Viruses Require Cells As Hosts, AIDS, HIV

Mutations, AIDS, HIV

Mutations Caused by Reverse Transcriptase, HIV, AIDS

• A virus is genetic information wrapped in a protein coat and sometimes surrounded by a membrane envelope.
• Genome of a virus is made up of either DNA or RNA (single or double-stranded).
• A virus is a parasite: it requires resources of a living cell to reproduce.
• Virus's genome is replicated within the host cell and under the control of the virus.
• Cell produces proteins and other molecules required to assemble new virus particles.
• New viral particles go on to infect more cells and the cycle repeats.
• Rounds of infection and virus reproduction can result in huge numbers of progeny virus particles.

• A virus undergoes mutations frequently in the course of viral infection and reproduction.
• Mutation may be a change, insertion or deletion of a nucleotide in the polynucleotide chain.
• A single nucleotide change may affect the virus's ability to infect cells and to survive the defenses of the host.
• Mutations can be caused by external agents, such as ultraviolet light, other forms of radiation and certain chemical substances, which physically damage DNA or RNA.
• Mutations are also introduced by molecular machinery that replicates the genetic material.

• Reverse transcriptase:
  - Prone to introducing errors into the viral DNA it synthesizes.
  - Used by retroviruses such as HIV.
  - Transcribes RNA genome into DNA.
  - Mutations occur randomly.
• HIV's reverse transcriptase is error-prone and introduces point mutations in nearly every copy of the HIV genome. This replicating enzyme is the primary cause of the rapid mutation rate of the HIV virus.
• Viral dynamics, mutation rate, and genome size predict that HIV will develop mutations at every position numerous times per day.
• Under drug selection pressures, complete replacement of wild-type (WT) virus by drug-resistant virus can occur in 14 to 28 days.
• Once a mutation has been introduced into the viral genome, it is replicated and passed on to all the progeny virus.
• Location of a mutation is critically important in determining its effect on the virus.
• Mutation inside a gene or in another important nucleotide sequence may dramatically affect the ability of the virus to survive and reproduce.
• Mutations may be:
  - Harmful
  - Silent (no effect on survival)
  - Beneficial (increase fitness)
• Survival of the fittest virus variant occurs through the process of natural selection.

Mutation - a change in the genetic material of a virus (or a cell).