Recording-2025-03-16T20:30:07.628Z Lecture 13.7

Retroviruses Overview

• Retroviruses are unique viruses characterized by their single-stranded RNA, typically positive sense.

• They use reverse transcriptase, an enzyme to convert viral RNA into double-stranded DNA via reverse transcription.

• This viral DNA can integrate into the host's DNA, a process known as integration, which is not common in most viruses.

Key Enzymes in Retroviruses

• Reverse Transcriptase

  • Enzyme that synthesizes DNA from an RNA template.

  • Converts the incoming RNA genome into complementary DNA (cDNA).

  • Degrades the original RNA strand after creating the DNA copy.

• Integrase

  • Enzyme that facilitates the integration of viral DNA into the host cell's DNA, allowing the virus to replicate within the host.

HIV as a Retrovirus

• HIV (Human Immunodeficiency Virus) is a prime example of a retrovirus with significant historical impact, especially during the AIDS pandemic.

• HIV Structure

  • The virus has a distinct shape, with a capsid that houses two identical single strands of RNA to increase replication accuracy.

  • Contains essential enzymes like reverse transcriptase and integrase, which aids in its life cycle.

HIV Replication Cycle

Key Steps in the HIV Life Cycle:

1. Attachment

   • The HIV spikes, specifically gp120, attach to the CD4 receptor on host cells (primarily helper T cells).

2. Entry

   • HIV enters the host cell via endocytosis, tricking the cell into internalizing it.

3. Uncoating

   • The viral RNA and enzymes are released into the cytoplasm after the viral envelope is dismantled.

4. Reverse Transcription

   • The viral RNA is reverse transcribed into DNA in the cytoplasm.

   • Resulting double-stranded DNA is then transported into the nucleus.

5. Integration

   • Integrase incorporates the viral DNA into the host's DNA, allowing it to become part of the host's genetic material.

6. Transcription and Translation

   • The integrated viral DNA is transcribed into RNA, which serves as both mRNA (for protein synthesis) and as the genome for new viral particles.

7. Assembly

   • Viral proteins and RNA genomes are assembled into new virions.

8. Budding and Maturation

   • Newly formed virions bud off from the host cell membrane.

   • Maturation occurs post-budding, with protease cleaving polypeptides into functional proteins to create a mature, infectious virus.

Biological Importance and Resistance to HIV

• Some individuals possess a mutated form of the CXCR4 co-receptor, which HIV uses for entry, providing genetic resistance to HIV infection.

• This phenomenon has implications for future treatments or cures for HIV.

Conclusion

• Retroviruses, particularly HIV, showcase a complex replication cycle that involves RNA to DNA conversion and subsequent integration, which allows them to persist within host organisms.