Viral replication and disease

MDCAT Biology · Biology · Acellular life. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A bacteriophage can multiply in two ways. In the lytic cycle the phage attaches, injects its DNA, takes over the bacterium, makes new phages and bursts (lyses) the cell. In the lysogenic cycle the phage DNA joins the bacterial chromosome as a prophage and is copied each time the bacterium divides. No harm is done until the prophage leaves the chromosome and starts a lytic cycle.

HIV is a retrovirus: its genetic material is RNA, and its enzyme reverse transcriptase copies this RNA into DNA. The DNA joins a chromosome of a helper T cell as a provirus. As helper T cells are destroyed, immunity fails and the person develops AIDS.

Rules to remember

Lytic cycle order: attachment (adsorption) → penetration → replication of phage DNA and proteins → assembly → lysis
Only the phage DNA enters the bacterium; the protein coat stays outside. Lysozyme from the phage tail makes the hole in the wall
Virulent phage = lytic cycle only; temperate phage (for example lambda) = can follow either cycle
Prophage = phage DNA inserted in the bacterial chromosome; the bacterium carrying it is lysogenic
HIV in a helper T cell: attachment to the CD4 receptor → reverse transcription (RNA → DNA) → integration as a provirus → transcription and translation → assembly → budding
HIV spreads by unprotected sexual contact, infected blood or shared needles, and from mother to child (placenta, birth, breast milk). It does not spread by touch, food, coughing or mosquitoes
Prevention: screened blood, sterile needles, safe sexual behaviour, treatment of infected mothers. There is no cure and no vaccine; antiretroviral drugs slow the virus

Common mistake

Thinking the whole phage enters the bacterium, or that a person with HIV but no symptoms cannot infect others. Only phage DNA enters, and HIV can be passed on during the long symptom-free period.

Worked example

One phage infects one bacterium. Each lytic cycle releases 200 new phages from every infected cell, and each new phage infects a fresh bacterium. How many phages are there after two cycles?

After cycle 1: 1 infected cell × 200 = 200 phages.
These infect 200 bacteria.
After cycle 2: 200 infected cells × 200 = 40 000 phages.

Answer: 40 000 phages (4 × 10⁴).

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