Reverse Transcription

In viruses, particularly retroviruses like HIV, reverse transcription is a critical step in their replication cycle. These viruses carry their genetic information in the form of single-stranded RNA and use reverse transcription to convert their RNA genome into double-stranded DNA, which can then integrate into the host's genome. Here's a detailed breakdown of the steps involved in reverse transcription within a virus:

1. Entry into the Host Cell

Step: The retrovirus enters the host cell through receptor-mediated endocytosis or membrane fusion.
Purpose: This allows the viral RNA genome and associated enzymes to be released into the host cell's cytoplasm.

2. Release of Viral RNA and Enzymes

Step: Once inside the host cell, the viral capsid is uncoated, releasing the viral RNA along with key enzymes, including reverse transcriptase, integrase, and protease.
Purpose: This uncoating step frees the viral RNA so it can serve as a template for reverse transcription.

3. Synthesis of Minus-Strand cDNA (First Strand)

Step: Reverse transcriptase uses a short tRNA molecule (provided by the host cell) as a primer, which binds to a complementary sequence on the viral RNA called the primer binding site (PBS).
Action: Reverse transcriptase begins synthesizing the complementary DNA (cDNA) strand from the RNA template in the 5' to 3' direction, creating the first DNA strand (minus strand).
Result: The RNA-dependent DNA polymerase activity of reverse transcriptase creates a complementary DNA strand, starting at the PBS.

4. RNA Degradation by RNase H

Step: The RNA strand paired with the newly synthesized cDNA is degraded by the RNase H activity of reverse transcriptase, except for a small segment called the polypurine tract (PPT).
Purpose: This degradation is necessary to allow the synthesis of the second DNA strand.

5. Plus-Strand DNA Synthesis (Second Strand)

Step: The remaining PPT serves as a primer for the synthesis of the second strand of DNA (plus strand). Reverse transcriptase now synthesizes the complementary DNA strand using the newly synthesized minus-strand cDNA as a template.
Action: This involves synthesizing DNA in the opposite direction, leading to the formation of a double-stranded DNA molecule.
Result: A complete double-stranded DNA (dsDNA) copy of the viral RNA genome is produced.

6. Formation of the Long Terminal Repeats (LTRs)

Step: During reverse transcription, long terminal repeats (LTRs) are duplicated at both ends of the viral DNA. These LTRs are crucial for integration into the host genome.
Purpose: LTRs contain promoter and enhancer sequences necessary for the transcription of viral genes after integration.

7. Nuclear Import of Viral DNA

Step: The newly synthesized double-stranded viral DNA is transported into the nucleus of the host cell.
Purpose: The viral DNA needs to enter the nucleus to integrate into the host genome, especially in non-dividing cells.

8. Integration into the Host Genome

Step: The viral enzyme integrase facilitates the insertion of the viral DNA into the host's chromosomal DNA.
Purpose: Integration is essential for the viral genome to be replicated along with the host's DNA during cell division and for the production of new viral RNA and proteins.

9. Transcription and Translation

Step: Once integrated, the host's transcription machinery transcribes the viral DNA into mRNA, which can be translated into viral proteins or packaged into new viral particles.
Purpose: This leads to the production of new virions, completing the viral replication cycle.

Key Enzyme Functions of Reverse Transcriptase

RNA-Dependent DNA Polymerase Activity: Synthesizes cDNA from the RNA template.
RNase H Activity: Degrades the RNA strand of RNA-DNA hybrids.
DNA-Dependent DNA Polymerase Activity: Synthesizes the second DNA strand using the first strand as a template.