01 / 20

Viral Genetics,
Transduction & Retroviruses

Master how viruses hijack host machinery, transfer genetic material between bacteria via transduction, and how retroviruses use reverse transcriptase to integrate into host genomes.

MCAT Biology

What Are Viruses?

Viruses are obligate intracellular parasites: they cannot replicate on their own and must hijack a host cell's machinery. A virus particle (virion) consists of a nucleic acid genome (DNA or RNA, single- or double-stranded) enclosed in a protein capsid, sometimes surrounded by a lipid envelope.

Genome
DNA or RNA; ss or ds; linear or circular
Capsid
Protein shell: icosahedral, helical, or complex
Envelope
Lipid bilayer from host membrane (some viruses)

Viral Replication Strategies

All viruses follow the same general steps: attachment, entry, replication, assembly, and release. However, the details depend on the genome type (the Baltimore classification).

1
Attachment: Viral surface proteins bind specific host cell receptors (determines tropism).
2
Entry: Genome (and sometimes enzymes) enter the host cell via fusion, endocytosis, or injection.
3
Replication & Translation: Host ribosomes and polymerases (or viral ones) produce viral proteins and copy the genome.
4
Assembly: New capsids form around newly replicated genomes.
5
Release: Lysis (non-enveloped) or budding (enveloped) frees new virions.

Lytic vs Lysogenic Cycle

LYTIC CYCLE 1. Phage attaches & injects 2. Host DNA degraded 3. Viral genome replicated 4. Assembly of new phage 5. Cell lysis & release LYSOGENIC CYCLE 1. Phage attaches & injects 2. DNA integrates (prophage) 3. Cell divides normally 4. Prophage replicated with host 5. Induction → lytic (stress) Induction

Bacterial Transduction

Transduction is the transfer of bacterial genes from one bacterium to another via a bacteriophage (phage). It is one of three mechanisms of horizontal gene transfer in bacteria, alongside transformation and conjugation.

Generalized
Random host DNA fragments packaged into phage heads during lytic cycle
Specialized
Only genes adjacent to prophage integration site transferred during imprecise excision

Generalized Transduction

Host DNA 1. Phage infects 2. Host DNA fragmented 3. Host DNA packaged (error) 4. New host gets donor genes = Host DNA

Specialized Transduction

Host Bacterial Chromosome Prophage gal bio Imprecise excision gal phage Infects new host gal prophage New host gains gal gene

Generalized vs Specialized

Generalized Specialized
Cycle involved Lytic Lysogenic (imprecise excision)
Genes transferred Any host gene (random) Only genes flanking prophage site
DNA in phage head Host DNA only Phage + host DNA (hybrid)
Mechanism Packaging error Imprecise excision
Classic example Phage P1 (E. coli) Phage lambda (gal/bio)

Horizontal Gene Transfer in Bacteria

Transformation Transduction Conjugation
Vector Naked DNA Bacteriophage F pilus (cell-cell)
Requires contact? No No (phage is vector) Yes (pilus bridge)
DNA type Free fragments Phage-packaged F plasmid or Hfr
Discovery Griffith (1928) Zinder & Lederberg (1952) Lederberg & Tatum (1946)

What Are Retroviruses?

Retroviruses are RNA viruses that reverse the central dogma: they convert their single-stranded RNA genome into double-stranded DNA using the enzyme reverse transcriptase. This DNA copy (provirus) then integrates into the host genome via integrase.

Genome
Two copies of (+)ssRNA (diploid)
Key Enzyme
Reverse transcriptase (RNA-dependent DNA polymerase)
Example
HIV (Human Immunodeficiency Virus)

Retroviral Life Cycle

1. Binding & Fusion 2. Reverse Transcription ssRNA → dsDNA 3. Integration (Integrase) dsDNA → Host genome 4. Transcription & Translation Host RNA Pol II 5. Assembly 6. Budding & Maturation Protease cleaves polyproteins

HIV Drug Targets

Understanding the retroviral life cycle reveals multiple drug targets. MCAT questions frequently connect viral enzymes to their pharmacological inhibitors.

1
Reverse Transcriptase Inhibitors: NRTIs (e.g., AZT/zidovudine) act as chain terminators; NNRTIs bind allosterically.
2
Integrase Inhibitors: Block proviral DNA insertion into the host genome (e.g., raltegravir).
3
Protease Inhibitors: Prevent cleavage of polyproteins into functional viral proteins (e.g., ritonavir).
4
Fusion/Entry Inhibitors: Block viral binding to CD4/CCR5 receptors or membrane fusion (e.g., enfuvirtide, maraviroc).

How to Identify Transduction Type

1
Is a phage involved? If no: it is transformation or conjugation, not transduction.
2
Was the phage in a lysogenic state? If yes and excision was imprecise: specialized transduction.
3
Was host DNA randomly packaged during lysis? If yes: generalized transduction.
4
Can any gene be transferred, or only specific ones? Any gene = generalized. Only flanking genes = specialized.
5
Does the transducing particle contain phage DNA? No = generalized (host DNA only). Yes (hybrid) = specialized.

Worked Example: Gene Transfer

1
Phage is involved (P1): this rules out transformation and conjugation.
2
P1 is a lytic phage: it does not normally integrate (no lysogenic cycle involved).
3
The leu+ gene is not near a prophage site: it was randomly packaged during host DNA degradation.
4
Answer: Generalized transduction via phage P1.

Worked Example: Retrovirus Life Cycle

1
AZT is a thymidine analog that lacks a \(3'\)-OH group on the deoxyribose sugar.
2
Reverse transcriptase incorporates AZT into the growing DNA chain instead of thymidine.
3
Without a \(3'\)-OH, no phosphodiester bond can form with the next nucleotide: chain termination.
4
Step blocked: Reverse transcription (step 2 of the retroviral life cycle: ssRNA to dsDNA conversion).

Common Mistakes

✕ Confusing Transduction Types
Generalized = random (lytic). Specialized = specific flanking genes (lysogenic excision). They are NOT interchangeable.
✕ Calling Retroviruses "DNA Viruses"
Retroviruses have an RNA genome. They make a DNA intermediate (provirus) but are classified as RNA viruses.
✕ Mixing Up HGT Mechanisms
Transformation = naked DNA uptake. Transduction = phage-mediated. Conjugation = pilus/cell contact. Each has a distinct vector.

Question 1 of 5

During generalized transduction, what does the transducing phage particle contain?

Question 2 of 5

Which enzyme allows retroviruses to synthesize DNA from an RNA template?

Question 3 of 5

A researcher discovers that a phage transfers only the gal gene from donor to recipient E. coli. Which mechanism is most likely?

Question 4 of 5

HIV has a high mutation rate primarily because reverse transcriptase:

Question 5 of 5

A temperate phage in the lysogenic cycle integrates into the host chromosome and is called a: