Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
Answers to self-assessment questions
Annual
incidence
worldwide
Annual
mortality
worldwide in
2011
over 20 million
158 000
16 Viruses do not have targets for antibiotics; for
example, viruses are not cells, so they have
no cell walls. Also they do not have the cell
machinery for making proteins, they use the
ribosomes and enzymes of their host cells
to make their proteins. The antibiotics that
we use to control bacterial infections do not
interfere with human proteins, so none of
them will inhibit the reproduction of viruses.
17 a n a thick cell wall that is impermeable to
antibiotics
■■
membrane proteins that inactivate
antibiotics
■■
enzymes that catalyse the breakdown
of antibiotics (e.g. β-lactamase enzymes
including penicillinases)
■■
■■
membrane proteins that pump antibiotics
out of the cell
changing the part of the protein to which
the antibiotic binds so this is not possible
b Among a population of bacteria are some
with a mutation that confers resistance
to the antibiotic. Susceptible bacteria are
killed by the antibiotic (or stop reproducing).
Bacteria resistant to the antibiotic survive
and reproduce. These bacteria pass on the
mutant gene that codes for a protein that
provides resistance so, in a short period of
time, there is a large population of antibioticresistant
bacteria.
c Vertical transmission – the DNA in the
bacterial chromosome is replicated and
copies are transferred to the two new
cells formed in binary fission (asexual
reproduction). Note that bacteria are
prokaryotic and do not divide by mitosis.
Horizontal transmission – either a copy of
a plasmid or a copy of part of the bacterial
chromosome passes through a conjugation
tube from one bacterium to another.
Vertical transmission is always within
bacteria of the same species; horizontal
transmission may be between members of
the same or different species.
18 MRSA evolved because antibiotics are
used in hospitals and act as a selection
pressure. Also, there has been transfer of
genes between different types of bacteria;
for example, Staphylococcus aureus and
Enterococcus faecalis, which is an intestinal
bacterium. E. faecalis was the source of
vancomycin resistance in S. aureus. MRSA is
common in prisons in the USA because many
people are kept in close proximity and once a
disease in present in such a community, it is
easily transmitted.
19 a The more frequently antibiotics are used,
the more frequently resistant bacteria will
be selected for. If antibiotic use is infrequent,
then other selection pressures will be
more important in bacterial populations,
decreasing the likelihood of resistant
bacteria surviving.
b Changing the antibiotic changes the
selection pressure. Different strains of
bacteria will be selected for when a different
antibiotic is used, decreasing the likelihood
of a resistant strain for each antibiotic
becoming widespread.
c It is far less likely that any individual
bacterium will be resistant to two antibiotics
than to any single antibiotic. Therefore,
using two antibiotics together decreases the
chance of any bacteria surviving.
20 B and E. These have inhibition zones larger
than the minimum required to be in the
sensitive range. These antibiotics could be
used together.
Cambridge International AS and A Level Biology © Cambridge University Press 2014