Viruses
General Biology · Section 3.4 · 18 study cards
Viral structure and genomes, lytic and lysogenic cycles, animal virus entry and exit, retroviruses and HIV, prions, vaccines and spillover.
Practice this set → Spaced repetition, card by card. No account needed.
Method
Ask first what the virus has to work with
Every detail of a viral life cycle follows from two facts: the genome type and whether there is an envelope. Genome type dictates what has to happen before proteins can be made, so an RNA virus must either be read directly as messenger RNA, be copied into messenger RNA, or, in a retrovirus, be copied into DNA first. The envelope dictates entry and exit, since an enveloped virus can fuse and can bud, while a naked virus is generally taken in by endocytosis and released by lysis. State those two facts about any virus in a question before answering anything else.
Use one skeleton for every life cycle
- Attachment, which depends on a match between a viral surface protein and a host receptor and therefore sets host range and tropism.
- Entry and uncoating, or, for a phage, injection of the genome alone.
- Genome replication and gene expression, the step where genome type matters most.
- Assembly of capsids around new genomes.
- Release by lysis or by budding.
Lysogeny is an optional pause inserted between steps two and three, in which the genome integrates and waits.
Treat lysogeny as a decision, not a stage
The lytic and lysogenic cycles are two strategies available to one temperate phage, not two kinds of virus. A repressor holds the lytic genes off; DNA damage destroys the repressor and the phage switches. Any question that describes stress, ultraviolet light or a chemical mutagen applied to lysogenic bacteria is asking you to predict induction and lysis.
Connect mutation rate to medicine
When a question asks why a treatment or a vaccine fails, check whether the genome is RNA. No proofreading means high mutation rates, which means antigenic change and rapid resistance, which in turn means combination therapy and reformulated vaccines. Almost every clinical part of this unit reduces to that chain of reasoning.
Definitions and theorems
Worked example
A virus isolated from a patient has an envelope, a single-stranded RNA genome, and cannot express its genes until that genome has been copied into DNA and inserted into the host chromosome. It infects only cells carrying a particular surface receptor. Classify the virus, predict four points at which a drug could act, and explain why a vaccine designed against last year's isolate may no longer protect.
Classify it. An RNA genome that must be converted to DNA and integrated identifies a retrovirus; HIV is the worked example. The requirement for a particular receptor explains its narrow tropism, as HIV requires CD4 with a co-receptor.
Identify the first drug target. Attachment and fusion depend on the viral envelope glycoprotein binding the receptor, so an entry or fusion inhibitor can block infection before the genome ever enters.
Identify the second. Copying RNA into DNA requires reverse transcriptase, an enzyme with no host counterpart, which makes it an unusually safe target; nucleoside and non-nucleoside inhibitors both act here.
Identify the third and fourth. Integrase inserts the DNA copy into the host chromosome, and a viral protease cleaves the long polyprotein precursors into functional proteins during maturation. Both are viral enzymes and both are blocked by licensed drugs.
Explain why combinations are used. Each drug alone selects for resistant variants, but the probability of one genome carrying resistance mutations to three drugs at once is very low, so the drugs are given together.
Explain the vaccine failure. Reverse transcriptase has no proofreading, so mutations accumulate at a high rate, especially in the envelope glycoprotein that antibodies recognise.
Complete the argument. Antibodies raised against last year's glycoprotein no longer bind the altered surface, so immune memory does not recognise the current isolate. The virus has escaped by antigenic change rather than by any change in how it replicates.
Common mistakes
- Saying viruses are alive because they reproduce and evolve. They reproduce only by commandeering a host cell, and they have no metabolism, no cells and no ribosomes. Answer with the criteria they meet and the criteria they fail, rather than with a single word.
- Treating lytic and lysogenic as two kinds of virus. They are two cycles available to one temperate phage, chosen by conditions. A question about ultraviolet light applied to lysogenic bacteria is asking about induction, which is the switch between them.
- Confusing a prophage with a provirus. A prophage is a phage genome in a bacterial chromosome and can excise itself on induction. A retroviral provirus is integrated permanently and is inherited by every descendant of that cell.
- Expecting antibiotics to help a viral infection. Antibiotics act on bacterial cell walls and bacterial ribosomes, neither of which a virus possesses. The correct answer names the missing target, not simply that viruses are too small.
- Saying enveloped viruses are hardier because they have an extra layer. The opposite is true: the lipid envelope dries out and is dissolved by soap and alcohol, so enveloped viruses are the fragile ones outside a host, while naked capsids persist on surfaces.
Practice it
Reading the method is not the same as being able to recall it under pressure. This set drills 18 cards one at a time and schedules each card separately, so the ones you keep missing come back sooner.