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AQA A LEVEL BIOLOGY EXAM QUESTIONS
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Exam questions and answers for section 8: mutations, gene technology also includes synoptic links from section 4
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Section 8Gene Mutations
Mutation – any change to the quantity or structure of the DNA of an organism
Gene mutation – any change to one or more nucleotide bases, or any rearrangement of the bases
Explain how the structure of DNA is related to its functions
Helix = compact
Long = carry lots of information
Double stranded = semi conservative replication
Many hydrogen bonds = stable/ unzipping
Base sequence = allow amino acids to be coded for// allows information to be stored
Phosphate backbone = provides strength
A mutation can lead to the production of a non-functional enzyme. Explain how
1. Change in nucleotide sequence/ triplet of DNA/ gene
2. Change in amino acid sequence/ primary structure
3. Change in hydrogen bonds/ionic/disulfide
4. Change in tertiary structure
5. Change in active site
6. Substrate not complementary to active site
7. No enzyme substrate complexes formed
Gene mutations occur simultaneously. During which part of the cell cycle are gene mutations most
likely to occur?
Interphase/ S (synthesis) phase
Suggest explanation
DNA replication occurs
Complete Figure 1 to show the sequence of amino acids coded for by the original DNA base
sequence
Ile, Gly, Val, Ser
Some gene mutations affect the amino acid sequence. Some mutations do not. Use the information
from Figure 1 and Figure 2 to explain: whether mutation 1 affects the amino acid sequence
Has no effect/ same amino acid sequence, still codes for glycine
How mutation 2 could lead to the formation of a non-functional enzyme
Leu replaces Val/ changes in amino acid sequence
Change in hydrogen/ ionic bonds which alter tertiary structure
Substrate cannot bind
A change from Glu to Lys at amino acid 300 had no effect on the rate of reaction catalysed by the
enzyme. The same change at amino acid 279 significantly reduced the rate of reaction catalysed by
the enzyme.
,Use all the information and your knowledge of protein structure to suggest reasons for the
differences between the effects of these two changes (3)
1. (Both) negatively charged to
positively charged change in
amino acid;
2. Change at amino acid 300 does
not change the shape of
the active site
OR
3. Change at amino acid 300 does
not change the tertiary structure
OR Change at amino acid 300
results in a similar tertiary
structure;
4. Amino acid 279 may have been
involved in a (ionic, disulfide or
hydrogen) bond and so the
shape of the active site changes
OR
5. Amino acid 279 may have been involved in a (ionic, disulfide or hydrogen) bond and so the
tertiary structure changed;
OR
6. Amino acid 279 may be in the active site and be required for binding the substrate;
Types of mutation
Substitution of bases*
Deletion of bases*
Addition of bases
Duplication of bases
Inversion of bases
Translocation of bases
Addition of bases
Extra base inserted
Similar effect to deletion
Frame shift to the right
If 3 bases added or multiple of three bases – no frame shift
Result: different polypeptide
Duplication of bases
One or more bases are repeated
Frame shift to the right
Inversion of bases
, Group of bases separated from DNA sequence and rejoin at the same position but in the
inverse order (back to front)
Result: amino acid sequence
Translocation of bases
Group of bases separated from DNA on one chromosome and inserted into DNA sequence
on different chromosome
Significant effect on gene expression
Abnormal phenotype
Cancer, reduced fertility
Causes of mutations
Arise spontaneously during DNA replication, rate: one or two mutations per 100000 genes
per generation
Permanent changes in DNA, occur without any outside influence
Random, occur with a predictable frequency
Natural rate varies from species to species
Mutagenic agents or mutagens – external influence, outside factors that increase rate of
basic mutation
Mutagenic agents are high energy ionising radiation or chemicals.
Name one mutagenic agent
X rays, UV light, alpha, beta particles, high energy radiation, carcinogen, tar.
In flax plants the flowers are white. Lilac or blue. The diagram shows the pathway by which flower
cells produce coloured pigments. A deletion mutation occurs in gene 1. Describe how a deletion
mutation alters the structure of a gene.
Removal of 1 or more bases causes base sequence to change
Describe and explain how the altered gene could result in flax plants with white coloured flowers
Sequence of bases in mRNA would change
Amino acids sequence different
Changes tertiary structure/ active site shape
White pigment does not bind
Lilac pigment not produce
If the DNA of a cell is damaged, a protein called p53 stops the cell cycle. Mutations in the gene for
p53 could cause cancer to develop. Explain how
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