33.1 Why Do Biologists Study Viruses?

• Viruses directly influence the genetic makeup of organisms.

  • They can introduce foreign genes into cellular genomes by lateral gene transfer-picking up genes from one organism and shuttling them to another organism of the same species or even a different species.

• Physicians and researchers use the term epidemic (literally,“upon-people”) to describe a disease that rapidly infects many individuals over a widening area.

• An epidemic of worldwide scope is called a pandemic.

• In terms of the total number of people affected, the measles and smallpox epidemics and the 1918 influenza pandemic mentioned earlier have been surpassed by the acquired immune deficiency syndrome (AIDS) pandemic.

  • AIDS is an affliction caused by the human immunodeficiency virus (HIV).

• Like other viruses, HIV parasitizes specific types of cells-most notably, the helper T cells of the immune system, which is the body’s defense system against disease.

  • Helper T cells are crucial to the immune system’s responses to invading pathogens.

33.2 How Do Biologists Study Viruses?

• The genome of the bacteriophage is then transferred to the cytosol in a process referred to as uncoating, which varies depending on the structure of the virus.

• n 1981-right at the start of 1e AIDS epidemic-biomedical researchers realized that people with AIDS had few or no helper T cells possessing a membrane protein called CD4.

  • These cells, symbolized as CD4+, have a key supportive role in the immune response

  • They used antibodies that bound specifically to CD4 and to other host membrane proteins.

• Although CD4 is necessary for HIV attachment, the viral genome will not be uncoated unless the virion also binds to a second membrane protein, a co-receptor.

• Drugs that interfere with viral infection or replication are called antivirals.

• Some of these viral enzymes, called RNA replicases, function as RNA-dependent RNA polymerases.

• But not all RNA viruses replicate using RNA replicases. In certain animal RNA viruses, the genome is first transcribed from RNA to DNA by a viral enzyme called reverse transcriptase.

  • This enzyme is an unusual DNA polymerase-one that can synthesize DNA from either an RNA or a DNA template.

  • It first synthesizes a complementary DNA, or cDNA, using a single-stranded RNA template.

• Viruses that reverse- transcribe their genome in this way are called retroviruses (“backward-viruses”).

• In bacteriophages, this alternate mode of infection is called lysogeny.

• In viruses that infect animal cells, the dormant state is called latency.

33.3 What Themes Occur in the Diversification of Viruses?

• HIV is an example of a virus responsible for an emerging disease: a new illness that suddenly affects significant numbers of individuals in a host population.