BIOLOGY WITH HAMZA · MDCAT REVISION
Acellular Life
Name: ____________________ Date: ______________
Recall first. Check second. Return to the ideas you missed.
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Learning targets
- Explain why viruses are obligate intracellular parasites.
- Relate genome type to the route used to make viral mRNA.
- Distinguish virions, viroids, prions and proviruses.
Core notes
- Viruses lack cellular metabolism and protein-synthesis machinery.
- Capsids are protein; envelopes are host-derived lipids containing viral proteins.
- Every virus must produce positive-sense mRNA for translation.
- Genome type determines the enzymes and intermediates required.
Read, cover and redraw
Stores viral genetic information.
Protects the genome and helps delivery.
Host-derived lipid layer in some viruses.
Recognise and attach to host receptors.
Written recall
1. Why must a negative-sense RNA virus carry or rapidly produce an RNA-dependent RNA polymerase?
2. What is the difference between a virion and a provirus?
Apply the idea
1. After unadsorbed phages are removed, 200 infected bacteria release 16,000 new infectious phages in one cycle. Assuming every infected cell lyses, what is the mean burst size?
- 80 phages per bacterium
- 200 phages per bacterium
- 8 phages per bacterium
- 16,000 phages per bacterium
My answer: _____ Reason: ____________________________________
2. A drug does not change the number of viral genomes inside cells just after entry, but it sharply reduces viral protein production. Which early step is most directly affected?
- Attachment to receptors
- Translation of viral mRNA
- Release of mature virions
- Adsorption to the cell surface
My answer: _____ Reason: ____________________________________
3. In the simplified retrovirus pathway, which enzyme performs step X?
- Viral RNA
- X
- Viral DNA
- Integration into host DNA
- DNA ligase
- RNA-dependent RNA polymerase
- Reverse transcriptase
- Restriction endonuclease
My answer: _____ Reason: ____________________________________
Answer key
Cover this section while attempting the questions.
Written recall
1. Its genome cannot be translated directly. The enzyme is needed to make complementary positive-sense mRNA that host ribosomes can read.
2. A virion is a complete extracellular infectious particle; a provirus is viral DNA integrated into the host genome.
Applied questions
1. A — Burst size is progeny released per infected cell: 16,000 ÷ 200 = 80. It is not the total number of particles in the culture.
2. B — Comparable intracellular genome numbers indicate entry occurred. Reduced viral protein production points most directly to translation, although later replication can also fall as a consequence.
3. C — Reverse transcriptase synthesizes DNA using viral RNA as a template. Integrase is involved later in inserting the DNA into the host genome.