Summary
Summary Infectious disease
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- CIE
Biology A level revision notes on infectious disease
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Infectious diseaseDisease: an illness or disorder of the body or mind that leads to poor health, each is
associated with a set of signs and symptoms
Infectious diseases are caused by pathogens and can be passed from person to person or
from an animal to a person.
Non-infectious diseases are not caused by pathogens and include lung cancer, a long-
term degenerative disease. Inherited or genetic diseases are also classified as non-
infectious and include sickle-cell anaemia.
Antibiotics
An antibiotic is a drug that kills or stops the growth of bacteria, without harming the cells
of the infected organism.
They can interfere with growth by stopping:
Synthesis of bacterial cell walls
Activity of proteins in the cell surface membrane
DNA synthesis (replication)
Protein synthesis
Penicillin prevents the synthesis of the cross-links between the peptidoglycans polymers
in the cell walls of bacteria by inhibiting the enzymes that build the cross-links. This
means that penicillin is only active against the bacteria when they are growing.
When a newly formed bacterial cell is growing it secretes enzymes called autolysins,
which make holes in its cell wall to allow it to stretch so the new peptidoglycan chains
can link together. Penicillin prevents these chins from linking and the cell wall becomes
weaker as more holes are made. The bacteria take up water by osmosis and eventually
the pressure potential exerted on the cell wall is too great so they burst.
Penicillin does not affect human cells, as they have no cell walls, or viruses as they have
no cells.
Resistance
Antibiotic resistance can arise when an existing gene within the bacterial genome
changes spontaneously to give rise to a nucleotide sequence that codes for a slightly
different protein that is not affected by the antibiotic. This change in the DNA is a
mutation.
Since bacteria only have one copy of each gene in a single loop of double-stranded DNA, a
mutation will have an immediate effect. The bacteria cells divide by binary fission so
when the chromosome is replicated, each daughter cell will receive the mutated gene
and will also be resistant. This is vertical transmission.
Genes often occur on plasmids, which are frequently transferred from one bacterium to
another. This happens during conjunction when a tube forms between two bacteria to
allow the movement of DNA. This is horizontal transmission and can happen between
different species.
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