BIO 112 Notes: Viruses

Viruses

A virus is a small nucleic acid genome enclosed in a protein capsid and sometimes a membranous viral envelope (derived from the membrane of the host cell) containing viral proteins that help the virus enter a cell.

  • The genome may be single or double-stranded DNA or RNA.

Viruses do not really fit our definition of living organisms.

  • An isolated virus is biologically inert, unable to replicate its genes or regenerate its own ATP. They cannot reproduce or carry out metabolic activities outside of a host cell.
  • Most biologists studying viruses today would probably agree that they are not alive but exist in a shady area between life-forms and chemicals.
  • Viruses replicate only in host cells. Once the viral genome is inside the host cell, the proteins it encodes can commandeer the host, reprogramming the cell to copy the viral genome and manufacture viral proteins. The host provides the nucleotides for making viral nucleic acids, as well as enzymes, ribosomes, tRNAs, amino acids, ATP, and other components needed for making the viral proteins.

Phages

Phages (viruses that infect bacteria) can replicate by two alternative mechanisms: the lytic cycle, resulting in the lysis (and death) of the host cell and the release of new phages, and the lysogenic cycle, in which the host cell survives and the bacterial genome is incorporated into the host chromosome and replicated with it.

  • They impact human health as a possible treatment for human bacterial diseases.
  • Bacteriophages attack only their host bacteria, not human cells, so they are potentially good candidates to treat bacterial diseases in humans.

Lytic cycle- kills host cell and releases new phages. Lysogenic cycle- host cell lives and bacterial genome is left in the host chromosome to be replicated.

HIV

HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome).

  • HIV and other retroviruses are enveloped viruses that contain two identical molecules of single-stranded RNA and two molecules of reverse transcriptase. After HIV enters a host cell, its reverse transcriptase molecules are released into the cytoplasm, where they catalyze synthesis of viral DNA. The newly made viral DNA then enters the cell’s nucleus and integrates into the DNA of a chromosome.
  • The integrated viral DNA, called a provirus, never leaves the host’s genome, remaining a permanent resident of the cell. The host’s RNA polymerase transcribes the proviral DNA into RNA molecules, which can function both as mRNA for the synthesis of viral proteins and as genomes for the new viruses that will be assembled and released from the cell.

Think: HIV enters the host, its reverse transcriptase molecules begin to synthesize viral DNA. New DNA enters the nucleus to reach the chromosome and viral DNA (provirus) never leaves the host’s genome. Host cell’s RNA polymerase transcribes proviral RNA into RNA molecules, which leads to making more viral proteins.

Viruses in Animals

Many animal viruses have an envelope. Whereas few bacteriophages have an RNA genome or envelope, many animal viruses have both. In fact, nearly all animal viruses with RNA genomes have an envelope, as do some with DNA genomes.

  • Retroviruses (such as HIV) use the enzyme reverse transcriptase to copy their RNA genome into DNA, which can be integrated into the host genome as a provirus. Vaccines stimulate the immune system to defend the host against specific viruses.

Emerging Viruses

Viruses that suddenly become apparent are often referred to as emerging viruses. HIV, the AIDS virus, is a classic example.

  • Outbreaks of emerging viral diseases in humans (epidemics) are usually caused by existing viruses that expand their host territory. We should be concerned about emerging viruses because they can mutate.
  • RNA viruses have a high rate of mutation because viral RNA polymerases do not correct errors in replicating their RNA genomes. Some mutations change viruses into new genetic varieties (strains) that can cause disease, even in people immune to the ancestral virus.

Plant Viruses

More than 2,000 types of viral diseases of plants account for an estimated annual loss of $15 billion worldwide due to the destruction of crops. Plant viruses have the same basic structure and mode of replication as animal viruses. Most plant viruses discovered thus far, including tobacco mosaic virus (TMV), have an RNA genome. Many have a helical capsid, like TMV, while others have an icosahedral capsid. Viruses enter plant cells through damaged cell walls (horizontal transmission) or are inherited from a parent (vertical transmission).

Prions

A prion is a misfolded form of a protein normally present in brain cells. They are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals.

  • When the prion gets into a cell containing the normal form of the protein, the prion somehow converts normal protein molecules to the misfolded prion versions.
  • Several prions then aggregate into a complex that can convert other normal proteins to prions, which join the chain. Prion aggregation interferes with normal cellular functions and causes disease symptoms.