Chapter 16 Adaptation and Selection
Common Questions and Mark-schemes
Question
Describe how resistance
to antibiotics arises and
how it spreads
Long answer Q – you
might not need all of
these sections. Read the
Q carefully
Explain why a deletion /
addition mutation would
have greater effect than
a substitution mutation
State a way an antibiotic
kills bacteria
If an antibiotic damages
the cell wall, why does
this kill bacteria?
Why do antibiotics not
affect human cells
Mark scheme
How resistance arises:
 Antibiotics kill bacteria
 Mutation in DNA of bacteria
 Resistant gene / allele
 <Produces an enzyme to break down antibiotic, or some other
mechanism>
How resistance spreads within same species:
 Resistant bacteria survive and reproduce
 Frequency of resistant allele increases each generation
 Gene spread by vertical gene transmission
 When cell divides by binary fission
How resistance spreads between species:
 Horizontal gene transmission can spread gene between species
 Copy of plasmid transferred via…
 Conjugation tube
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Deletion / addition removes / adds a base where as substitution swaps
one for another
Frame shift occurs
Triplet codes from point of mutation onwards change
Primary, secondary, tertiary structure of proteins change
Changes 3D shape of protein / enzyme
Try and say how this would impact, e.g. active site changes shape
Prevents cell wall synthesis; bacteria cells damaged by osmosis
Binds to ribosomes; prevents protein synthesis
Inhibit DNA synthesis; prevents cell division
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If bacterial cell has a lower water potential than surroundings
Water enters by osmosis
Cell bursts
No cell wall / different sizes ribosomes
There have also been a couple of question about competitive inhibitors – some antibiotics act as
competitive inhibitors.
Key points on mark scheme: Same shape; fits into active site of enzyme; competitive inhibitor