Retroviruses are a special case of viral replication, distinct from lytic and lysogenic cycles.
Example discussed: HIV
Characteristics of Retroviruses
Enveloped, single-stranded RNA virus.
Carries three special proteins inside the envelope:
Reverse Transcriptase
Integrase
Protease (discussed later)
Entry and Uncoating
Entry Method: Direct fusion with the host cell membrane.
Uncoating: Nucleocapsid dissolves, releasing RNA and proteins into the host cell.
Reverse Transcription
Reverse Transcriptase: Converts viral RNA to complementary DNA (cDNA).
Reads RNA from 5’ to 3’ end.
First cDNA strand is created, followed by a second complementary strand.
Forms a double-stranded DNA from cDNA.
Integration into Host DNA
Integrase: Clips 3’ ends of cDNA, forming sticky ends.
Viral RNA is degraded by ribonuclease.
Integrase integrates viral DNA into host DNA.
Travels through nuclear membrane to the genome.
Enters provirus stage (similar to lysogenic cycle).
Active Transcription
Unlike typical lysogenic viruses, no repressor gene present.
Viral DNA is actively transcribed with host DNA.
Viral mRNA is produced and transported to the cytosol.
Protein Translation and Virus Assembly
Viral mRNA is translated into proteins:
Capsid proteins
Reverse transcriptase, integrase, and protease
Components self-assemble into immature viruses in the cytosol.
Budding and Maturation
Immature viruses bud off from the host cell, acquiring an envelope.
Protease: Cleaves proteins, making them functional before the virus infects new cells.
Complexity of Retroviral Replication
Combines elements of both lysogenic and lytic cycles.
More complex than traditional viral replication processes.
Summary
Retroviruses like HIV have a unique replication process that involves reverse transcription, integration into host DNA, and maturation through protease activity.
Understanding this process highlights the complexity and adaptability of retroviruses.