Lesson 5 of 6 / Viral and bacterial genetics
Antigenic drift and shift
How does reassortment differ from gradual mutation?
I can explain sources of viral variation, including influenza antigenic drift and shift.
I can explain viral genetic variation, including antigenic drift and shift.Syllabus
Syllabus 9477, Core 2(f). Explain viral genetic variation, including antigenic drift and shift.
About 6 min
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Two strains swapped RNA segments in one cell.
Reassortment of segments can create a new antigen combination at once. That is antigenic shift.
The key idea
Drift accumulates antigen-altering sequence changes; shift can create a major new antigen combination through reassortment of segmented influenza genomes.
Explore the mechanism
Change a sequence or mix genome segments?
Accumulating sequence changes can alter antigenic properties over time: drift. Selection may favour some variants, but need does not direct the mutations.
Original qualitative models. Segment colours show origin; the chosen segment numbers are illustrative and do not assign actual antigen genes.
Explanation
Replication errors introduce mutations into viral genomes. If a change alters a surface antigen while retaining viral function, existing antibodies may bind less effectively. Selection can favour variants in an immune population. Mutations do not arise because the virus decides it needs escape.
Antigenic drift refers to the accumulation of changes in antigenic properties over time, often through mutations affecting influenza surface proteins. A small sequence change can matter if it changes an antibody-binding region; many other mutations are neutral or harmful.
When different influenza strains infect the same cell, genome segments can be packaged in new combinations. This reassortment can produce an abrupt major antigenic change, termed antigenic shift in the influenza A context. The mechanism requires segmented genomes and co-infection; it is not simply one unusually large point mutation.
Recombination can also generate variation in some viruses by exchange or copying between related genomes. Distinguish the broad origin of variants from their later increase through selection. A new variant will not necessarily transmit efficiently or cause a pandemic; host susceptibility and transmission biology also matter.
Step by step
- 1
Identify the genetic event
Sequence change or segment reassortment?
- 2
Connect it to antigen shape
Not every mutation changes antibody recognition.
- 3
Separate origin from spread
Fitness and host immunity influence which variants increase.
Worked example
Work through the evidence
Two influenza A strains co-infect a cell. Progeny contain some whole RNA segments from each. Classify the mechanism.
One way to explain it
Genome-segment reassortment. If it creates a major new antigenic combination, it can cause antigenic shift.
Why this answer works
- Whole segments are being exchanged.
- The antigenic consequence must be established, not assumed for every reassortment.
Watch out for this
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Copying errors can contribute to drift; shift involves a major antigenic change, often from reassortment.
Check your understanding
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Two compatible segmented influenza strains co-infecting a cell
Segments can be packaged into new combinations.
Quick recall
How does reassortment differ from gradual mutation?