This comprehensive set of A-Level Biology notes is meticulously crafted to provide students with essential information and streamlined content that reinforces understanding of key concepts. The notes are organized thematically, covering all major topics in the A-Level Biology curriculum, from 1-8. ...
Topic 8
The control of
gene expression
Model answer notes by @biologywitholivia
Topic Sub-topic Understand Memorise Practise
8.1 Alteration of the sequence of bases in DNA can alter the
structure of proteins
8.2 Gene expression is 8.2.1 Most of a cell’s DNA is not translated
controlled by a
number of features 8.2.2 Regulation of transcription and
translation
8.2.3 Gene expression and cancer
8.3 Using genome projects
8.4 Gene technologies 8.4.1 Recombinant DNA technology
allow the study and
alteration of gene 8.4.2 Differences in DNA between
function allowing a individuals of the same species can be
better understanding exploited for identification and diagnosis
of organism function of heritable conditions
and the design of new
8.4.3 Genetic fingerprinting
industrial and medical
processes
,AQA A Level Biology Topic 8 Control of gene expression stan.store/biologywitholivia
8.1 The alteration of the sequence of
bases in DNA can alter the structure of
proteins
What is a gene mutation?
● A change in the base sequence of DNA
● Can arise spontaneously during DNA replication (interphase)
What is a mutagenic agent?
A factor that increases rate of mutation, eg. ultraviolet (UV) light or alpha particles.
Explain how a gene mutation can lead to the production of a non-functional
protein or enzyme (general)
1. Changes sequence of base triplets in DNA so changes sequence of codons on mRNA
2. So changes sequence of amino acids in the encoded polypeptide
3. So changes position of hydrogen / ionic / disulphide bonds (between amino acids)
4. So changes tertiary structure (shape) of protein
5. Enzymes - active site changes shape so substrate can’t bind, enzyme-substrate complex can’t form
Describe the different types of gene mutations
Substitution A base / nucleotide is replaced by a different base / nucleotide in DNA Students
should be able
Addition 1 or more bases / nucleotides are added to the DNA base sequence to relate the
nature of a
Deletion 1 or more bases / nucleotides are lost from the DNA base sequence
gene mutation
Duplication A sequence of DNA bases / nucleotides is repeated / copied to its effect on
the encoded
Inversion A sequence of bases / nucleotides detaches from the DNA sequence, polypeptide.
then rejoins at the same position in the reverse order
Translocation A sequence of DNA bases / nucleotides detaches and is inserted at a
different location within the same or a different chromosome
Explain why not all gene mutations affect the order of amino acids
● Some substitutions change only 1 triplet code / codon which could still code for the same amino acid
○ As the genetic code is degenerate (an amino acid can be coded for by more than one triplet)
● Some occur in introns which do not code for amino acids
2
, AQA A Level Biology Topic 8 Control of gene expression stan.store/biologywitholivia
Explain why a change in amino acid sequence is not always harmful
● May not change tertiary structure of protein (if position of ionic / disulphide / H bonds don’t change)
● May positively change the properties of the protein, giving the organism a selective advantage
Explain what is meant by a frameshift
● A frameshift occurs when gene mutations (eg. addition, deletion,
duplication or translocation) change the number of nucleotides /
bases by any number not divisible by 3
● This shifts the way the genetic code is read, so all the DNA triplets
/ mRNA codons downstream from the mutation change
● The sequence of amino acids encoded changes accordingly and
the effects on the encoded polypeptide are significant
If a multiple of 3 bases is added / removed there won’t be a frameshift, but extra / less triplets will result in extra /
less amino acids in the encoded polypeptide.
A frameshift could also lead to production of a stop codon (that doesn’t code for amino acids so terminates
translation), resulting in a shorter polypeptide.
Exam insight: common mistakes ❌
Mistake Explanation
“A mutation is a change in Genes are made of DNA, which has a nucleotide base sequence that codes
amino acid sequence of a for a sequence of amino acids in a polypeptide. Mutations change this base
gene” sequence and this results in a change in amino acid sequence
of a polypeptide.
“A gene mutation can lead to Genes do not determine which amino acids are produced - amino acids
a new amino acid being already exist in cells. Instead, each gene has the code for the order in which
formed or produced.” these amino acids need to be joined together in translation to form a
particular polypeptide. So, a gene mutation can lead to a different sequence
of amino acids being formed, but not new amino acids themselves.
“Mutations change the 3D Always mention the term tertiary structure and the change in the position of
structure of a protein.” hydrogen bonds, ionic bonds and disulphide bridges.
“All mutations are harmful.” A mutation may result in the formation of a new, advantageous allele that
results in change in polypeptide that positively affects the properties of the
protein, giving an organism a selective advantage.
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