Chapter summary
Viral and bacterial genetics, at a glance
Scan the key ideas, or hide the answers and try to recall them.
01
Virus structure and the boundary of life
Why is a virus not simply a very small cell?
Key idea and reminders
Viruses package a genome in protein, sometimes with an envelope, but lack the autonomous cellular machinery needed for reproduction.
- Genome plus capsid; some also have an envelope.
- Viruses lack ribosomes and independent metabolism.
- Evolutionary properties do not make them cellular.
Keep in mind: Viruses are acellular particles; bacteria are cells with ribosomes and their own metabolic systems.
02
Lytic and lysogenic inheritance
How can phage DNA persist while the host keeps dividing?
Key idea and reminders
Lytic infection makes and releases new phages; lysogeny maintains a prophage copied with the host genome until induction.
- Lytic: replication, assembly, lysis.
- Lysogenic: integrated prophage copied with host DNA.
- Induction can switch to lytic development.
Keep in mind: It is copied with the host chromosome, even while lytic particle production is repressed.
03
Influenza: copy RNA and bud
How does an RNA virus use a cell that normally transcribes DNA?
Key idea and reminders
Influenza supplies viral RNA-copying machinery, uses host translation, and packages new genomes in budding enveloped particles.
- Negative-sense RNA needs viral polymerase to make mRNA.
- Host ribosomes make viral proteins.
- Budding supplies the envelope.
Keep in mind: That describes a retroviral strategy such as HIV; influenza uses RNA-dependent RNA synthesis.
04
HIV: RNA through a DNA stage
How can an RNA virus leave a persistent DNA copy?
Key idea and reminders
HIV reverse-transcribes RNA into DNA, integrates a provirus, then uses host expression machinery to make progeny.
- Reverse transcriptase: RNA to DNA.
- Provirus: integrated viral DNA.
- Host transcription and translation produce viral components.
Keep in mind: It synthesises DNA using RNA as a template. Ribosomes translate mRNA into polypeptides.
05
Antigenic drift and shift
How does reassortment differ from gradual mutation?
Key idea and reminders
Drift accumulates antigen-altering sequence changes; shift can create a major new antigen combination through reassortment of segmented influenza genomes.
- Drift: accumulated antigen-changing mutations.
- Shift: major change, often segment reassortment in influenza A.
- Selection acts on variants; it does not direct mutations.
Keep in mind: Copying errors can contribute to drift; shift involves a major antigenic change, often from reassortment.
06
Bacterial reproduction and gene transfer
How can bacteria gain variation without meiosis?
Key idea and reminders
Binary fission transmits an existing genome; horizontal transfer introduces DNA by uptake, phages or direct cell contact.
- Transformation: free DNA uptake.
- Transduction: phage-mediated transfer.
- Conjugation: contact and plasmid transfer.
Keep in mind: It transfers DNA between cells. Bacteria reproduce by binary fission in this model.