Animal Viruses: Reverse-Transcribing Virus Synthesis & Replication: Videos & Practice Problems

Reverse transcriptase is an enzyme crucial for retroviruses, such as HIV. It functions as an RNA-dependent DNA polymerase, meaning it uses the viral RNA genome as a template to synthesize complementary DNA (cDNA). This process is the reverse of normal transcription, hence the name 'reverse transcriptase.' After the retrovirus injects its RNA and reverse transcriptase into a host cell, the enzyme converts the single-stranded RNA (ssRNA) into single-stranded DNA (ssDNA). This ssDNA is then used to create a double-stranded DNA (dsDNA) molecule, which can integrate into the host cell's genome, allowing the virus to replicate and persist within the host.

Retroviruses integrate their DNA into the host genome through a multi-step process. After the retrovirus injects its RNA and reverse transcriptase into the host cell, the reverse transcriptase converts the viral RNA into double-stranded DNA (dsDNA). This dsDNA is then transported into the host cell's nucleus. An enzyme called integrase, also encoded by the retrovirus, facilitates the integration of the viral dsDNA into the host cell's chromosomal DNA. Once integrated, the viral DNA can be transcribed and translated by the host cell's machinery, leading to the production of new viral particles and potentially causing latent infections.

The plus single-stranded RNA (ssRNA) genome in retroviruses is significant because it serves as the initial template for reverse transcription. Upon entry into the host cell, the retrovirus injects this plus ssRNA along with the enzyme reverse transcriptase. The reverse transcriptase uses the plus ssRNA to synthesize a complementary minus single-stranded DNA (ssDNA). This minus ssDNA is then used to create a double-stranded DNA (dsDNA) molecule. The plus ssRNA genome is crucial for the initial steps of viral replication and for the eventual production of new viral RNA genomes and proteins.

The integration of viral DNA into the host genome is a key factor in viral latency. Once the double-stranded DNA (dsDNA) of the retrovirus integrates into the host cell's chromosomal DNA, it can remain dormant or silent for extended periods. During this latent phase, the viral DNA is replicated along with the host cell's DNA during cell division, but it does not produce new viral particles. This allows the virus to evade the host's immune system and persist in the host. When conditions become favorable, the integrated viral DNA can be reactivated, leading to the production of new viral particles and potentially causing disease symptoms.

The replication cycle of retroviruses involves several key steps: 1) Entry: The retrovirus binds to and enters the host cell. 2) Reverse Transcription: The viral RNA genome and reverse transcriptase are released into the host cell. Reverse transcriptase converts the viral RNA into double-stranded DNA (dsDNA). 3) Integration: The dsDNA is transported into the nucleus and integrated into the host cell's genome by the enzyme integrase. 4) Transcription and Translation: The integrated viral DNA is transcribed into viral RNA by the host's machinery. This RNA serves as both mRNA for protein synthesis and as new viral genomes. 5) Assembly: New viral particles are assembled from the synthesized proteins and RNA genomes. 6) Release: The new viral particles are released from the host cell to infect other cells.