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Chapter 17
DOMAIN BACTERIA, DOMAIN ARCHAEA, VIRUSES
Prokaryotes are everywhere.
They have a tremendous impact on life on Earth.
There are two branches of prokaryote evolution: Archaea and Bacteria (Eubacteria).
3.5 billion years ago ----- prokaryotic type cells.
1.5 billion years ago ----- eukaryotic cells called acritarchs.
CHARACTERISTICS OF THE EUBACTERIA AND ARCHAEA
Prokaryote Characteristics
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Prokaryote cells are smaller than those of eukaryotes.
They lack membrane bound organelles.
Their chromosomes are circular DNA molecules lacking protein.
They divide by binary fission.
They lack mitosis and meiosis.
Occasionally undergo genetic recombination through conjugation (exchange of DNA) by
means of pili (sing. pilus).
They often have slimy capsules and a flagellum, which allows limited motility.
Many are colonial forming filaments but the cells remain independent without any
cytoplasmic connections.
Nutrition is by the absorption of food in solution through their cell wall and plasma
membrane; some obtain energy through chemical reactions involving inorganic substances
(e.g. H2S or S2032- thiosulfate ion) and others are photosynthetic.
DIFFERENCES BETWEEN PROKARYOTES AND EUKARYOTES
Prokaryotes: lack nuclear envelope; reproduce by binary fission; lack membrane-bound organelles;
one chromosome, circular; lack split genes (no introns).
Recent revisions of the prokaryotic Kingdom Monera have resulted in creation of two groups of
prokaryotes. These two taxa have been elevated to the status of Domain.
Eubacteria: peptidoglycan with muramic acid cell wall.
Archaebacteria: cell wall incorporates other substances but no muramic acid.
There are other differences between these two groups mostly in RNA sensitivity and structure.
Eukaryotes: have a nuclear membrane; mitotic mechanism; membrane-bound organelles; several
chromosomes, free ends; split genes (introns and exons), cytoskeleton.
CELLULAR DETAIL AND REPRODUCTION OF BACTERIA
The DNA is concentrated in the nucleoid region.
One circular, double-stranded DNA molecule that contains genes responsible for metabolism, cell
growth, and cell replication.
This circular DNA molecule is usually attached at one point to the plasma membrane.
The bacterial DNA is sometimes called the genophore.
The chromosome of bacteria contains about 1/1000 of the DNA material found in eukaryotes, and
has relatively little protein associated with it.
Replication and translation is similar to eukaryotes.
Plasmids are small circular DNA molecules found in bacteria.
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Plasmids are separate and independent from the chromosome
Plasmids are capable of replication.
Plasmids have small number of genes.
These genes are not normally needed for reproduction or survival of the bacterium.
These often encode antibiotic resistance genes, and genes encoding unusual metabolisms.
Plasmids often consist of copies of one or at most very few different plasmids.
Plasmids can be transferred between cells by conjugation.
Bacteria reproduce by binary fission.
Binary fission replication that is the splitting of single cell into two cells.
DNA replicates and a transverse wall is formed by the ingrowth of the plasma membrane and cell
wall.
In budding, a cell develops a bulge or bud that enlarges and eventually separates from the mother
cell.
In fragmentation, walls develop within the cell that eventually separates into several cells.
Genetic material is sometimes exchange between bacterial cells.
Transformation occurs when a cell takes in fragments of DNA released by another cell into their
environment.
Conjugation occurs when two cells of different mating types come together and genetic material is
transferred from one cell to the other by means of a sex pilus.
In transduction viruses transfer genes between bacterium cells.
SIZE, FORM AND CLASSIFICATION OF BACTERIA
Except for the cyanobacteria, most bacteria are between 2 and 3 micrometers in diameter, and a few
approach 0.15 micrometer.
Their unique shapes are useful in classification and identification.
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coccus (sphere) e.g.: Staphylococcus aureus {common skin bacterium}
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bacillus (rod) e.g.: Clostridium tetanus {produces tetanus toxin}
spirochete (corkscrew, flexible) e.g.: Treponema pallidum {cause of syphilis}
spirillum (rigid helix) e.g.
vibrio (comma shape) e.g.: Vibrio cholera {cause of cholera}
In addition to individual shape bacteria can be classified in a variety of ways:
1. Colony shape in culture.
2. Motility.
3. Morphological characteristics other than shape, e.g. multiple flagella.
4. Metabolic activity and biochemistry, e. g. What sugars they ferment.
5. DNA sequence.
6. Clinical bacteria are classified according to how dangerous they are.
7. Where they grow.
Endospores are specialized structures formed by some bacteria.
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Formed inside bacterial membrane, consisting of thick peptidoglycan layers surrounding
DNA, RNA, ribosomes, and essential enzymes.
Allows the cell to survive under adverse conditions.
Highly dehydrated, no metabolism occurring until hydrated again. Special endospore
staining needed to visualize.
It is not a reproductive spore.
Most bacteria have a cell wall made of peptidoglycans, sugars linked to short polypeptides.
The walls of archaea lack peptidoglycans.
The cell wall structure varies with the species.
1. Gram-positive bacteria absorb and retain crystal violet stain; their wall is structurally
simpler and with a large amount of peptidoglycans than the next group.
2. Gram-negative bacteria do not absorb crystal violet stain; their cell wall is structurally
more complex and has fewer amounts of peptidoglycans.
The cell wall of gram-negative bacteria is made of a layer of peptidoglycan and proteins surrounded
by an outer membrane of lipopolysaccharides, carbohydrates bounded to lipids.
These lipopolysaccharides are often toxic.
Many prokaryotes secrete a sticky substance that forms another protective layer called the capsule or
glycocalyx outside the cell wall.
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Composed of polysaccharides and / or peptidoglycans.
Sometimes called the capsule or slime layer.
Conveys virulence! {example: Streptococcus pneunoniae, Bacillus anthracis}
Facilitates attachment {example: attachment of encapsulated bacteria to epithelium}.
Some bacteria have hundreds of hair-like appendages known as fimbriae (sing. fimbria) and pili.
Fimbriae are more numerous and shorter than pili.
Fimbriae: attachment to surfaces {example: Neisseria gonorrhea use pili for attachment to mucous
membranes of the penis, vagina, and urinary tract}
"Sex pili" allow for joining of two bacteria during conjugation exchange of DNA.
In a heterogeneous medium, many prokaryotes exhibit taxis, that is attraction to a stimulus or a
movement away from it.
Most motile bacteria move by means of rotating flagella.
Bacterium flagella have a structure different from that of eukaryotes; it consists of a basal body, a
hook and the filament.
Spirochetes move with a corkscrew motion.
The motion is created by helical filaments under the outer layer of the cell wall, similar in structure
to the flagella.
When the filaments rotate, the cells moves in a corkscrew motion.
Some bacteria secrete a slimy thread that anchor to the substratum. As the cells continue to secrete
jets of slime, the filamentous prokaryote glides along the growing end of the filament.
DOMAIN EUBACTERIA - KINGDOM BACTERIA
PHYLUM BACTERIOPHYTA
Class Bacteriae
True bacteria have muramic acid as a constituent of their cell wall.
Most of them are heterotrophic.
Saprobes: obtain their food from non-living organic matter.
Parasites: depend on living organisms for their food.
Autotrophic bacteria are capable of synthesizing organic compounds from simple inorganic
compounds.
“In 1993, scientists found many new species of chemoautotrophic bacteria living in fissured rock far
below the ocean floor. These bacteria take in carbon dioxide and water and convert the chemical
energy in sulfur compounds to run metabolic processes that create carbohydrates and sugars. A
unique characteristic of these chemoautotrophic bacteria is that they thrive at temperatures high
enough to kill other organisms. Some scientists assert that these unique bacteria should be classified
in their own new taxonomic kingdom.”
http://www.bookrags.com/sciences/biology/autotrophic-bacteria-wmi.html
Nutrition is varied in bacteria.
According to the source of carbon:
Heterotrophic species are saprophytic (saprotrophs) and parasitic. They require at least one organic
nutrient.
Autotrophic bacteria are either photosynthetic or chemosynthetic. They need inorganic CO2 as the
source of carbon.
According to the source of energy:
1. Phototrophs obtain energy from light.
2. Chemotrophs obtain energy from chemicals taken from the environment.
Combining the sources of carbon and energy:
Autotrophs:
1. Photoautotrophs obtain energy from light and CO2 is carbon source.
2. Chemoautotrophs obtain their energy by oxidizing inorganic chemicals and CO2 is carbon
source, e.g. such as hydrogen sulfide, sulfur, ammonia, nitrites, hydrogen gas, or iron.
The purple and green sulfur bacteria take the energy of sunlight, carbon dioxide and hydrogen
sulfide to produce carbohydrates. They are photoautotrophs.
Bacteriochlorophyll pigment instead of chlorophyll.
The purple and green sulfur bacteria use hydrogen sulfide instead of water, and the equation is:
CO2 + 2H2S = CH2O + H2O + 2S
The iron bacteria are chemoautotrophs.
Heterotrophs:
1. Photoheterotrophs obtain energy from light and carbon from organic molecules.
2. Chemoheterotrophs must consume organic molecules for both energy and carbon.
Most bacteria require oxygen to live. They are aerobes (obligate aerobes).
Some bacteria carry their metabolic functions always in the absence of oxygen. They are obligate
anaerobes (obligate anaerobes).
Facultative anaerobes use oxygen if it is available but can also carry on their metabolic functions
anaerobically.
Some prokaryotes are capable of using atmospheric nitrogen as the source of nitrogen.
Nitrogen fixation: N2 is converted to NH4+.
This is the only known biological mechanism that uses N2.
Some soil bacteria convert NH4+ to NO2-; others convert NO2- to nitrates, NO3_.
HUMAN RELEVANCE OF THE UNPIGMENTED, PURPLE AND GREEN
SULFUR BACTERIA
Composting and compost
Nonliving organic material is converted to compost (humus) by decomposers: bacteria and fungi.
Compost is good for gardening and as a soil conditioner.
True bacteria and disease
Bacteria are parasites of plants, animals and humans.
Diseases are transmitted through tiny particles in the air.
Cough, sneeze, shouting release tiny drops of saliva containing bacteria.
Moisture evaporates and bacteria adhere to minute protein flakes left behind.
These flakes are inhaled by those nearby.
Open sewers and unsanitary conditions for food preparation cause bacterial diseases.
Cholera, typhoid, dysentery, Staphylococcus and Salmonella poison.
Botulism is a poison caused by Clostridium botulinum, a bacterium.
Syphilis, gonorrhea, anthrax and brucellosis enter the body through the skin or mucous membranes.
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Syphilis and gonorrhea are transmitted through sexual contact.
Anthrax is a disease of cattle, farm animals and wild animals.
Brucellosis is also a disease of farm animals; it can be transmitted through contaminated milk.
There is effective treatment for these diseases if done early enough.
Bacteria can get access into the body through open wounds: Tetanus and gangrene.
Insects and other organisms can act as vectors of bacterial diseases.
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Bubonic plague was caused by ticks, fleas and other ectoparasites of animals.
Tularemia, rickettsias, Lyme disease and Mycoplasmas are transmitted by fleas and ticks.
KOCH’S POSTULATES
In 1890 the German physician and bacteriologist Robert Koch set out his celebrated criteria for
judging whether a given bacteria is the cause of a given disease. Koch's criteria brought some muchneeded scientific clarity to what was then a very confused field.
Koch's postulates are as follows:
1. The bacteria must be present in every case of the disease.
2. The bacteria must be isolated from the host with the disease and grown in pure culture.
3. When bacteria from a pure culture are inoculated into a healthy susceptible host, it must
produce the disease in the host.
4. The bacteria must be recovered from the experimentally infected host and grown in pure
culture for comparison with the original culture.
http://www.medterms.com/script/main/art.asp?articlekey=7105
True Bacteria Useful To Humans
Bacillus thuringiensis or Bt.
Unlike typical nerve-poison insecticides, Bt acts by producing proteins (delta-endotoxin, the "toxic
crystal") that reacts with the cells of the gut lining of susceptible insects. These Bt proteins paralyze
the digestive system, and the infected insect stops feeding within hours. Bt-affected insects generally
die from starvation, which can take several days.
The gene for the Bt toxin has been introduced into several crops through genetic engineering. There
is no need to spray these crops.
Bacillus thuringiensis var. israelensis.
To control mosquito larvae, formulations containing the israelensis strain are placed into the
standing water of mosquito breeding sites. It is very specific; it attacks only mosquito larvae and two
other minor pests.
Bacillus popilliae
It attacks the larva of the Japanese beetle. This beetle was introduced accidentally in the US and is
very destructive to ornamental and agricultural crop plants.
The cause of death in insects infected with B. popilliae is not fully known. Physiological starvation
caused by the growth of bacterial cells in the hemolymph seems the most likely explanation, and fat
reserves of diseased larvae have been shown to be much reduced compared with those of healthy
larvae. However, toxins also may be involved because they have been detected in culture filtrates of
the bacteria and shown to be lethal on injection.
http://www.nysaes.cornell.edu/ent/biocontrol/pathogens/bacillus_popilliae.html
BIOREMEDIATION
Bioremediation is the use of organisms to cleanup the environment of hazardous wastes.
Usually plants and microbes are used in this process.
Bacteria capable of cleaning up oil spills, munitions factories, explosives, Agent Orange Defoliant,
creosote, etc.
OTHER USEFUL BACTERIA
The most important role of bacteria is in the decomposition of organic matter. Bacteria and fungi are
the major decomposers in the environment.
Saprophytic bacteria are major components of waste water treatment plants where raw sewage is
treated before releasing into waterways.
Nitrogen-fixing bacteria are essential in the recycling of nitrogen in ecosystems.
Lactic acid bacteria are Gram-positive, nonsporeforming rods and cocci which produce lactic acid as
a sole or major end product of fermentation. They are important in the food industry as fermentation
organisms in the production of cheese, yogurt, buttermilk, sour cream, pickles, sauerkraut, sausage
and other foods. Important genera are Streptococcus and Lactobacillus.
Bacteria of the genus Brevibacterium are responsible for the making and odor of Brie and Limburger
cheese.
Bacteria are used in the manufacture of industrial chemicals, vitamins, flavorings, food stabilizers,
and a blood plasma substitute; they play a role in the curing of vanilla, cocoa beans, coffee, and tea
and in the production of vinegar and sauerkraut; they aid in the extraction of linen fibers from flax
stems, in the production of ensilage for cattle feed, and in the production of several important amino
acids.
Class Cyanobacteriae
Cyanobacteria are also known as blue-green algae.
Characteristics
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Prokaryotic cells.
Chlorophyll a present
Blue phycocyanin and red phycoerythrin pigments
Can fix nitrogen
Produce oxygen by splitting water
Lack flagella but some species have gliding movements.
May form three different kinds of cells: vegetative, akinetes (thick walled) and heterocysts
(nitrogen fixing).
Single cells or colonial; colonies may be filamentous, sheets or hollow balls.
Reproduction is by binary fission, budding or fragmentation.
Forms symbiotic associations with protists, plants and animals.
Genetic recombination has been reported but is apparently rare; method similar to other
bacteria.
Studies of metabolic similarities and ribosomal RNA sequence suggest that cyanobacteria form a
good, monophyletic taxon.
Cyanobacteria have an elaborate and highly organized system of internal membranes which function
in photosynthesis.
Chlorophyll a and several accessory pigments (phycoerythrin and phycocyanin) are embedded in
these photosynthetic lamellae, the analogs of the eukaryotic thylakoid membranes
Cyanobacteria are virtually ubiquitous in their occurrence: marine and fresh water habitat, hot
springs to Antarctica; terrestrial: walls, tree trunks, lava flows, soil, etc.
They produce cyanophycin, a nitrogenous food reserve.
Fragmentation may occur at heterocysts.
Akinetes may also be produced during periods of stress.
Cyanobacteria cells may have been the origin of chloroplasts, since they divide as chloroplasts do
when their host cells divide.
Cyanobacteria may become very abundant in bodies of polluted fresh water.
Toxic substances are produced when the bacteria die and are decomposed.
At least 40 species of cyanobacteria are known to fix nitrogen.
Cyanobacteria, chloroplasts and oxygen
Symbiotic cyanophyta lack cell wall and appear to function like chloroplasts.
When the cell of the symbiont divides, the cyanobacteria also divide just like chloroplasts do.
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3.5 billion years ago ----- prokaryotic type cells.
1.5 billion years ago ----- eukaryotic cells called acritarchs.
Molecular oxygen was a by product of the splitting of water. An oxidizing aerobic environment
began to form.
We don't know at what speed oxygen began to accumulate in the atmosphere.
3.5 billion-year-old cyanobacteria fossils have been found in Warrawoona, Australia.
About 2 billion years ago the concentration of oxygen in the atmosphere began to gradually increase
to its present level.
By the Cambrian (545 m.y.a.) the levels of O2 had increased enough to allow the rapid evolution of
aerobic organisms.
Antioxidant compounds like vitamins K and E, carotenes, coenzyme Q, and others, began to appear.
Ozone layer also was formed that prevents short wave UV rays from reaching the earth's surface.
Human relevance of cyanobacteria
Cyanobacteria are at the bottom of the food chain.
More than 40 species are nitrogen fixers.
They cause algal blooms that give bad taste and odor to the water.
They can produce toxic substances that can kill animals and humans.
Blue-green algae produce three main types of toxin:
• Endotoxins are thought to produce allergic reactions, skin rashes, irritation of the eyes, and
gastroenteritis.
• Neurotoxins damage nerves and can cause muscle tremors, especially in the muscles
animals and people need to breath.
• Hepatotoxins damage the liver. They may also increase the risk of certain types of cancer.
To put the toxicity of blue-green algae in perspective, Microcystis, one of the blue-green algae found
in Australian waters, produces a toxin called microcystin LR. Microcystin LR is 200 times more
toxic than cyanide.
Some species are edible; others produce antibiotics, and other cause allergic reactions.
Class Prochlorobacteriae
Prochlorobacteria are similar in form to cyanobacteria but have pigmentation similar to that of
higher plants and lack phycobilins.
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They have chlorophyll a and b.
Lack phycobilins.
Have carotenoids.
Double membrane.
Double membrane makes their internal membrane system.
The cell wall possesses both Gram-positive and Gram-negative structural features.
All the known species are either free living or are associated with invertebrate hosts.
Molecular evidence suggests a close relationship with the cyanobacteria.
Two species have been described, Prochloron (symbiotic) and Prochlorothrix (free living,
filamentous).
“Prochloron is a genus of prokaryotic algae with photosynthetic pigments like those of chlorophytes.
Prochlorophytes are almost invariably found associated as symbionts with marine protochordates
(didemnid ascidians), and so far none has been successfully grown in sustained culture away from in
host. Based on materials collected from nature, information of various sorts (biochemical,
physiological, cytological and fine-structural) has been obtained, indicating many resemblances
(and probably close phylogenetic affinities) between prochlorophytes and cyanophytes.”
Phycologia. 23(2):203-8. 1984. Lewin RA. Scripps Institution of Oceanography, University of California, La Jolla 92093,
USA.
“Prochloron cells have been found in extracellular symbiosis with 20 species of ascidians, chiefly
didemnids and one holothurian (Cox, 1986). The associations were mainly found throughout the
tropical Pacific and Indian Oceans.” LUUC R. MUR and TINEKE BURGER-WIERSMA
http://141.150.157.117:8080/prokPUB/chaphtm/099/COMPLETE.htm
DOMAIN (KINGDOM) ARCHAEA
Characteristics
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Domain Archaea is mostly composed of cells that live in extreme environments.
Species of the domain Archaea are not inhibited by antibiotics.
Lack peptidoglycan in their cell wall (unlike bacteria, which have this sugar/polypeptide
compound).
Can have branched carbon chains in their membrane lipids of the phospholipid bilayer.
Eukaryotes share with the Archaea many mRNA sequences, have similar RNA polymerases, and
have introns.
It is believed that the domains Archaea and Bacteria branched from each other very early in
history, and membrane infolding produced eukaryotic cells in the archaean branch approximately
1.7 billion years ago.
There are three main groups of Archaea: extreme halophiles, methanogens, and
hyperthermophiles.
Source: http://www.sidwell.edu/us/science/vlb5/Labs/Classification_Lab/Archaea/
Methanogens
Bacillus and coccus are the most common cell forms; spiral and sarcina forms exist.
Methanogens produce methane gas from simple carbon compounds.
They live in anaerobic environments like swamp, marine sediments and digestive track of animals.
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4H2 + CO2 → CH4 + 2 H2O
Δ -130.4 kJ/mole of methane
Methanogens can live in a variety of environments; there are many freshwater and marine
methanogens but very few hypersaline.
They can be found in temperatures up to 100ºC but methanogenesis is inhibited below 15ºC.
Halophiles – Salt bacteria
They can live in salty environments exceeding 15% salinity.
Most are photosynthetic autotrophs.
They often color the rocks on which they live with a purple-reddish color due bacteriorhodopsin, a
photosynthetic pigment.
“The exact chemical explanation for the extreme salt tolerance of these bacteria, and their need for salinity at least three to
four times that of sea water, is very complicated. The cells themselves contain a very high internal salt concentration
(primarily potassium and sodium), equal to or higher than their environment, otherwise, they would be rapidly dehydrated
(plasmolyzed) in the brine. It has also been shown that the highly saline environment is essential for normal enzyme
function within the cells, and to maintain the fragile protein coating or "wall" around the delicate cell membrane. In fact, if
the salt concentration drops too low, the outer protein "wall" actually dissolves and the inner cell membrane disintegrates,
thus destroying the cell (Larsen, 1967).
Halobacteria can thrive in concentrated brine nine times the salinity of sea water, and can even remain alive in dry salt
crystals for years.”
http://www.resa.net/nasa/otherextreme.htm#halo
The Sulfolobus Bacteria
These bacteria occur in sulfur hot springs
Extreme thermophiles live in temperatures up to 110oC and pH of 1 or 2; their optimum temperature
is 75-80º.
They can tolerate very acidic environments pf pH < 2
Human Relevance of the Archaea
Methanogens can be used in large scale to produce methane, a source of energy.
VIRUSES
Viruses are subcellular particles that cannot metabolize on their own and are not considered to be
truly living organisms.
Viruses consist of nucleic acid enclosed in a protein coat and, sin some cases, a membranous
envelope. A virus is a genome enclosed in a protective coat.
Viruses are infectious agents and are not assigned to any of the six kingdoms.
Viruses are organized associations of macromolecules
Viral genomes, capsids and envelopes.
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Nucleic acid, DNA or RNA, which carries the blueprint for the replication of progeny
viruses.
The genomes may consist of a double stranded DNA, a single stranded DNA, a double
stranded RNA or a single stranded RNA.
The smallest virus has only four genes; the largest viruses have several hundreds.
Contained within a protective shell of protein units called the capsid.
The capsid could be made of a single type of protein or of several hundred types of proteins.
The capsids may have a variety of shapes: rod shaped, polyhedral, spherical, etc.
Some viruses have viral envelopes, membranes that cover the capsid and help them to infect
their hosts.
Viral envelopes are derived from the host cell and it contains host cell phospholipids and
membrane proteins.
Viruses cannot replicate outside of a living cell.
Once it has invaded a cell it is able to direct the host cell machinery to synthesize new intact
infectious virus particles (virions).
Because viruses are non-motile, they are entirely dependent on external physical factors for chance
movement and spread to infect other susceptible cells.
Viruses are between 20 and 300 nanometers long.
The genome ( DNA or RNA) codes for the few proteins necessary for replication.
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Some proteins are functional, e.g. nucleic acid polymerases,
and some are structural, i.e. they become incorporated and form part of the virion.
Phages or bacteriophages are viruses that attack bacteria cells.
Phages have the most complex capsids. They have a polyhedral head, a tail piece and fibers for
attachment to the bacterium.
VIRAL REPRODUCTION
Viruses lack enzymes, ribosomes and other equipment for making proteins.
Each type of virus can infect only a limited range of host cells called its host range.
The host specificity depends on the recognition system of the virus for the identification of the host
cell.
Lytic cycle.
1. Attachment to the host cell using its tail fibers to stick to specific receptor sites on the host's
membrane.
2. Penetration of the nucleic acid into the cytoplasm of the cell.
3. Bacterial DNA is destroyed; replication of viral macromolecules.
4. Cell's metabolism is directed by the viral genome.
5. Assembly of the newly synthesized viral components (viral nucleotides, proteins, etc).
6. Release of the new viruses by destroying the host cell with lytic enzymes, lysozymes.
Cells of higher animals that are invaded by viruses produce a protein called interferon, which is
released into the fluid around the cells into the bloodstream.
Interferon causes cells to produce protective proteins that prevent or inhibit the propagation of many
types of viruses.
Animal viruses often have an envelope made from the host's cell membrane, which allows them to
enter or exit the cell.
Retroviruses are a special category of RNA viruses that require reverse transcription of their RNA
to DNA and then integration of that DNA into the host cell genome before replication can take place.
They carry a reverse transcriptase enzyme as part of the virion.
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HIV, human immunodeficiency virus, is a retrovirus.
Vaccines are harmless variants or derivatives of pathogenic microbes that stimulate the immune
system to mount defenses against the actual pathogen.
Effective vaccines have been developed against many viral diseases, e. g. polio, smallpox, mumps,
hepatitis, and rubella (German measles).
Tumor viruses
Tumor viruses insert their DNA in the host cell's DNA triggering cancerous changes through their
own or host cell's genes.
The virus responsible for hepatitis B also seems to cause liver cancer in individuals with chronic
hepatitis.
Viral genes that directly cause cancer are called oncogenes.
Proto-oncogenes are found in normal cells and produce proteins associated with the cell cycle.
In some cases, the virus transforms the cell by turning on or increasing the expression of the cell's
oncogenes.
Plant viruses
Plant viruses cause much damage to agriculture.
Most plant viruses are single stranded RNA viruses.
The viruses may enter the plant through damaged cell wall or could be inherited from a parent during
asexual propagation.
Viruses may have evolved from cell fragments containing DNA and that were able to move from one
cell to another.
VIROIDS AND PRIONS.
A viroid consists of a short circular strand of RNA with no protein coat.
They are made of several hundred nucleotides.
Viroids do not encode proteins but can replicate in host plant cell and disrupt the metabolism of the
entire plant.
Prions are small polypeptide chains consisting of 208 amino acids.
They are abnormal versions of normal brain proteins. When prions contact a similar protein, it may
induce the normal protein to adopt an abnormal shape.
A chain reaction will follow until the prions reach a dangerous level causing cellular malfunction and
possibly degeneration of the brain.
These particles have been associated to animal and plant diseases.
http://www.kcom.edu/faculty/chamberlain/Website/Lects/PRIONS.HTM