Describe retroviruses and reverse transcription

Overview

Retroviruses are a unique class of RNA viruses that reverse the usual flow of genetic information by converting their RNA genome into DNA. This process, called reverse transcription, fundamentally challenged the Central Dogma of Molecular Biology (DNA → RNA → Protein) when discovered in 1970 by Howard Temin and David Baltimore.

The Retroviral Lifecycle

1. Entry and Uncoating

WHAT happens: The retrovirus binds to specific host cell receptors (e.g., HIV-1 binds CD4 on T cells) and fuses its envelope with the cell membrane. The viral core enters the cytoplasm and uncoats, releasing:

• Two copies of +ssRNA genome (diploid)
• Reverse transcriptase enzyme
• Integrase enzyme
• Protease enzyme

WHY two RNA copies? Redundancy for error correction during reverse transcription and template switching to repair damage.

2. Reverse Transcription – The Core Innovation

Step-by-Step Derivation of the Process

Step 1: Primer Binding

• A host tRNA (e.g., tRNA-Lys3 in HIV) binds to the Primer Binding Site (PBS) near the 5' end of viral RNA
• WHY tRNA? RT cannot initiate synthesis de novo – it needs a 3'-OH group to extend. The host tRNA provides this primer, exploiting the cell's own machinery.

Step 2: First-Strand Synthesis (Minus-Strand DNA)

• RT extends the tRNA primer in 5' → 3' direction, synthesizing DNA complementary to the RNA template
• Creates a short DNA segment called minus-strand strongstop DNA (extends to 5' end of RNA)
• WHY "strong-stop"? It stops at the 5' end of the template – can't go further without template switching.

Step 3: First Template Jump

• RNase H degrades the RNA template except for specific purine-rich sequences (PPT – polypurine tract)
• The minus-strand strong-stop DNA has a complementary sequence (R region) at BOTH ends of the RNA genome (because retroviruses have terminal repeats)
• The DNA "jumps" to the 3' end of the RNA andneals to the complementary R region
• WHY is this jump necessary? To continue synthesis beyond the template end and create full-length DNA. The identical R regions at both ends make this template switching possible.

Step 4: Completion of Minus-Strand

• RT continues extending, copying the entire RNA genome into DNA
• RNase H progressively degrades the RNA template, except the PPT region

Step 5: Plus-Strand Synthesis Initiation

• The remaining PPT serves as primer for second strand (plus-strand) DNA synthesis
• RT now acts as DNA-dependent DNA polymerase, using the minus-strand DNA as template

Step 6: Second Template Jump

• Plus-strand synthesis creates plus-strand strong-stop DNA
• This DNA jumps to the other end of the minus-strand template (again using complementary PBS sequences)
• WHY two jumps? To create the Long Terminal Repeats (LTRs) – duplicated sequences at both ends essential for integration.

Step 7: Completion

• Both strands are fully synthesized, creating linear dsDNA with LTRs at both ends
• The DNA is longer than the original RNA due to LTR duplication

Derivation of LTR formation:

• Original RNA has: U5 - [genes] - U3-R at 5' end and R-U3 at 3' end (note: order is 5' R-U5... U3-R-U3 3')
• Actually: 5' structure is R-U5-PBS... 3' structure is PPT-U3-R

After two template jumps and synthesis:

• 5' LTR: U3-R-U5 (from 3' end of RNA → 5' end synthesis → template jump)
• 3' LTR: U3-R-U5 (from 5' end synthesis → second jump → completion)

WHY this structure? U3 contains the promoter and enhancer sequences. R contains signals for transcription termination/polyadenylation. U5 contains sequences for integration. Having complete copies at BOTH ends ensures bidirectional regulatory control.

After reverse transcription:

• Proviral DNA: ~9.8 kb (slightly longer due to LTR duplication)
• Each LTR: ~634 bp
  • U3: 454 bp (promoter, enhancers)
  • R: 97 bp (transcription signals)
  • U5: 83 bp (integration signals)

Why this step matters: RT error rate is ~1 error per 10,000 nucleotides (no proofreading!). For 9.7 kb genome, that's nearly 1 mutation per replication cycle. This high mutation rate is WHY HIV evolves drug resistance so rapidly and why vaccine development is so challenging.

3. Integration

WHAT happens: The viral DNA enters the nucleus and integrase enzyme catalyzes insertion into a host chromosome.

HOW integration works:

1. Integrase removes 2 nucleotides from each 3' end of viral DNA (creates recessed 3'-OH)
2. Integrase makes a staggered cut in host DNA (5bp apart)
3. The 3'-OH of viral DNA attacks the host DNA phosphodiester bonds (transesterification)
4. Host repair machinery fills gaps and ligates

Result: Viral DNA is now a provirus, flanked by 5 bp duplications of host DNA (target site duplications).

WHY is integration permanent? The proviral DNA is replicated along with host chromosomes during cell division. In non-dividing cells (like neurons or resting T-cells), it can remain latent indefinitely – this is the major barrier to curing HIV.

4. Transcription and Translation

• Host RNA Polymerase II reads the provirus like a normal gene (using the U3 promoter in 5' LTR)
• Produces full-length genomic RNA (packaged into new virions) and spliced mRNAs (translated into proteins)
• Viral proteins are processed by viral protease

5. Assembly and Budding

• New virions assemble at plasma membrane and bud off, acquiring envelope from host membrane
• Protease cleaves polyproteins into mature functional proteins AFTER budding (maturation)

Visual checkpoint: Each immature virion has a smooth, round core. Mature virions have a distinctive cone-shaped core. Electron microscopy can distinguish them.

Key Retroviruses and Their Impact

Why this matters for treatment:

• Any drug that targets a single viral protein will face resistance
• Pre-existing resistant mutants are likely already present
• SOLUTION: HART (Highly Active Antiretroviral Therapy) uses≥3 drugs targeting different stages
  • NRTI (nucleoside RT inhibitor): fake DNA building block
  • NNRTI (non-nucleoside RT inhibitor): jams RT enzyme
  • Protease inhibitor: blocks maturation
  • Integrase inhibitor: prevents integration

Probability calculation: If each drug independently reduces viral replication 100-fold, three drugs together reduce it 100³ = 1,000,000-fold. The chance a single virus is resistant to all three simultaneously is incredibly low (~10^-15), preventing resistance emergence.

Common Mistakes and Misconceptions

Why it's wrong: Reverse transcription is RNA → DNA, which never occurs in the standard central dogma. The "reverse" refers to the typical flow DNA → RNA, not running transcription backwards (which would be RNA → DNA in retroviruses is indeed reverse transcription, but we're not making RNA from DNA and then going back – we're starting with RNA).

The fix: Think of it as "reverse of the usual DNA-to-RNA direction." The virus reverses the direction of information flow: template RNA → product DNA, instead of the usual template DNA → product RNA.

Why it's wrong: Reverse transcription occurs in the cytoplasm immediately after uncoating. Only the completed DNA enters the nucleus.

The fix: Timeline: Cytoplasm (reverse transcription) → Transport to nucleus → Nuclear import → Integration. RT is packaged in the virion precisely because it must work in the cytoplasm before nuclear entry.

Why it's wrong: Many proviruses enter latency, producing no viral proteins and causing no immune response. In HIV, latently infected resting CD4+ T cells can harbor provirus for years without producing virus.

The fix: The provirus is a molecular time bomb. It can reactivate when the cell is stimulated (e.g., by another infection). This latent reservoir is WHY stopping antiretroviral therapy leads to viral rebound – the hidden proviruses restart production. Current estimate: ~1 million latently infected cells per HIV patient on suppressive therapy.

Why it's wrong: The error rate is a feature, not a bug. High mutation rates generate genetic diversity, allowing rapid adaptation to:

• New host immune responses (escape mutants)
• Antiretroviral drugs (resistance mutations)
• Different cell types (tropism changes)

The fix: RT lacks3'→5' exonuclease (proofreading) activity BY DESIGN. The virus trades replication fidelity for evolutionary flexibility. It's an evolutionary strategy optimized for survival in a hostile (immune-active) environment.

Quantitative insight: HIV diversifies WITHIN a single patient into a "quasispecies" – a cloud of related sequences. After years of infection, HIV variants in one person can be more diverse than influenza variants globally in a single year.

Reverse Transcriptase as a Molecular Tool

1. Gene cloning: mRNA lacks introns (already spliced). Making cDNA from mRNA gives you intron-free coding sequences you can express in bacteria (which can't splice eukaryotic introns).

2. RT-PCR: Amplify RNA by first converting to cDNA, then using standard PCR. This enables:

   • Gene expression analysis
   • RNA virus detection (COVID-19 tests!)
   • Developmental biology (tracking which genes are "on")

3. RNA sequencing: Modern RNA-seq protocols use RT to convert cellular RNA to DNA libraries for sequencing.

The irony: A viral enzyme that causes disease became one of the most essential tools in biological research and medical diagnostics.

Why not PCR the RNA directly? Standard DNA polymerases can't use RNA as template. We NEED reverse transcriptase to make the DNA intermediate.

Active Recall Practice

Now, a retrovirus is like a sneaky spy. It carries its instructions written in RNA (pencil), but it wants to permanently hide its instructions in your factory's DNA manual (permanent marker). How does it do this?

The virus has a special tool called reverse transcriptase – it's like a "pencil-to-permanent-marker converter." It reads the RNA instructions (pencil) and writes them into DNA (permanent marker). Once the viral instructions are in DNA, the virus uses another tool (integrase – like a stapler) to stick its page into your cell's instruction manual.

Now here's the scary part: every time your cell copies its instruction manual to divide, it also copies the virus's page! The viral instructions spread to every new cell. Your body can't just erase pencil (destroy RNA) anymore – the instructions are in permanent marker (DNA) inside your manual.

That's why HIV is so hard to cure – the virus's instructions become a permanent part of some cells' DNA, hiding quietly until they're activated.

• Long terminal repeats require
• Two
• Recombination events
• Primer binding starts the race
• Strong-stop DNA jumps at each end

Or: "FIRST jump = Five-to-three flip, SECOND jump = Plus-strand position" – the first jump moves minus-strand DNA from 5' to 3' end of template; second jump positions plus-strand to complete the structure.

Connections

• Central Dogma of Molecular Biology – retroviruses violate the typical unidirectional flow
• DNA Replication and Repair – compare fidelity mechanisms (RT lacks proofreading)
• Viral Structure and Classification – retroviruses are +ssRNA, enveloped, Class VI
• HIV/AIDS Pathogenesis – reverse transcription is step 2 of 7 in HIV lifecycle
• Antiretroviral Therapy – drugs target RT (NRTIs, NNRTIs), integrase, protease
• Molecular Cloning Techniques – RT is used to make cDNA libraries
• PCR and RT-PCR – RT-PCR extends PCR to detect RNA targets
• Transposons and Retrotransposons – retroviruses evolutionarily related to retrotransposons
• Gene Therapy Vectors – modified retroviruses deliver therapeutic genes
• RNA World Hypothesis – RT might be ancient, dating to RNA-based life

Summary

Retroviruses execute a molecular reversal of the central dogma by converting RNA genomes to DNA through reverse transcription, then integrating into host chromosomes as permanent proviruses. The reverse transcriptase enzyme performs RNA→DNA synthesis with high error rates that generate diversity but enable rapid evolution and drug resistance. Integration via integrase makes retroviral infections persistent and, in cases like HIV, incurable with current therapies. Understanding reverse transcription is essential for developing antiretroviral therapies, using RT-PCR diagnostics, and engineering viral vectors for research and gene therapy.

#flashcards/biology

What are retroviruses? :: RNA viruses that reverse-transcribe their RNA genome into DNA using reverse transcriptase, then integrate into the host chromosome as a provirus

Explain the two template jumps in reverse transcription :: First jump: minus-strand strong-stop DNA transfers from 5' to 3' end of RNA using complementary R regions. Second jump: plus-strand strong-stop DNA transfers to the other end of minus-strand DNA. Both jumps enable creation of complete LTRs at both ends

Concept Map

Hinglish (regional understanding)