AP Syllabus focus:
‘Retroviruses use reverse transcriptase to copy their RNA genome into DNA, which integrates into host chromosomes and directs production of new viral particles.’
Retroviruses are RNA viruses that replicate through a DNA intermediate.
This diagram summarizes the retroviral (HIV) replication cycle, emphasizing the key reversal of information flow when reverse transcriptase converts viral RNA into DNA. It also shows integrase-mediated insertion of viral DNA into the host genome (provirus formation) and how host transcription/translation machinery produces viral RNA and proteins for assembly and release. Source
Their key adaptation is reverse transcriptase, which allows viral genetic information to become part of a host cell’s genome and be expressed using host machinery.
Core idea: information flow can be reversed
In most cells, genetic information flows from DNA to RNA to protein. Retroviruses temporarily reverse this pattern by making DNA from RNA, enabling long-term persistence inside the host.
Reverse transcriptase: A viral enzyme that synthesizes DNA from an RNA template, producing DNA that can ultimately integrate into a host chromosome.
Retroviral genome and enzymes (what the virus brings)
Retroviruses typically package:
These molecular renderings depict three HIV enzymes central to the steps described in the notes: reverse transcriptase synthesizes a DNA copy from the RNA genome, integrase inserts that DNA into the host genome, and protease processes viral polyproteins during maturation. The figure helps connect each enzyme to its specific role in making an integrated provirus and producing infectious progeny. Source
An RNA genome (often two copies of single-stranded RNA)
Reverse transcriptase (for RNA → DNA synthesis)
Enzymes needed to embed viral DNA into host chromosomes (commonly integrase)
A protein coat and, in many retroviruses, a membrane envelope that helps with cell entry
Reverse transcription: RNA genome → DNA
Once a retrovirus enters a host cell and uncoats, reverse transcriptase begins copying the viral RNA into DNA. This conversion is essential because host chromosomes are DNA, and stable genomic integration requires DNA.
Key features of reverse transcription for AP Biology:
Reverse transcriptase uses the viral RNA as a template to synthesize a complementary DNA strand.
The viral RNA template is then removed or degraded as DNA synthesis proceeds, allowing formation of a DNA version of the viral genome.
A second DNA strand is synthesized to form double-stranded viral DNA, the form that can integrate into host chromosomes.
Important implications:
Because reverse transcriptase is not a host DNA polymerase, it is typically more error-prone, which can increase viral variation and help viral populations adapt within hosts.
Why the DNA intermediate matters
Producing DNA from RNA is not just a workaround; it changes the biology of infection:
DNA can be integrated into a chromosome.
Integrated DNA can be replicated whenever the host cell replicates its DNA.
Integrated viral DNA can be transcribed to make viral RNA, including RNA that serves as genomes for new viruses.
Integration into host chromosomes: forming a provirus
After double-stranded viral DNA is made, it can be inserted into a host chromosome.
This schematic focuses on how newly synthesized viral DNA becomes integrated into host chromosomal DNA, a defining step that creates a provirus. It highlights the preintegration complex and the role of integrase in joining viral DNA ends to target sites in host DNA (often presented in the context of chromatin/nucleosomes). Source
Integration is a defining step because it links viral replication to the host genome.
Key points:
Integrated viral DNA is called a provirus (viral DNA embedded in host chromosomal DNA).
The provirus may persist for long periods, because it is copied along with host DNA during cell division.
Once integrated, viral genes can be expressed when the host cell transcribes the proviral DNA into RNA.
From integrated DNA to new viral particles
After integration, the proviral DNA can direct production of new viral particles by serving as a template for transcription.
A high-level sequence consistent with the specification:
Provirus (viral DNA) in host chromosome is transcribed by host enzymes to produce viral RNA.
Viral RNA molecules function as:
mRNA to make viral proteins
New genomic RNA copies to be packaged into assembling viruses
Viral proteins and viral RNA assemble into new viral particles.
New viral particles exit the cell (often by budding in enveloped retroviruses), carrying reverse transcriptase to infect additional cells.
Biological consequences of integration
Integration has several consequences that connect directly to how retroviruses spread and persist:
Long-term infection potential: A provirus can remain in the genome and be expressed later.
Genomic disruption risk: Integration into or near a host gene can disrupt gene function or alter regulation, which can damage cells.
Stable inheritance in dividing cells: If an infected cell divides, daughter cells can inherit the provirus because it is part of the chromosome.
FAQ
They bind to reverse transcriptase and prevent DNA synthesis from the viral RNA template.
This blocks formation of double-stranded viral DNA, so integration into the host chromosome cannot occur.
Many reverse transcriptases have limited proofreading capacity during DNA synthesis.
Higher error rates generate more mutations in viral DNA, increasing diversity in the viral population.
LTRs are repeated sequences at the ends of retroviral DNA formed during reverse transcription.
They contain regulatory regions that can promote transcription of viral genes once integrated.
Engineered retroviral vectors are altered to remove key viral genes required for replication.
A therapeutic or reporter gene is inserted, and packaging systems supply missing functions so the vector can enter cells without producing fully infectious progeny.
If a retrovirus integrates into a germline cell (egg or sperm precursor), the provirus can be inherited.
Over generations, these sequences can accumulate mutations and become inactive, leaving retroviral DNA fragments embedded in the genome.
Practice Questions
State the role of reverse transcriptase in a retrovirus. (2 marks)
Converts (copies) viral RNA into DNA (1)
Produces DNA that can integrate into the host chromosome / become part of host genomic DNA (1)
Describe how a retrovirus uses reverse transcriptase to produce new viral particles after entering a host cell. (6 marks)
Viral RNA genome enters the host cell (1)
Reverse transcriptase synthesises a DNA strand complementary to viral RNA (1)
A second DNA strand is made to form double-stranded viral DNA (1)
Viral DNA integrates into a host chromosome (provirus formation) (1)
Proviral DNA is transcribed to produce viral RNA (1)
Viral RNA and viral proteins assemble into new viral particles (1)
