very large molecules).

Viruses vary considerably in size and shape. Three basic structural groups exist: isometric; rod shaped or elongated; and tadpole-like, with head and tail (as in some bacteriophages). The smallest viruses are icosahedrons (20-sided polygons) that measure about 18 to 20 nanometres wide (one-millionth of a millimetre = 1 nanometre). The largest viruses are rod shaped. Some rod-shaped viruses may measure several microns in length, but they are still usually less than 100 nanometres in width. Thus, the widths of even the largest viruses are below the limits of resolution of the light microscope, which is used to study bacteria and other large micro-organisms.

Many of the viruses with helical internal structure have outer coverings (also known as envelopes) composed of lipoprotein or glycoprotein, or both. These viruses appear roughly spherical or in various other shapes, and they range from about 60 to more than 300 nanometres in diameter. Complex viruses, such as some bacteriophages, have heads and a tubular tail, which attaches to host bacteria. The pox viruses are brick shaped and have a complex protein composition. Complex and pox viruses are exceptions, however; most viruses have a simple shape.

Replication

Viruses do not contain the enzymes and metabolic precursors necessary for self-replication. They have to get these from the host cells that they infect. Viral replication, therefore, is a process of separate synthesis of viral components and assembly of these into new virus particles. Replication begins when a virus enters the cell. The virus coat is removed by cellular enzymes, and the virus RNA or DNA comes into contact with ribosomes (cell organs that synthesize proteins) inside the cell. There the virus RNA or DNA directs the synthesis of proteins specified by the viral nucleic acid. The nucleic acid replicates itself, and the protein subunits constituting the viral coat are synthesized. Thereafter, the two components are assembled into a new virus. One infecting virus can give rise to thousands of progeny viruses. Some viruses are released by destruction of the infected cell. Others are released by budding through cell membranes and do not kill the cell. In some instances, infections are “silent”—that is, viruses may replicate within the cell but cause no obvious cell damage.

Lytic and Lysogenic Cycles of a Bacteriophage. All bacteriophages (viruses that parasitize bacteria) have a lytic or infectious cycle, in which the virus, incapable of replicating itself, injects its genetic material into a bacterium. By pirating its host’s enzymes and protein-building capacities, the virus can reproduce and repackage, making about 100 new copies before it bursts from and destroys the bacterium. Some bacteriophages, however, behave differently when they infect a bacterium. The injected genetic material instead integrates itself into its host DNA, passively replicating with it to be inherited by bacterial daughter cells. In about 1 in 100,000 of these lysogenic cells, the viral DNA spontaneously activates and starts a new lytic cycle.

The RNA-containing viruses are unique among replicative systems in that the RNA can replicate itself independently of DNA. In some cases, the RNA can function as messenger RNA (see Genetics), indirectly replicating itself using the cell's ribosomal and metabolic precursor systems. In other cases, RNA viruses carry within the coat an RNA-dependent enzyme that directs the synthesis of virus RNA. Some RNA viruses, which have come to be known as retroviruses, may produce an enzyme that can synthesize DNA from the RNA molecule. The DNA thus formed then acts as the viral genetic material.

Viral Replication Outside of a host cell, a virus is an inert particle. Once inside a cell, a virus can replicate many times, creating thousands of viruses that leave the cell to find host cells of their own. Viruses that cause disease do so by destroying or damaging cells as they leave them.

Bacterial viruses and animal viruses differ somewhat in their interaction with the cell surface during infection. The “T even” bacteriophage that infects the bacterium Escherichia coli, for instance, first attaches to the surface and injects its DNA directly into the bacterium. No absorption and uncoating take place. The basic events of virus replication, however, are the same after the nucleic acid enters the cell.

Viruses in Medicine

Viruses represent a major challenge to medical science in combating infectious diseases. Many cause diseases that are of major importance to humans and that are extraordinary in their diversity.

Included among viral diseases is the common cold, which affects millions of people every year. Recent research has even indicated that the AD-36 virus, which causes cold-like symptoms, affects food-energy absorption and more than doubles the normal layer of body fat in animals. About 30 per cent of obese people had contracted AD-36 compared with 5 per cent of lean people, and so this virus may contribute to obesity in a percentage of people. Other viral diseases are important because they are frequently fatal. These diseases include rabies, haemorrhagic fevers, encephalitis, poliomyelitis, and yellow fever. Most viruses, however, cause diseases that usually only create acute discomfort unless the patient develops serious complications from the virus or from a bacterial infection. Some of these diseases are influenza, measles, mumps, cold sores (also known as herpes simplex), chickenpox, shingles (also known as herpes zoster), respiratory diseases, acute diarrhoea, warts, and hepatitis. Still others, such as rubella (also known as German measles) virus and cytomegalovirus, may cause serious abnormalities or death in unborn infants. Acquired immune deficiency syndrome (AIDS) is caused by a retrovirus. Only two retroviruses are unequivocally linked with human cancers (see Leukaemia and HTLV), but some papilloma virus forms are suspected. Increasing evidence also indicates that other viruses may be involved in some types of cancer and in chronic diseases such as multiple sclerosis and other degenerative diseases. Some of the viruses take a long time to cause disease; kuru and Creutzfeldt-Jakob disease, both of which gradually destroy the brain, are slow virus diseases.

Viruses that cause important human disease are still being discovered. Most can be isolated and identified by laboratory methods, but these usually take several days to complete. One of the most recently discovered viruses is rotavirus, the causal agent of infant gastroenteritis.

Spread

To cause new cases of disease, viruses must be spread from person to person. Many viruses, such as those causing influenza and measles, are transmitted by the respiratory route when virus-containing droplets are put into the air by people coughing and sneezing. Other viruses, such as those that cause diarrhoea, are spread by the faecal-oral route. Still others, such as yellow fever and viruses called arboviruses, are spread by biting insects. Viral diseases are either endemic (present most of the time), causing disease in susceptible people, or epidemic—that is, they come in large waves and attack thousands of people. An example of an epidemic viral disease is the worldwide occurrence of influenza almost every year.

Treatment

Smallpox Vaccination This drawing shows a doctor administering the smallpox vaccine, first discovered in 1796 by British physician Edward Jenner. Jenner found that infecting a patient with cowpox, a minor disease, produced immunity to smallpox, which can cause disfigurement or death. His discoveries won him worldwide renown.

Currently, no completely satisfactory treatments exist for viral infections, because most drugs that destroy viruses also damage the cell. The drug amantadine is used extensively in some countries for treatment of respiratory infections caused by influenza-A viruses, and the drug AZT is used in the treatment of HIV.

One promising antiviral agent, interferon, is produced by the cell itself. This non-toxic protein, which is produced by some animal cells infected with viruses, can protect other cells against such infection. The use of interferon for treating cancer is under intensive study. Until recently, study of the use of interferon has been restricted by its limited availability in pure form. However, new techniques of molecular cloning of genetic material (see Genetic Engineering) now make it possible for scientists to obtain the protein in larger quantities. Its relative value as an antiviral agent has already been established.
The only effective way to prevent viral infection is by the use of vaccines. For example, vaccination for smallpox on a worldwide scale in the 1970s eradicated this disease. Many antiviral vaccines have been developed for humans and other animals. Those for humans include vaccines for rubeola (also known as measles), rubella, poliomyelitis, and influenza. Immunization with a virus vaccine stimulates the body's immune mechanism to produce a protein—called an antibody—that will protect against infection with the immunizing virus. The viruses are always altered before they are used for immunization so that they cannot themselves produce disease.

Plant Diseases

Viruses cause a wide variety of diseases in plants and frequently cause serious damage to crops. Common plant-disease viruses are turnip yellow mosaic virus, potato leaf roll virus, and tobacco mosaic virus. Plants have rigid cell walls that plant viruses cannot penetrate, so the most important means of plant-virus spread is provided by animals that feed on plants. Often, healthy plants are infected by insects that carry on their mouthparts viruses acquired while feeding on other infected plants. Nematodes (also known as roundworms) may also transmit viruses while feeding on the roots of healthy plants.

Plant viruses can accumulate in enormous quantities within infected cells. For instance, tobacco mosaic virus may represent as much as 10 per cent of the dry weight of infected plants. Studies on the interaction of plant viruses with plant cells are limited, because plants often cannot be infected directly, but only by means such as an insect vector. Cell cultures in test tubes, which can be infected with plant viruses, are not generally available.

Role in Research

The study of viruses and their interaction with host cells has been a major motivation for the host of fundamental biological studies at a molecular level. For example, the existence of messenger RNA, which carries the genetic code from DNA to define what proteins are made by a cell, was discovered during studies of bacteriophages replicating in bacteria. Studies of bacteriophages have also been instrumental in delineating the biochemical factors that start and stop the utilization of genetic information. Knowledge of how virus replication is controlled is fundamental to understanding biochemical events in higher organisms.

The reason that viruses are so useful as model systems for studying events that control genetic information is that viruses are, in essence, small pieces of genetic information that is different from the genetic information of the cell. This allows scientists to study a smaller and simpler replicating system, but one that works on the same principle as that of the host cell. Much of the research on viruses is aimed at understanding their replicative mechanism in order to find ways to control their growth, so that viral diseases can be eliminated. Studies on viral diseases have also contributed greatly to understanding the body's immune response to infectious agents. Antibodies in blood serum, as well as secretions of the mucous membranes, all of which help the body eliminate foreign elements such as viruses, have been more thoroughly characterized by studying their responses to viral infection. Intense scientific interest is now concentrated on studies designed to isolate certain viral genes. These genes can be used in molecular-cloning systems to produce large amounts of particular virus proteins, which can in turn be used as vaccines.

Kingdom Protista

Protists

-these are eukaryotes with nuclei and other membrane-bound organelles

-most of them are unicellular organisms, however, some of the protists are multicellular organisms, and few of them are very large

Algae

-plantlike protists that perform photosynthesis

-algae contain chlorophyll and produce glycogen as a by-product of photosynthesis

-many algae contain cellulose cell walls, just as plants do.

-they live whereever there is sufficient water. They grow in ponds, in salt water, in moist soil and even on the surface of the ice

-both plants and algae capture the energy of the sun through the photosynthesis and store it, making the energy usable to heterotrophs

-they provide food for other organisms, they also produce oxygen

Colonial organisms- are clusters, or colonies.

Unicellular Algae

-include the dinoflagellates, diatoms and euglenoids.

Dinoflagellates

Dinoflagellates are algae with two flagella that spin the cells through the water. Generally, the two flagella arise from two grooves and at right angles to one another . One flagellum circles the cell like a head and the other sticks out like a tail. The rhythmic beating of its flagella propel a dinoflagellate through the water like a spinning top. The majority of dinoflagellates grow in saltwater habitats. Others are free-living while some have symbiotic relationships with jellyfish, sea anemones, corals and other organisms hat live near the coral reefs. Symbiotic dinoflagellates supply nutrients to the animals which they live. Symbiotic dinoflagellates do not have a flagella.

- a dinoflagellate Spineferites

Diatoms

Diatoms are algae that lack both cilia and flagella and have glass walls containing silica. The cell walls of diatoms consist of two halves that fit together like a lid on Petri dish. Each diatom has a pair of pores that allows gases and other materials to pass through cell walls. There are many patterns of diatoms. Diatoms are among the most abundant organism in the Earth. Probably because they contain oil, diatoms float in the water. When diatoms die, their shells sink to the bottom of the sea. Soil deposits contanining shells are known diatomaceous Earth. Its tough, gritty-structures makes diatomaceous Earth deposits useful for products such as lotion, detergents, and cleaners and cleaners, abrasives, polishing agents and toothpaste.

- diatoms

Euglenoids

The Euglenoids are organisms similar to the organism Euglena. Euglenoid has no rigid cell wall. Instead, Euglena has a flexible protein covering called pellicle. Eulena has a flagella. They are difficult to classify because they are both algae and protozoa . Since many of them have chloroplasts and perform photosynthesis, in the past they were called as algae. They have flagella like some protozoans, they have pellicle like many ciliates. Euglenas that have grown in darkness lose theirchloroplastsand fuction as heterotrophs. Some scientists hink Euglenas should belong to protozoans.

Multicellular Algae

-Many algae have multicelled bodies.

-A body of a multicellular alga is called a thallus. A thallus can have many specialized structures including stringlike filaments and leaflike sheets, or rootlike holdfasts.

-Microbiologists have now classifid algae as protists because they have different reproductive structuresfom plants.

Green Algae

Green Algae ore Chlorophyta are green and multicellular. However some algae are unicellular and some are colonial. In stressful conditions Green Algaes contract their flagella and becomes dormant.When their is water present, they will regrow their flagella, increase their size and reproduces. After rainfall, algae that have been dormant grow in puddles and drainage ditches.

Volvox is a common colonial green algae. In Volvox, clusters of cells with flagella live together in a ball-shaped colony. The colonies may contain several thousand cell. Some cells are specialized for specific functions such as reproduction.

The multicellular green algae either grow as filaments, with cells hooked from end to end, or as flat, leaflike sheets of cells.

Most green algae live in fresh water and moist soil. A few green algae live symbiotic relationships with organisms such as Paramecium, Hydra and fungi. A symbiotic association between an alga and a fungus is called a lichen.

Red Algae

Most of the red algae or rhodophyta are muticellular organismsand grow mainly in saltwater habitats. Typically, red algae have a thalli with branched filaments and are less than 1m long. However, not all red algae are red. Besides chlorophyll, the red algae have other pigments that trap sunlight. Their accesory pigments allow the red algaeto use the light that penetrates into deep water for photosynthesis. Consequently, red algae can live where there is too little moist for most other plants and algae.

Coralline red algae are an important component of coral reefs. Coralline red algae have calcium carbonate in their cell walls. Besides cellulose, the coralline algae have calcium carbonate in their cell walls. Calcium carbonate makes the branching red algae stiff.

to be continued..

Kingdom Monera

Monerans

Characteristics:

• no membrane-covered nuclei and organelles


• mostly unicellular


• reproduce asexually by binary fission


• produce food through photosynthesis but use a wider variety of substances as raw materials than eukaryotes
  
• tiny organisms


• a cell wall, usually surrounded by a layer of slime, encloses the cell

Bacteria
-simplest microorganisms, single-celled or noncellular spherical or spiral or rod-shaped organisms lacking chlorophyll that produced by fission

Classification according to shape:

1.coccus-spherical



2.bacillus-rodlike



3.spirillus-spiral



Functions of Monerans:

1. Sewage disposal


2. Production of cheese and vinegar


3. Used in tanning leather and curing meat


4. production of anibiotics like neomycin


5. Biological control of harmful insects
   

Characteristics of Monerans:

1.cell wall-peptidoglycan

2.flagellum-for movement

3.pili-for attachment

4.mode of reproduction

Asexual:Binary Fission

Sexual:Conjugation, Transduction and Transformation

Cyanobacteria


-predominantly photosynthetic prokaryotic organisms containing blue pigment in addition to chlorophyll
-occur singly or in colonies in diverse habitats that can form filaments that they split up in 2 or break into fragments for reproduction
-examples:Anacbaena, oscillatoria, nostoc
-can carry out photosynthesis and absorb food from surroundings





Two Prokaryotic Kingdoms:

Archaebacteria


Kingdom of prokaryotes more like eukaryotic cells than eubacteria.



Major Groups of Archaebacteria:


Methanogens(methane maker)

• live at swamps, sewage, stockyards, animal guts and other oxygen free habitats
  
• their anaerobic pathway ends in methane


• they release 2 billion tons of methane from termite guts, ruminant guts, wetlands, rice paddies and landfills
  
• this tremendous quantities of this by-product influence carbon dioxide levels in the atmosphere and the global carbon dioxide cycle
  

Extreme halophiles(salt lovers)

• live in very salty water, as in brackish ponds and salt lakes, and near hydrotherml vents
  
• they spoil salted fish, animal hides, and commercial sea salt


• most of them make ATP by aerobic pathways


• some also switch to a photosynthetic pathway when oxygen is low

Extreme thermophiles(heat lovers)

• live in highly acidic soils, hot springs, even coal mine wastes


• some start the food webs at hydrothermal vents, where water reaches 110 degrees Celsius


• they get electrons from hydrogen sulfide


• they are cited as evidence that life originated deep in the oceans
  

Chemosynthesizers


Instead of using the Sun's energy, chemosynthesizers absorb compounds that contain sulfur, iron and nitrogen, and get their energy through a process called oxidation. They use the energy to change carbon dioxide into organic food molecules, which support a whole community of other organisms. Chemosynthesizers can live in harsh environments where no other producer could survive, like the hot sulfur vents on the ocean floor.






Eubacteria


Unlike archaebacteria, they have fatty acids in their plasma membrane. In most cases their cell wall incorporates peptidoglycan.


Modes of Nutrition:

Photoautotrophs

• cyanobacteria, or blue-green algae, are common oxygen releasing, photosynthetic types


• you may see them at ponds and lakes where mucus-sheathed chains of cells form slimy mats in nutrient enriched water


• Anabaena and other types can convert nitrogen to ammonia for biosynthesis


• if nitrogen compounds dwindle, modified cells call heterocysts synthesize a nirogen fixing enzyme. they produce and share nitrogen compounds with other cells in the chains and get carbohydrates in return.


• anaerobic photosynthesizers get electrons from hydrogen sulfide or hydrogen gas, not water. They may resemble anaerobic bacteria in which the cyclicathway of photosynthesis is involved.

Chemoautotrophs

• they have mighty roles in the cycling of nitrogen and other nutrients

Chemoheterotrophs

• many have roles as decomposers and as human helpers


• Lactobacillus is used to help make pickles, buttermilk and yoghurt


• Actinomycetes to make antibiotics
  
• E. coli in our gut produces Vitamin K and compounds that help us digest fat. It also helps newborns digest milk, and it prevents many food-borne pathogens from colonizing the human gut


• sugarcane and corn rely on the nirogen-fixing spirochete Azospirillum. They take up some nitrogen fixed by this symbiont and give some sugars to it.

Gram Positive Bacteria

-the bacteria's cell wall is mad eup of a protein-sugar complex that takes on a purple color during the Gram Staining .

Gram Negative Bacteria

-the gram negative bacteriahas an extra layer of lipid on the outside of lipid on the outside of the cell wall and appear pink during the Gram Staining

Gram Staining- a test on cell walls developed by Hans Christian Gram

Geologic Timetable..

Precambrian Era

The Precambrian (or Pre-Cambrian) is an informal name for the eons of the geologic timescale that came before the current Phanerozoic eon. It spans from the formation of Earth around 4500 Ma (million years ago) to the evolution of abundant macroscopic hard-shelled fossils, which marked the beginning of the Cambrian, the first period of the first era of the Phanerozoic eon, some 542 Ma.

The SubDivisions of Precambrian

A diverse terminology has evolved covering the early years of the Earth's existence, but it is tending to settle out and come into greater use as radiometric dating allows plausible real dates to be assigned to specific formations and features. The terms Archean (older than about 2500 Ma), Proterozoic (2500-600 Ma), and Neoproterozoic (600-542 Ma) appear to have general currency. Some additional terms are included in the geological time line. See Timetable of the Precambrian.

• Proterozoic : Modern use is most often the period from the beginning of the lower Cambrian boundary, through 2500 Ma. The boundary has been placed at various times by various authors, but has now been settled at 542 Ma. As originally used, it was a synonym for Precambrian and hence included everything prior to the Cambrian boundary.
  
  
  
  
  
  • Neoproterozoic : the earliest subdivision of the Proterozoic roughly from the Cambrian boundary back to as far as 900 Ma, although modern use tends to represent a shorter interval : 542-600 Ma. The Neoproterozoic corresponds to Precambrian Z rocks of older North American geology.
    
    
    
    
    
    • Ediacaran : In March 2004, the International Union of Geological Sciences officially defined the term to describe this geologic period. The period begins at the time of deposition of a particular stratigraphic boundary, about 620 Ma. The period ends at the beginning of the Cambrian, 542 Ma. In this period the Ediacaran fauna appeared.
    
    
    
    • Cryogenian a proposed subdivision of the Neoproterozoic.
    
    
    
    • Tonian a proposed subdivision of the Neoproterozoic.
    
    
    
    
  
  
  
  • Mesoproterozoic : the middle division of the Proterozoic. Roughly from 900-1600 Ma. Corresponds to Precambrian Y rocks of older North American geology.
  
  
  
  • Paleoproterozoic : The oldest subdivision of the Proterozoic. Roughly from 1600-2500 Ma. Corresponds to Precambrian X rocks of older North American geology.
  
  
  
  



• Archaean : Roughly from 2500-3800 Ma.



• Hadean : Prior to 3800 Ma. This term was intended originally to cover the time before any preserved rocks were deposited, although a very few old rock beds seem to be slightly older than 3800 Ma. Some zircon crystals from about 4400 Ma demonstrate the existence of crust in the Hadean Eon. Other records from Hadean time come from the moon and meteorites.

It has been proposed that the Precambrian should be divided into eons and eras that reflect stages of planetary evolution, rather than the current scheme based upon numerical ages. Such a system could rely on events in the stratigraphic record and be demarcated by GSSPs. The Precambrian could be divided into five "natural" eons, characterized as follows.

1. Accretion and differentiation: a period of planetary formation until giant Moon-forming impact event.



2. Hadean: the Late Heavy Bombardment period.



3. Archean: a period defined by the first crustal formations (the Isua greenstone belt) until the deposition of banded iron formations due to increasing atmospheric oxygen content.



4. Transition: a period of continued iron banded formation until the first continental red beds.



5. Proterozoic: a period of modern plate tectonics until the first animals.
   

Paleozoic Era

-a major division (era) of geologic time (see Geologic Timescale, table) occurring between 570 to 240 million years ago. It is subdivided into six periods, the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian (see each listed individually). During the hiatus between the late Precambrian and Paleozoic eras most of the evidence of the earth's early history was destroyed by erosion. From the beginning of the Paleozoic, shallow seas began to encroach on the continents. In North America, the era began with submerged geosynclines, or downward thrusts of the earth's crust, along the eastern, southeastern, and western sides of the continent, while the interior was dry land. As the era proceeded, the marginal seas periodically washed over the stable interior, leaving sedimentary deposits to mark their incursions. During the early part of the era, the area of exposed Precambrian, or shield, rocks in central Canada were eroding, supplying sediment to the geosynclines from the interior. Beginning in the Ordovician period, mountain building intermittently proceeded in the eastern part of the Appalachian geosyncline throughout the rest of the era, bringing in new sediments. Sediments washing from the Acadian Mts. filled the western part of the Appalachian geosyncline to form the famous coal swamps of the Carboniferous period. Uplift of the Appalachians caused the region to be never again inundated by vast marginal seas. Paleoclimatic studies and evidence of glaciers indicate that central Africa was most likely in the polar regions during the early Paleozoic. During the early Paleozoic, the huge continent Gondwanaland had either formed or was forming. By mid-Paleozoic, the collision of N America and Europe produced the Acadian-Caledonian uplifts, and a subduction plate uplifted eastern Australia. By late Paleozoic, continental collisions formed the supercontinent Pangaea and resulted in some of the great mountain chains, including the Appalachians, Urals, and Tasmans. The most noteworthy feature of Paleozoic life is the sudden appearance of nearly all of the invertebrate animal phyla in great abundance at the beginning of the Cambrian. A few primitive fishlike invertebrates, and then vertebrates, appeared in the Cambrian and Ordovician, scorpions in the Silurian period, land invertebrates and amphibians in the Devonian, land reptiles in the Carboniferous, and marine reptiles in the Permian. All reptiles increased in number and in variety by the late Permian. The plant life of the Paleozoic era reached its climax in the Carboniferous and was much contracted in the Permian.

Mesozoic Era

The Mesozoic Era is one of three geologic eras of the Phanerozoic eon. The division of time into eras dates back to Giovanni Arduino, in the 18th century, although his original name for the era now called the 'Mesozoic' was 'Secondary' (making the modern era the 'Tertiary'). Lying between the Paleozoic and the Cenozoic, Mesozoic means 'middle animals', derived from Greek prefix meso-/μεσο- for 'between' and zoon/ζωον meaning animal or 'living being'. It is often called the 'Age of Medieval Life' or the 'Age of the Dinosaurs', after the dominant fauna of the era.
The Mesozoic was a time of tectonic, climatic and evolutionary activity. The continents gradually shifted from a state of connectedness into their present configuration; the rifting provided for speciation and other important evolutionary developments. The climate was exceptionally warm throughout the period, also playing an important role in the evolution and diversification of new animal species. By the end of the era, the basis of modern life was in place.

Mesozoic Era

Geologic periods

Following the Paleozoic, the Mesozoic extended roughly 180 million years: from 251 million years ago (Mya) to when the Cenozoic era began 65 Mya. This time frame is separated into three geologic Periods. From oldest to youngest:

• Triassic (251.0 Ma to 199.6 Ma)



• Jurassic (199.6 Ma to 145.5 Ma)



• Cretaceous (145.5 Ma to 65.5 Ma)

The lower (Triassic) boundary is set by the Permian-Triassic extinction, during which approximately 90% to 96% of marine species and 70% of terrestrial vertebrates became extinct. It is also known as the "Great Dying" because it is considered the largest mass extinction in history. The upper (Cretaceous) boundary is set at the Cretaceous-Tertiary (KT) extinction, which may have been caused by the meteor that created the Chicxulub Crater on the Yucatán Peninsula. Approximately 50% of all genera became extinct, including all of the non-avian dinosaurs.

Life in the Mesozoic Period

The extinction of nearly all animal species at the end of the Permian period allowed for the radiation of many new lifeforms. In particular, the extinction of the large herbivorous and carnivorous dinocephalia left those ecological niches empty. Some were filled by the suriving cynodonts and dicynodonts, the latter of which subsequently became extinct. Animal life during the Mesozoic was dominated, however, by large archosaurian reptiles that appeared a few million years after the Permian extinction: dinosaurs, pterosaurs, and aquatic reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs.
The climatic changes of the late Jurassic and Cretaceous provided for further adaptive radiation. The Jurassic was the height of archosaur diversity, and the first birds and placental mammals also appeared. Angiosperms radiated sometime in the early Cretaceous, first in the tropics, but the even temperature gradient allowed them to spread toward the poles throughout the period. By the end of the Cretaceous, angiosperms dominated tree floras in many areas, although some evidence suggests that biomass was still dominated by cycad and ferns until after the KT extinction.
Some have argued that insects diversied with angiosperms because insect anatomy, especially the mouth parts, seems particularly well-suited for flowering plants. However, all major insect mouth parts preceded angiosperms and insect diversification actually slowed when they arrived, so their anatomy originally must have been suited for some other purpose.
As the temperatures in the seas increased, the larger animals of the early Mesozoic gradually began to disappear while smaller animals of all kinds, including lizards, snakes, and perhaps the ancestor mammals to primates, evolved. The KT extinction exacerbated this trend. The large archosaurs became extinct, while birds and mammals thrived, as they do today.

Cenozoic Era

The Cenozoic Era (IPA pronunciation: sometimes Caenozoic Era in the United Kingdom) meaning "new life" (Greek kainos = new + zoe = life) is the most recent of the three classic geological eras. It covers the 65.5 million years since the Cretaceous-Tertiary extinction event at the end of the Cretaceous that marked the demise of the last dinosaurs and the end of the Mesozoic Era. The Cenozoic era is ongoing.
The Cenozoic is divided into two periods, the Palaeogene and Neogene, and they are in turn divided into epochs. The Palaeogene consists of the Paleocene, Eocene, and Oligocene epochs, and the Neogene consists of the Miocene, Pliocene, Pleistocene, and Holocene epochs, the last of which is ongoing. Historically, the Cenozoic has been divided into periods (or sub-eras) named the Tertiary (Paleocene to Pliocene) and Quaternary (Pleistocene and Holocene), although most geologists no longer recognize them.

Cenozoic Era's Life

The Cenozoic is the age of new life. During the Cenozoic, mammals diverged from a few small, simple, generalized forms into a diverse collection of terrestrial, marine, and flying animals. The Cenozoic is just as much the age of savannas, or the age of co-dependent flowering plants and insects. Birds also evolved substantially in the Cenozoic.
Geologically, the Cenozoic is the era when continents moved into their current positions. Australia-New Guinea split from Gondwana to drift north and, eventually, abut South-east Asia; Antarctica moved into its current position over the South Pole; the Atlantic Ocean widened and, later in the era, South America became attached to North America.

Geologic Timetable

The Female Reproductive System..

What Is the Female Reproductive System?

All living things reproduce. Reproduction - the process by which organisms make more organisms like themselves - is one of the things that sets living things apart from nonliving matter. But even though the reproductive system is essential to keeping a species alive, unlike other body systems, it's not essential to keeping an individual alive.

In the human reproductive process, 2 kinds of sex cells, or gametes, are involved. The male gamete, or sperm, and the female gamete, the egg or ovum, meet in the female's reproductive system to create a new individual. Both the male and female reproductive systems are essential for reproduction. The female needs a male to fertilize her egg, even though it is she who carries offspring through pregnancy and childbirth.

Humans, like other organisms, pass certain characteristics of themselves to the next generation through their genes, the special carriers of human traits. The genes that parents pass along to their children are what make children similar to others in their family, but they are also what make each child unique. These genes come from the male's sperm and the female's egg, which are produced by the male and female reproductive systems.

What Is the Female Reproductive System?

Most species have 2 sexes: male and female. Each sex has its own unique reproductive system. They are different in shape and structure, but both are specifically designed to produce, nourish, and transport either the egg or sperm.

Unlike the male, the human female has a reproductive system located entirely in the pelvis. The external part of the female reproductive organs is called the vulva, which means covering. Located between the legs, the vulva covers the opening to the vagina and other reproductive organs located inside the body.

The fleshy area located just above the top of the vaginal opening is called the mons pubis. Two pairs of skin flaps called the labia (which means lips) surround the vaginal opening. The clitoris, a small sensory organ, is located toward the front of the vulva where the folds of the labia join. Between the labia are openings to the urethra (the canal that carries urine from the bladder to the outside of the body) and vagina. Once girls become sexually mature, the outer labia and the mons pubis are covered by pubic hair.

A female's internal reproductive organs are the vagina, uterus, fallopian tubes, and ovaries.

The vagina is a muscular, hollow tube that extends from the vaginal opening to the uterus. The vagina is about 3 to 5 inches (8 to 12 centimeters) long in a grown woman. Because it has muscular walls, it can expand and contract. This ability to become wider or narrower allows the vagina to accommodate something as slim as a tampon and as wide as a baby. The vagina's muscular walls are lined with mucous membranes, which keep it protected and moist. The vagina serves 3 purposes: It's where the penis is inserted during sexual intercourse, and it's also the pathway that a baby takes out of a woman's body during childbirth, called the birth canal, and it provides the route for the menstrual blood (the period) to leave the body from the uterus.

A thin sheet of tissue with 1 or more holes in it called the hymen partially covers the opening of the vagina. Hymens are often different from person to person. Most women find their hymens have stretched or torn after their first sexual experience, and the hymen may bleed a little (this usually causes little, if any, pain). Some women who have had sex don't have much of a change in their hymens, though.

The vagina connects with the uterus, or womb, at the cervix (which means neck). The cervix has strong, thick walls. The opening of the cervix is very small (no wider than a straw), which is why a tampon can never get lost inside a girl's body. During childbirth, the cervix can expand to allow a baby to pass.

The uterus is shaped like an upside-down pear, with a thick lining and muscular walls - in fact, the uterus contains some of the strongest muscles in the female body. These muscles are able to expand and contract to accommodate a growing fetus and then help push the baby out during labor. When a woman isn't pregnant, the uterus is only about 3 inches (7.5 centimeters) long and 2 inches (5 centimeters) wide.

At the upper corners of the uterus, the fallopian tubes connect the uterus to the ovaries. The ovaries are 2 oval-shaped organs that lie to the upper right and left of the uterus. They produce, store, and release eggs into the fallopian tubes in the process called ovulation. Each ovary measures about 1 1/2 to 2 inches (4 to 5 centimeters) in a grown woman.

There are 2 fallopian tubes, each attached to a side of the uterus. The fallopian tubes are about 4 inches (10 centimeters) long and about as wide as a piece of spaghetti. Within each tube is a tiny passageway no wider than a sewing needle. At the other end of each fallopian tube is a fringed area that looks like a funnel. This fringed area wraps around the ovary but doesn't completely attach to it. When an egg pops out of an ovary, it enters the fallopian tube. Once the egg is in the fallopian tube, tiny hairs in the tube's lining help push it down the narrow passageway toward the uterus.

The ovaries are also part of the endocrine system because they produce female sex hormones such as estrogen and progesterone.

What Does the Female Reproductive System Do?

The female reproductive system enables a woman to:

• produce eggs (ova)





• have sexual intercourse





• protect and nourish the fertilized egg until it is fully developed





• give birth

Sexual reproduction couldn't happen without the sexual organs called the gonads. Although most people think of the gonads as the male testicles, both sexes actually have gonads: In females the gonads are the ovaries. The female gonads produce female gametes (eggs); the male gonads produce male gametes (sperm). After an egg is fertilized by the sperm, the fertilized egg is called the zygote.

When a baby girl is born, her ovaries contain hundreds of thousands of eggs, which remain inactive until puberty begins. At puberty, the pituitary gland, located in the central part of the brain, starts making hormones that stimulate the ovaries to produce female sex hormones, including estrogen. The secretion of these hormones causes a girl to develop into a sexually mature woman.

Toward the end of puberty, girls begin to release eggs as part of a monthly period called the menstrual cycle. Approximately once a month, during ovulation, an ovary sends a tiny egg into 1 of the fallopian tubes. Unless the egg is fertilized by a sperm while in the fallopian tube, the egg dries up and leaves the body about 2 weeks later through the uterus. This process is called menstruation. Blood and tissues from the inner lining of the uterus combine to form the menstrual flow, which in most girls lasts from 3 to 5 days. A girl's first period is called menarche.

It's common for women and girls to experience some discomfort in the days leading to their periods. Premenstrual syndrome (PMS) includes both physical and emotional symptoms that many girls and women get right before their periods, such as acne, bloating, fatigue, backaches, sore breasts, headaches, constipation, diarrhea, food cravings, depression, irritability, or difficulty concentrating or handling stress. PMS is usually at its worst during the 7 days before a girl's period starts and disappears once it begins.

Many girls also experience abdominal cramps during the first few days of their periods. They are caused by prostaglandin, a chemical in the body that makes the smooth muscle in the uterus contract. These involuntary contractions can be either dull or sharp and intense.

It can take up to 2 years from menarche for a girl's body to develop a regular menstrual cycle. During that time, her body is adjusting to the hormones puberty brings. On average, the monthly cycle for an adult woman is 28 days, but the range is from 23 to 35 days

If a female and male have sex within several days of the female's ovulation, fertilization can occur. When the male ejaculates (which is when semen leaves a male's penis), between 0.05 and 0.2 fluid ounces (1.5 to 6.0 milliliters) of semen is deposited into the vagina. Between 75 and 900 million sperm are in this small amount of semen, and they "swim" up from the vagina through the cervix and uterus to meet the egg in the fallopian tube. It takes only 1 sperm to fertilize the egg.

About a week after the sperm fertilizes the egg, the fertilized egg (zygote) has become a multicelled blastocyst. A blastocyst is about the size of a pinhead, and it's a hollow ball of cells with fluid inside. The blastocyst burrows itself into the lining of the uterus, called the endometrium. The hormone estrogen causes the endometrium to become thick and rich with blood. Progesterone, another hormone released by the ovaries, keeps the endometrium thick with blood so that the blastocyst can attach to the uterus and absorb nutrients from it. This process is called implantation.

As cells from the blastocyst take in nourishment, another stage of development, the embryonic stage, begins. The inner cells form a flattened circular shape called the embryonic disk, which will develop into a baby. The outer cells become thin membranes that form around the baby. The cells multiply thousands of times and move to new positions to eventually become the embryo. After approximately 8 weeks, the embryo is about the size of an adult's thumb, but almost all of its parts - the brain and nerves, the heart and blood, the stomach and intestines, and the muscles and skin - have formed.

During the fetal stage, which lasts from 9 weeks after fertilization to birth, development continues as cells multiply, move, and change. The fetus floats in amniotic fluid inside the amniotic sac. The fetus receives oxygen and nourishment from the mother's blood via the placenta, a disk-like structure that sticks to the inner lining of the uterus and connects to the fetus via the umbilical cord. The amniotic fluid and membrane cushion the fetus against bumps and jolts to the mother's body.

Pregnancy lasts an average of 280 days - about 9 months. When the baby is ready for birth, its head presses on the cervix, which begins to relax and widen to get ready for the baby to pass into and through the vagina. The mucus that has formed a plug in the cervix loosens, and with amniotic fluid, comes out through the vagina when the mother's water breaks.

When the contractions of labor begin, the walls of the uterus contract as they are stimulated by the pituitary hormone oxytocin. The contractions cause the cervix to widen and begin to open. After several hours of this widening, the cervix is dilated (opened) enough for the baby to come through. The baby is pushed out of the uterus, through the cervix, and along the birth canal. The baby's head usually comes first; the umbilical cord comes out with the baby and is cut after the baby is delivered. The last stage of the birth process involves the delivery of the placenta, which is now called the afterbirth. After it has separated from the inner lining of the uterus, contractions of the uterus push it out, along with its membranes and fluids.

Things That Can Go Wrong With the Female Reproductive System

Your child may sometimes experience reproductive system problems. Below are some examples of disorders that affect the female reproductive system.

Things That Can Go Wrong With the Vulva and Vagina

• Vulvovaginitis is an inflammation of the vulva and vagina. It may be caused by irritating substances (such as laundry soaps or bubble baths). Poor personal hygiene (such as wiping from back to front after a bowel movement) may also cause this problem. Symptoms include redness and itching in the vaginal and vulvar areas and sometimes vaginal discharge. Vulvovaginitis can also be caused by an overgrowth of Candida, a fungus normally present in the vagina.





• Nonmenstrual vaginal bleeding is most commonly due to the presence of a vaginal foreign body, often wadded-up toilet paper. It may also be due to urethral prolapse, a condition in which the mucous membranes of the urethra protrude into the vagina and form a tiny, doughnut-shaped mass of tissue that bleeds easily. It can also be due to a straddle injury (such as when falling onto a beam or bicycle frame) or vaginal trauma from sexual abuse.





• Labial adhesions, the sticking together or adherence of the labia in the midline, usually appear in infants and young girls. Although there are usually no symptoms associated with this condition, labial adhesions can lead to an increased risk of urinary tract infection. Sometimes topical estrogen cream is used to help separate the labia.

Things That Can Go Wrong With the Ovaries and Fallopian Tubes

• Ectopic pregnancy occurs when a fertilized egg, or zygote, doesn't travel into the uterus, but instead grows rapidly in the fallopian tube. If a female has this condition, she can develop severe abdominal pain and should see a doctor because surgery may be necessary.





• Endometriosis occurs when tissue normally found only in the uterus starts to grow outside the uterus - in the ovaries, fallopian tubes, or other parts of the pelvic cavity. It can cause abnormal bleeding, painful periods, and general pelvic pain.





• Ovarian tumors, although they're rare, can occur. Girls with ovarian tumors may have abdominal pain and masses that can be felt in the abdomen. Surgery may be needed to remove the tumor.





• Ovarian cysts are noncancerous sacs filled with fluid or semisolid material. Although they are common and generally harmless, they can become a problem if they grow very large. Large cysts may push on surrounding organs, causing abdominal pain. In most cases, cysts will disappear on their own and treatment is unnecessary. If the cysts are painful, a doctor may prescribe birth control pills to alter their growth, or they may be removed by a surgeon.





• Polycystic ovary syndrome is a hormone disorder in which too many male hormones (androgens) are produced by the ovaries. This condition causes the ovaries to become enlarged and develop many fluid-filled sacs, or cysts. It often first appears during the teen years. Depending on the type and severity of the condition, it may be treated with drugs to regulate hormone balance and menstruation.





• Ovarian torsion, or the twisting of the ovary, can occur when an ovary becomes twisted because of a disease or a developmental abnormality. The torsion blocks blood from flowing through the blood vessels that supply and nourish the ovaries. The most common symptom is lower abdominal pain. Surgery is usually necessary to correct the condition.

Menstrual Problems

There are a variety of menstrual problems that can affect girls. Some of the more common conditions are:

• Dysmenorrhea is when a girl has painful periods.





• Menorrhagia is when a girl has a very heavy periods with excess bleeding.





• Oligomenorrhea is when a girl misses or has infrequent periods, even though she's been menstruating for a while and isn't pregnant.





• Amenorrhea is when a girl has not started her period by the time she is 16 years old or 3 years after starting puberty, has not developed signs of puberty by age 14, or has had normal periods but has stopped menstruating for some reason other than pregnancy.

Infections of the Female Reproductive System

• Sexually transmitted diseases. These include infections and diseases such as pelvic inflammatory disease (PID), human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS), human papillomavirus (HPV, or genital warts), syphilis, chlamydia, gonorrhea, and genital herpes. Most are spread from one person to another by sexual intercourse.





• Toxic shock syndrome. This uncommon illness is caused by toxins released into the body during a type of bacterial infection that is more likely to develop if a tampon is left in too long. It can produce high fever, diarrhea, vomiting, and shock.

Male Reproductive System

Unlike the female, whose sex organs are located entirely within the pelvis, the male has reproductive organs, or genitals, that are both inside and outside the pelvis. The male genitals include:

the testicles
the duct system, which is made up of the epididymis and the vas deferens
the accessory glands, which include the seminal vesicles and prostate gland
the penis

In a guy who has reached sexual maturity, the 2 testicles, or testes, produce and store millions of tiny sperm cells. The testicles are oval-shaped and grow to be about 2 inches (5 centimeters) in length and 1 inch (3 centimeters) in diameter. The testicles are also part of the endocrine system because they produce hormones, including testosterone. Testosterone is a major part of puberty in boys, and as a guy makes his way through puberty, his testicles produce more and more of it. Testosterone is the hormone that causes boys to develop deeper voices, bigger muscles, and body and facial hair, and it also stimulates the production of sperm.

Alongside the testicles are the epididymis and the vas deferens, which make up the duct system of the male reproductive organs. The vas deferens is a muscular tube that passes upward alongside the testicles and transports the sperm-containing fluid called semen. The epididymis is a set of coiled tubes (one for each testicle) that connects to the vas deferens.

The epididymis and the testicles hang in a pouch-like structure outside the pelvis called the scrotum. This bag of skin helps to regulate the temperature of testicles, which need to be kept cooler than body temperature to produce sperm. The scrotum changes size to maintain the right temperature. When the body is cold, the scrotum shrinks and becomes tighter to hold in body heat. When it's warm, the scrotum becomes larger and more floppy to get rid of extra heat. This happens without a guy ever having to think about it. The brain and the nervous system give the scrotum the cue to change size.

The accessory glands, including the seminal vesicles and the prostate gland, provide fluids that lubricate the duct system and nourish the sperm. The seminal vesicles are sac-like structures attached to the vas deferens to the side of the bladder. The prostate gland, which produces some of the parts of semen, surrounds the ejaculatory ducts at the base of the urethra, just below the bladder. The urethra is the channel that carries the semen to the outside of the body through the penis. The urethra is also part of the urinary system because it is also the channel through which urine passes as it leaves the bladder and exits the body.

The penis is actually made up of 2 parts: the shaft and the glans. The shaft is the main part of the penis and the glans is the tip (sometimes called the head). At the end of the glans is a small slit or opening, which is where semen and urine exit the body through the urethra. The inside of the penis is made of a spongy tissue that can expand and contract.

All boys are born with a foreskin, a fold of skin at the end of the penis covering the glans. Some boys have a circumcision, which means that a doctor or clergy member cuts away the foreskin. Circumcision is usually performed during a baby boy's first few days of life. Although circumcision is not medically necessary, parents who choose to have their children circumcised often do so based on religious beliefs, concerns about hygiene, or cultural or social reasons. Boys who have circumcised penises and those who don't are no different: All penises work and feel the same, regardless of whether the foreskin has been removed.

What Does the Male Reproductive System Do?

The male sex organs work together to produce and release semen into the reproductive system of the female during sexual intercourse. The male reproductive system also produces sex hormones, which help a boy develop into a sexually mature man during puberty.

When a baby boy is born, he has all the parts of his reproductive system in place, but it isn't until puberty that he is able to reproduce. When puberty begins, usually between the ages of 10 and 14, the pituitary gland - which is located in the brain - secretes hormones that stimulate the testicles to produce testosterone. The production of testosterone brings about many physical changes. Although the timing of these changes is different for every guy, the stages of puberty generally follow a set sequence.

During the first stage of male puberty, the scrotum and testes grow larger.
Next, the penis becomes longer, and the seminal vesicles and prostate gland grow.
Hair begins to appear in the pubic area and later it grows on the face and underarms. During this time, a male's voice also deepens.
Boys also undergo a growth spurt during puberty as they reach their adult height and weight.
Once a male has reached puberty, he will produce millions of sperm cells every day. Each sperm is extremely small: only 1/600 of an inch (0.05 millimeters long). Sperm develop in the testicles within a system of tiny tubes called the seminiferous tubules. At birth, these tubules contain simple round cells, but during puberty, testosterone and other hormones cause these cells to transform into sperm cells. The cells divide and change until they have a head and short tail, like tadpoles. The head contains genetic material (genes). The sperm use their tails to push themselves into the epididymis, where they complete their development. It takes sperm about 4 to 6 weeks to travel through the epididymis.

The sperm then move to the vas deferens, or sperm duct. The seminal vesicles and prostate gland produce a whitish fluid called seminal fluid, which mixes with sperm to form semen when a male is sexually stimulated. The penis, which usually hangs limp, becomes hard when a male is sexually excited. Tissues in the penis fill with blood and it becomes stiff and erect (an erection). The rigidity of the erect penis makes it easier to insert into the female's vagina during sexual intercourse. When the erect penis is stimulated, muscles around the reproductive organs contract and force the semen through the duct system and urethra. Semen is pushed out of the male's body through his urethra - this process is called ejaculation. Each time a guy ejaculates, it can contain up to 500 million sperm.

When the male ejaculates during intercourse, semen is deposited into the female's vagina. From the vagina the sperm make their way up through the cervix and move through the uterus with help from uterine contractions. If a mature egg is in 1 of the female's fallopian tubes, a single sperm may penetrate it, and fertilization, or conception, occurs.

This fertilized egg is now called a zygote and contains 46 chromosomes - half from the egg and half from the sperm. The genetic material from the male and female has combined so that a new individual can be created. The zygote divides again and again as it grows in the female's uterus, maturing over the course of the pregnancy into an embryo, a fetus, and finally a newborn baby.

Things That Can Go Wrong With the Male Reproductive System
Boys may sometimes experience reproductive system problems. Below are some examples of disorders that affect the male reproductive system:

Disorders of the Scrotum, Testicles, or Epididymis
Conditions affecting the scrotal contents may involve the testicles, epididymis, or the scrotum itself.
Testicular trauma. Even a mild injury to the testicles can cause severe pain, bruising, or swelling. Most testicular injuries occur when the testicles are struck, hit, kicked, or crushed, usually during sports or due to other trauma. Testicular torsion, when 1 of the testicles twists around, cutting off the blood supply, is also a problem that some teen males experience - although it's not common. Surgery is needed to untwist the cord and save the testicle.
Varicocele. This is a varicose vein (an abnormally swollen vein) in the network of veins that run from the testicles. Varicoceles commonly develop while a boy is going through puberty. A varicocele is usually not harmful, although in some people it may damage the testicle or decrease sperm production, so it helps for you to take your child to see his doctor if he is concerned about changes in his testicles.
Testicular cancer. This is one of the most common cancers in men younger than 40. It occurs when cells in the testicle divide abnormally and form a tumor. Testicular cancer can spread to other parts of the body, but if it's detected early, the cure rate is excellent. Teen boys should be encouraged to learn to perform testicular self-examinations.
Epididymitis is inflammation of the epididymis, the coiled tubes that connect the testes with the vas deferens. It is usually caused by an infection, such as the sexually transmitted disease chlamydia, and results in pain and swelling next to 1 of the testicles.
Hydrocele. A hydrocele occurs when fluid collects in the membranes surrounding the testes. Hydroceles may cause swelling of the testicle but are generally painless. In some cases, surgery may be needed to correct the condition.
Inguinal hernia. When a portion of the intestines pushes through an abnormal opening or weakening of the abdominal wall and into the groin or scrotum, it is known as an inguinal hernia. The hernia may look like a bulge or swelling in the groin area. It can be corrected with surgery.

Disorders of the Penis
Disorders affecting the penis include the following:
Inflammation of the penis. Symptoms of penile inflammation include redness, itching, swelling, and pain. Balanitis occurs when the glans (the head of the penis) becomes inflamed. Posthitis is foreskin inflammation, which is usually due to a yeast or bacterial infection.
Hypospadias. This is a disorder in which the urethra opens on the underside of the penis, not at the tip.
Phimosis. This is a tightness of the foreskin of the penis and is common in newborns and young children. It usually resolves itself without treatment. If it interferes with urination, circumcision (removal of the foreskin) may be recommended.
Paraphimosis. This may develop when a boy's uncircumcised penis is retracted but doesn't return to the unretracted position. As a result, blood flow to the penis may be impaired, and your child may experience pain and swelling. A doctor may try to use lubricant to make a small incision so the foreskin can be pulled forward. If that doesn't work, circumcision may be recommended.
Ambiguous genitalia. This occurs when a child is born with genitals that aren't clearly male or female. In most boys born with this disorder, the penis may be very small or nonexistent, but testicular tissue is present. In a small number of cases, the child may have both testicular and ovarian tissue.
Micropenis. This is a disorder in which the penis, although normally formed, is well below the average size, as determined by standard measurements.
Sexually transmitted diseases. Sexually transmitted diseases (STDs) that can affect boys include human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS), human papillomavirus (HPV, or genital warts), syphilis, chlamydia, gonorrhea, genital herpes, and hepatitis B. They are spread from 1 person to another mainly through sexual intercourse.
If your child has symptoms of a problem with his reproductive system or he has questions about growth and development, talk to your child's doctor - many problems with the male reproductive system can be treated.