AQA A Level Biology - Mutations and Gene Expression (2021-22)

Year 2




Mutations and
Gene Expression




Name:


Teacher:

1

, Mutations
What is a mutation?
A mutation is a random change in the base sequence of DNA. The order of DNA bases in a gene
determines the sequence of amino acids in a particular polypeptide. If a mutation occurs in the gene, the
sequence of amino acids in the polypeptide that it codes for could be changed.
Types of mutations

Type Description
One or more bases are swapped for another

One or more bases are removed

One or more bases are added/inserted

One or more bases are repeated

A sequence of bases is reversed

A sequence of bases is moved from one location in the genome to
another. This could be movement within the same chromosome or
movement to a different chromosome


Example: Here is a sequence of DNA bases on one strand of DNA:


ATG TTT CCT GTT AAA TAC CAT CGC


Below are five possible mutations of this piece of DNA. Name the type of mutation for each.
a. Mutation 1: ATG TTT CCT GTT AAG TAC CAT CGC

Type of Mutation: ……………………………

b. Mutation 2: ATG TTC CTG TTA AAT ACC AT C GC

Type of Mutation: ……………………………

c. Mutation 3: ATG TTT CCT GTT GTT AAA TAC CAT CGC

Type of Mutation: ……………………………

d. Mutation 4: GTA TTT CCT GTT AAA TAC CAT CGC

Type of Mutation: ……………………………

e. Mutation 5: ATG TTT CCT GTT AAA CTA CCA TCG C

Type of Mutation: ……………………………




2

,Mutations and proteins

If a mutation occurs in the gene, the sequence of amino acids in the polypeptide that it codes for could be
changed. However, not all DNA mutations do affect the sequence of amino acids. As the genetic code is
degenerate, some amino acids are coded for by more than one triplet code. This means that not all types of
mutation will result in a change in the amino acid sequence of a polypeptide. If a mutation doesn’t cause a
change in the sequence of amino acids, it is called a __________________.




Some DNA mutations have a very large effect on the amino acid sequence they code for. ___________
________________ and _____________ within a gene almost always cause a change in the amino acid
sequence of a polypeptide. The often cause a shift in the base triplets that follow the mutation so that the
triplet code is read in a different way. This is known as a ______________. The base triplets that follow on
from the mutation are said to be _____________ of the mutation.




3

, Mutagenic agents
Gene mutations can occur spontaneously, eg. when DNA is misread during DNA replication. However,
factors, known as mutagenic agents, can increase the rate of mutations.
Egs include:

• Ultraviolet (UV) radiation
• Ionising radiation
• Some chemicals
• Some viruses
They can increase the rate of mutation in different ways:
1. ACTING AS A BASE – chemicals called base analogs can substitute for a base during DNA
replication, changing the base sequence in the new DNA.

E.g.______________ (5BU) is a base analog that can substitute for thymine (T). When a nucleotide
containing 5-bromouracil is incorporated into the DNA, it is most likely to pair with adenine; however,
it can spontaneously shift into another __________ which pairs with a
different nucleobase, ___________ (G). If this happens during DNA replication, a guanine will be
inserted as the opposite base analog, and therefore in the next DNA replication, that guanine will pair
with a cytosine.




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