Mutations
2.12 i) Understand how errors in DNA replication can give rise to mutations.
ii) Understand how cystic fibrosis results from one of a number of possible gene mutations.
2.14 Understand how the expression of a gene mutation in people with cystic fibrosis impairs the
functioning of the gaseous exchange, digestive and reproductive systems.
Effect of Mutations on Proteins:
• The order of DNA bases in a gene determines the order of amino acids in a particular protein.
• If a mutation occurs in a gene, the primary structure of the protein could be altered.
• This could change the final 3D shape of the protein so it doesn’t work properly.
1. Where in the polypeptide does the mutation exert its effect?
o I.e. where is the resulting amino acid change located in the polypeptide?
o E.g. a mutation in the active site of the enzyme can change the active site’s shape, thus it cannot form
an enzyme-substrate complex or catalyse the reaction.
o If the mutation occurs in a non-critical part of the enzyme, it may still be able to carry out its function,
but perhaps less efficiently.
2. What are the chemical properties of the new amino acid coded for?
o I.e. what is the R-group of the new amino acid?
o E.g. channel proteins have hydrophilic amino acids facing outwards; if these were hydrophobic, it
may change the shape of the channel protein and reduce its efficiency.
Degenerate Amino Acids:
• If a mutation occurs in the 3rd base of a triplet, it may have no effect on the amino acids it codes for.
• This is because some amino acids are degenerate (can be coded for by multiple codons).
Cystic Fibrosis
Inheritance:
• Cystic fibrosis (CF) is an inherited disorder that mainly affects the respiratory, digestive and reproductive
systems.
• It is caused by a recessive allele, so a person will only inherit the disorder if they are homozygous for the
recessive allele.
• If a person is heterozygous, they won’t have CF, but they will be a carrier.
CFTR Protein:
• Cystic fibrosis is caused by a mutation in the gene that codes for the CFTR (cystic fibrosis transmembrane
conductance regulator) protein.
• CFTR is a channel protein that transports chloride ions out of cells into the mucus.
• This causes water to move into the mucus by osmosis, making the mucus less sticky.
• Mutant CFTR protein is much less efficient at transporting chloride ions, so less water moves out by osmosis.
• People with CF have abnormally thick and sticky mucus.
• The role of mucus is to catch microorganisms and dust.
• Thick and sticky mucus can cause problems in the respiratory, digestive and reproductive systems.
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