Anatomy of a Virus

• The tiniest viruses are 20 nm in diameter. (smaller than a ribosome)

• They consist of nucleic acids enclosed in a protein coat and sometimes a membranous envelop.

• The genomes (sets of genes) maybe– Double stranded DNA– Single stranded DNA– Double stranded RNA– Single stranded RNA

• They are called either a DNA or RNA virus depending on the type of nucleotide in the make-up.

• They may be linear or circular• The smallest have only 4 genes and largest

have several hundred.

• Capsid – a protein shell that covers the viral genome. They may be– Rod-shaped– Polyhedral– More complex

Capsids are built from large numbers of protein subunits called CAPSOMERES

The most complex capsids are found in viruses that infect bacteria – BACTERIOPHAGES (T1-T7). They have a protein tail piece with tail fibers that attach to the bacterium

Reproduction

• Viruses are obligate intracellular parasites that can reproduce only within a host cell.

• They do not have– Enzymes for metabolism

– Do not have ribosomes

– Do not have the equipment to make proteins

Each type of virus can infect and parasitize only a limited range of host cells called its

HOST RANGE.

• Some are broad based while others are not.– Swine flu virus can infect swine or humans– Rabies can infect may mammals

• Some can parasitize only E. coli

• Eukaryote viruses are usually tissue specific

• Viruses use a “lock and key” fit to identify hosts.

Reproduction occurs using lytic or lysogenic cycles

• The Lytic Cycle– Culminates in the death

of the host cell– Virulent viruses

reproduce only by lytic cyle.

– Natural selection favors bacterial mutations with receptor sites that are resistant to a particular phage or that have restriction enzymes to destroy the phages.

• The Lysogenic Cycle– Replication of the viral

genome without destroying the host cell.

– A temperate virus may reproduce by either cycle.

– Lambda virus: resembles T4 but only has a single short tail fiber

• While phages have the potential to wipe out a bacterial colony in just hours, bacteria have defenses against phages.– Natural selection favors bacterial mutants with

receptors sites that are no longer recognized by a particular type of phage.

– Bacteria produce restriction nucleases that recognize and cut up foreign DNA, including certain phage DNA.

• Modifications to the bacteria’s own DNA prevent its destruction by restriction nucleases.

– But, natural selection favors resistant phage mutants

• In the lysogenic cycle, the phage genome replicates without destroying the host cell.

• Temperate phages, like phage lambda, use both lytic and lysogenic cycles.

• Within the host, the virus’ circular DNA engages in either the lytic or lysogenic cycle.

• During a lytic cycle, the viral genes immediately turn the host cell into a virus-producing factory, and the cell soon lyses and releases its viral products.

The lambda phage which infects E. coli demonstrates the cycles of a temperate phage.

Lambda reproduction• Infects an E. coli cell by injecting its DNA• The lambda DNA molecule forms a circle.• Lytic or lysogenic cycles begin• In a lytic cycle, the cell is turned into a lambda

producing factory, the cell lyses and releases its products.

• In a lysogenic cycle, the viral genome is incorporated into by genetic recombination into a specific site on the host cell’s chromosome.

• It is now known as a prophage

• Every time the E. coli divides, it replicated the phage DNA and passes it along to the daughter cells.

• This enables the phage to replicate without destroying the host.

• The phages may at some point in time become active phages that lyse their host cell and releasing infectious particles.

• There is usually an environment trigger.• There may be other prophages released as well and

this may change the phenotype of the host. This is of medical importance. Examples: diphtheria, botulism and scarlet fever.

• Regardless of the type of virus, the parasite diverts the host cell’s resources for viral production.

• The host cell provides:• Nucleotides for nucleic acid production• Enzymes• Ribosomes• tRNA• Amino acids• ATP

Modes of infection and replication of animal viruses

• Focus on animals viruses with a viral envelop– The envelop is outside the capsid

and helps the virus enter the host cell.

– Generally a lipid bilayer with glycoprotein spikes

– The envelop fuses with the cell membrane

– The ER of the host cell makes the membrane proteins which are transported to the membrane

– New viruses exits the host in a process similar to exocytosis.

This reproductive cycle does not kill the host.

• Some viruses have envelopes that are not derived from the plasma membrane.

• Herpesvirus has an envelop that is derived from the nuclear membrane.

• These become integrated into the host genome as a provirus. Once these viruses are acquired they tend to reoccur through out a person’s life.

RNA as Viral Genetic Material

• The broadest variety of RNA genomes is found among viruses are those that infect animals.

• There are three types of single stranded RNA genomes

• The genome of class IV can directly serve as mRNA and can be translated into viral protein immediately after infection

RETROVIRUSES• Most complicated• Genetic information flows

in the reverse direction• Have the enzyme reverse

transcriptase– Transcribes DNA from an

RNA template

• The newly made DNA than integrates as a provirus into the nucleus of the animal cell

• The host’s RNA polymerase transcribes the virual DNA into RNA molecules.

Viral Diseases in Animals

• The damage caused by a viral disease depends on the ability of the tissue infected to regenerate by cell division.– Cold virus – we recover from– Poliovirus - attacks

• Vaccines are harmless variants of pathogenic microbes that stimulate the immune system to defenses against the pathogen.

• The link between viral infection and the symptoms it produces is often obscure.– Some viruses damage or kill cells by triggering the

release of hydrolytic enzymes from lysosomes.– Some viruses cause the infected cell to produce

toxins that lead to disease symptoms.– Other have molecular components, such as envelope

proteins, that are toxic.

• In some cases, viral damage is easily repaired (respiratory epithelium after a cold), but in others, infection causes permanent damage (nerve cells after polio).

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• The first vaccine was developed in the late 1700s by Edward Jenner to fight smallpox.– Jenner learned from his patients that milkmaids who

had contracted cowpox, a milder disease that usually infects cows, were resistant to smallpox.

– In his famous experiment in 1796, Jenner infected a farmboy with cowpox, acquired from the sore of a milkmaid with the disease.

– When exposed to smallpox, the boy resisted the disease.

– Because of their similarities, vaccination with the cowpox virus sensitizes the immune system to react vigorously if exposed to actual smallpox virus.

• Effective vaccines against many other viruses exist.

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• Vaccines can help prevent viral infections, but they can do little to cure most viral infection once they occur.

• Antibiotics which can kill bacteria by inhibiting enzyme or processes specific to bacteria are powerless again viruses, which have few or no enzymes of their own.

• Some recently-developed drugs do combat some viruses, mostly by interfering with viral nucleic acid synthesis.– AZT interferes with reverse transcriptase of HIV.– Acyclovir inhibits herpes virus DNA synthesis.

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• Plant viruses can stunt plant growth and diminish crop yields.

• Most are RNA viruses with rod-shaped capsids produced by a spiral of capsomeres.

6. Plant viruses are serious agricultural pests

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Fig. 18.9a

• In recent years, several very dangerous “emergent viruses” have risen to prominence.– HIV, the AIDS virus, seemed to appear suddenly in

the early 1980s.– Each year new strains of influenza virus cause

millions to miss work or class, and deaths are not uncommon.

– The deadly Ebola virus has caused hemorrhagic fevers in central Africa periodically since 1976.

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Fig. 18.8a

• Viroids, smaller and simpler than even viruses, consist of tiny molecules of naked circular RNA that infect plants.

• Their several hundred nucleotides do not encode for proteins but can be replicated by the host’s cellular enzymes.

• These RNA molecules can disrupt plant metabolism and stunt plant growth, perhaps by causing errors in the regulatory systems that control plant growth.

7. Viroids and prions are infectious agents even simpler than viruses

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• Viruses are in the semantic fog between life and nonlife.

• An isolated virus is biologically inert and yet it has a genetic program written in the universal language of life.

• Although viruses are obligate intracellular parasites that cannot reproduce independently, it is hard to deny their evolutionary connection to the living world.

8. Viruses may have evolved from other mobile genetic elements

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• Because viruses depend on cells for their own propagation, it is reasonable to assume that they evolved after the first cells appeared.

• Most molecular biologists favor the hypothesis that viruses originated from fragments of cellular nucleic acids that could move from one cell to another.– A viral genome usually has more in common with the

genome of its host than with those of viruses infecting other hosts.

– Perhaps the earliest viruses were naked bits of nucleic acids that passed between cells via injured cell surfaces.

– The evolution of capsid genes may have facilitated the infection of undamaged cells.Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Candidates for the original sources of viral genomes include plasmids and transposons.– Plasmids are small, circular DNA molecules that are

separate from chromosomes.– Plasmids, found in bacteria and in the eukaryote

yeast, can replicate independently of the rest of the cell and are occasionally be transferred between cells.

– Transposons are DNA segments that can move from one location to another within a cell’s genome.

• Both plasmids and transposons are mobile genetic elements.

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