Kai Khokhar 08/01/2026
OCR A A-level Biology-Module
6-Cellular Control and
expression
1-Genetic Code and
mutations:
Triplet Code: 3 DNA bases code for one amino
acid
3 bases on an mRNA molecule called a codon.
3 bases on tRNA called an anticodon.
Genetic Mutation: Random change in genetic info of an
individual. Some of these mutations may also involve changes
to structure/no of chromosomes, i.e. Down’s syndrome.
Point Mutations (Substitutions): One base swapped for another-
affects ONLY ONE amino acid. Hen -> men
Insertion/deletion: One or more nucleotide pairs inserted or
deleted from a length of DNA. This can cause a ‘frame shift.’
Men -> Mean, Men -> En
Types of Point mutation/Substitution:
1. Silent Mutation: leads to change in base triplet, but were triplet still
codes for same amino acid
2. Missense Mutation: Leads to change in amino acid sequence in a
protein. In turn, the tertiary structure is altered.
3. Nonsense Mutation: Leads to a base triplet code becoming a ‘stop’
Insertion/deletion:
If Nucleotide base
pairs are inserted
or deleted, all the
subsequent base
pairs are altered
(known as a
frame shift) 1
, Kai Khokhar 08/01/2026
Sickle Cell Anaemia:
Substitution of one base causes a huge and damaging change
in an organism’s physiology. This is caused by a single
substitution:
Caused by mutation in gene coding for B-chain of
Haemoglobin (Hb).
Mutation is caused by a base SUBSTITUTION.
Therefore, glutamate becomes valine.
This causes a huge effect on the Hb, it crystallises within
the erythrocytes, causing them to become sickle shaped.
In sickle cell Hb, the valine interacts with each other
rather than water.
When O2 levels are low, the valines bond with each other,
sticking Hb molecules together.
The Hb forms long fibres, which pull erythrocytes out of
usual shape.
Mutations with neutral effects:
Mutation is an intron of DNA
If silent mutation-some triplets code for same amino
acid/code is degenerate, so amino acid sequence is not
altered.
Some alternative amino acids will not alter shape of
protein.
Causes change in protein structure but gives no
advantage/disadvantage, i.e. rolling tongue
Beneficial Mutations: Mutations can drive evolution= i.e. ability
to digest lactose in European populations allows drinking of
milk, preventing diseases alike to osteoporosis.
2-Regulatory Mechanisms:
The entire genome of an organism is present in every
prokaryotic cell, or eukaryotic cell that contains a nucleus. This
includes genes not required by that cell. Genes can be turned
2
OCR A A-level Biology-Module
6-Cellular Control and
expression
1-Genetic Code and
mutations:
Triplet Code: 3 DNA bases code for one amino
acid
3 bases on an mRNA molecule called a codon.
3 bases on tRNA called an anticodon.
Genetic Mutation: Random change in genetic info of an
individual. Some of these mutations may also involve changes
to structure/no of chromosomes, i.e. Down’s syndrome.
Point Mutations (Substitutions): One base swapped for another-
affects ONLY ONE amino acid. Hen -> men
Insertion/deletion: One or more nucleotide pairs inserted or
deleted from a length of DNA. This can cause a ‘frame shift.’
Men -> Mean, Men -> En
Types of Point mutation/Substitution:
1. Silent Mutation: leads to change in base triplet, but were triplet still
codes for same amino acid
2. Missense Mutation: Leads to change in amino acid sequence in a
protein. In turn, the tertiary structure is altered.
3. Nonsense Mutation: Leads to a base triplet code becoming a ‘stop’
Insertion/deletion:
If Nucleotide base
pairs are inserted
or deleted, all the
subsequent base
pairs are altered
(known as a
frame shift) 1
, Kai Khokhar 08/01/2026
Sickle Cell Anaemia:
Substitution of one base causes a huge and damaging change
in an organism’s physiology. This is caused by a single
substitution:
Caused by mutation in gene coding for B-chain of
Haemoglobin (Hb).
Mutation is caused by a base SUBSTITUTION.
Therefore, glutamate becomes valine.
This causes a huge effect on the Hb, it crystallises within
the erythrocytes, causing them to become sickle shaped.
In sickle cell Hb, the valine interacts with each other
rather than water.
When O2 levels are low, the valines bond with each other,
sticking Hb molecules together.
The Hb forms long fibres, which pull erythrocytes out of
usual shape.
Mutations with neutral effects:
Mutation is an intron of DNA
If silent mutation-some triplets code for same amino
acid/code is degenerate, so amino acid sequence is not
altered.
Some alternative amino acids will not alter shape of
protein.
Causes change in protein structure but gives no
advantage/disadvantage, i.e. rolling tongue
Beneficial Mutations: Mutations can drive evolution= i.e. ability
to digest lactose in European populations allows drinking of
milk, preventing diseases alike to osteoporosis.
2-Regulatory Mechanisms:
The entire genome of an organism is present in every
prokaryotic cell, or eukaryotic cell that contains a nucleus. This
includes genes not required by that cell. Genes can be turned
2
