Retroviruses such as HIV replicate by copying their RNA genome into DNA and inserting it into the host's DNA. Using HIV as the example:1. Attachment: the glycoprotein gp120 on the envelope binds to CD4 receptors on T helper cells (with a co-receptor).
2. Entry: the envelope fuses with the host cell membrane and the capsid enters the cytoplasm, where the RNA and enzymes are released.
3. Reverse transcription: reverse transcriptase uses the viral RNA as a template to make a single strand of complementary DNA (cDNA). It then makes a second DNA strand, giving double-stranded DNA.
4. Integration: the dsDNA enters the nucleus and integrase inserts it into a host chromosome. The inserted viral DNA is called a provirus.
5. Transcription: host RNA polymerase transcribes the provirus, making viral mRNA and new copies of the RNA genome.
6. Translation: host ribosomes make viral proteins, which protease cuts into their working forms.
7. Assembly and budding: two copies of the RNA genome and the enzymes are packaged in capsids, and new virions bud from the cell, taking some of the host membrane as their envelope.
RNA → cDNA → dsDNA (provirus) → mRNA → proteins The provirus is copied every time the host cell divides. It can stay latent for years, making no new viruses, which is why HIV infection cannot be cured. When the provirus is active, repeated rounds of budding and the immune response destroy T helper cells, weakening the immune system.Each stage involving a viral enzyme (reverse transcriptase, integrase, protease) is a possible target for drugs.
2. Entry: the envelope fuses with the host cell membrane and the capsid enters the cytoplasm, where the RNA and enzymes are released.
3. Reverse transcription: reverse transcriptase uses the viral RNA as a template to make a single strand of complementary DNA (cDNA). It then makes a second DNA strand, giving double-stranded DNA.
4. Integration: the dsDNA enters the nucleus and integrase inserts it into a host chromosome. The inserted viral DNA is called a provirus.
5. Transcription: host RNA polymerase transcribes the provirus, making viral mRNA and new copies of the RNA genome.
6. Translation: host ribosomes make viral proteins, which protease cuts into their working forms.
7. Assembly and budding: two copies of the RNA genome and the enzymes are packaged in capsids, and new virions bud from the cell, taking some of the host membrane as their envelope.