Biology A Level Revision (AQA)
Paper 1 - Year 12 / AS Level
Biological Molecules
Cells
Organisms Exchanging Substances
Genetic Information, Variation
Paper 2 - Year 13 / A Level
Energy Transfers In and Between Organisms
Organisms Responses to their Environment
Genetics, Evolution and Ecosystems
The Control of Gene Expression
Some diagrams used in this document are sourced from Cognito (https://cognitoedu.org).
All rights to those diagrams belong to Cognito.
, Biology Revision A Level AQA
The Control of Gene Expression
Mutations
- Mutations are changes in the base sequence of DNA.
- Mutations can be caused by errors in DNA
replication.
● Types of Mutations
- Addition - where one or more bases are added to the
DNA sequence.
ATCGTT → ATCCGTT
- Deletion - where one or more bases are removed from the DNA sequence.
ATCGTT → ATCTT
- Substitution - where one or more bases are changed in the DNA sequence.
ATCGTT → ACCGTT
- Inversion - where the order of bases are reversed in the DNA sequence.
ATCGTT → TTGCTA
- Duplication - where one or more bases are duplicated in the DNA sequence.
ATCGTT → ATATCGTT
- Translocation - where a section of the DNA sequence is moved to another section of
the DNA sequence and can be within and across chromosomes.
● Effects of Mutations
- Amino acids & DNA → DNA has four bases (A, T, C, G). Sequences of three bases
(codons) code for specific amino acids, which join to form proteins.
- Protein structure → Each amino acid has a unique R group. Interactions between R
groups create a protein’s 3D shape, which is crucial for its function (active sites).
- Mutations → Changes in DNA bases can alter amino acids. This can disrupt a
protein’s 3D structure and function.
- Consequences → Mutations can be harmful and cause genetic disorders, such as
cystic fibrosis, by affecting protein function.
- Degenerate code → Some amino acids are coded by more than one codon. For
example, tyrosine is coded by TAT and TAC.
- Implication for mutations → Because of this redundancy, a base change (CAA → CAG)
might not change the amino acid so not all mutations affect protein structure.
- Frameshift mutation → Occurs when bases are inserted, deleted, or duplicated,
shifting all downstream codons.
- Impact → Alters many amino acids, often severely disrupting protein function. Single
amino acid changes might not matter, but frameshifts usually do.
- For example, Crohn’s disease is caused by a frameshift in the NOD2 gene, producing
a shortened, nonfunctional protein.
● Causes of Mutations
- Mistakes during DNA replication (copying) can create spontaneous mutations.
- Mutagenic agents are factors like ionising or ultraviolet radiation that increase
mutation rates.
Page 1
, Biology Revision A Level AQA
Stem Cells
- Stem cells are unspecialised cells.
- Specialised cells only express some of the genes in their DNA.
- Stem cells can divide by mitosis to produce more undifferentiated cells or they can
differentiate into specialised cells.
● Totipotent Stem Cells
- Totipotent cells are the most unspecialised cell and exist for a limited time during
embryonic development in mammals (the first few cell divisions).
- Totipotent cells translate only part of their DNA during development causing the
cells to remain unspecialised.
- Totipotent cells are able to produce any type of body cell including cells of
supportive structures (like the placenta).
- These can differentiate into any cell type and go on to form whole organisms.
- When the totipotent cells become specialised, only some genes in the cell are
activated and expressed.
● Pluripotent Stem Cells
- Totipotent cells can develop into
pluripotent cells in embryos.
- Pluripotent cells are able to
divide an unlimited number of
times to produce any type of cell
that makes up the body.
- Pluripotent cells can be used to
treat human disorders.
- These can differentiate into
most cell types, but cannot form
whole organisms.
● Multipotent Stem Cells
- Multipotent cells are found in
mature mammals and can only
differentiate into a few different
cell types.
- For example, multipotent cells
in the bone marrow can produce a range of blood cells.
● Unipotent Stem Cells
- Unipotent cells are also found in mature mammals and can only differentiate into
one type of cell.
- For example, cardiomyocytes are unipotent cells that can produce new muscle cells.
● Proteins in Differentiation
- When a gene is expressed, it is transcribed into mRNA to be translated into a protein.
- Differentiation occurs as certain proteins are made and the presence of certain
proteins means the cell has become specialised.
Page 2
Paper 1 - Year 12 / AS Level
Biological Molecules
Cells
Organisms Exchanging Substances
Genetic Information, Variation
Paper 2 - Year 13 / A Level
Energy Transfers In and Between Organisms
Organisms Responses to their Environment
Genetics, Evolution and Ecosystems
The Control of Gene Expression
Some diagrams used in this document are sourced from Cognito (https://cognitoedu.org).
All rights to those diagrams belong to Cognito.
, Biology Revision A Level AQA
The Control of Gene Expression
Mutations
- Mutations are changes in the base sequence of DNA.
- Mutations can be caused by errors in DNA
replication.
● Types of Mutations
- Addition - where one or more bases are added to the
DNA sequence.
ATCGTT → ATCCGTT
- Deletion - where one or more bases are removed from the DNA sequence.
ATCGTT → ATCTT
- Substitution - where one or more bases are changed in the DNA sequence.
ATCGTT → ACCGTT
- Inversion - where the order of bases are reversed in the DNA sequence.
ATCGTT → TTGCTA
- Duplication - where one or more bases are duplicated in the DNA sequence.
ATCGTT → ATATCGTT
- Translocation - where a section of the DNA sequence is moved to another section of
the DNA sequence and can be within and across chromosomes.
● Effects of Mutations
- Amino acids & DNA → DNA has four bases (A, T, C, G). Sequences of three bases
(codons) code for specific amino acids, which join to form proteins.
- Protein structure → Each amino acid has a unique R group. Interactions between R
groups create a protein’s 3D shape, which is crucial for its function (active sites).
- Mutations → Changes in DNA bases can alter amino acids. This can disrupt a
protein’s 3D structure and function.
- Consequences → Mutations can be harmful and cause genetic disorders, such as
cystic fibrosis, by affecting protein function.
- Degenerate code → Some amino acids are coded by more than one codon. For
example, tyrosine is coded by TAT and TAC.
- Implication for mutations → Because of this redundancy, a base change (CAA → CAG)
might not change the amino acid so not all mutations affect protein structure.
- Frameshift mutation → Occurs when bases are inserted, deleted, or duplicated,
shifting all downstream codons.
- Impact → Alters many amino acids, often severely disrupting protein function. Single
amino acid changes might not matter, but frameshifts usually do.
- For example, Crohn’s disease is caused by a frameshift in the NOD2 gene, producing
a shortened, nonfunctional protein.
● Causes of Mutations
- Mistakes during DNA replication (copying) can create spontaneous mutations.
- Mutagenic agents are factors like ionising or ultraviolet radiation that increase
mutation rates.
Page 1
, Biology Revision A Level AQA
Stem Cells
- Stem cells are unspecialised cells.
- Specialised cells only express some of the genes in their DNA.
- Stem cells can divide by mitosis to produce more undifferentiated cells or they can
differentiate into specialised cells.
● Totipotent Stem Cells
- Totipotent cells are the most unspecialised cell and exist for a limited time during
embryonic development in mammals (the first few cell divisions).
- Totipotent cells translate only part of their DNA during development causing the
cells to remain unspecialised.
- Totipotent cells are able to produce any type of body cell including cells of
supportive structures (like the placenta).
- These can differentiate into any cell type and go on to form whole organisms.
- When the totipotent cells become specialised, only some genes in the cell are
activated and expressed.
● Pluripotent Stem Cells
- Totipotent cells can develop into
pluripotent cells in embryos.
- Pluripotent cells are able to
divide an unlimited number of
times to produce any type of cell
that makes up the body.
- Pluripotent cells can be used to
treat human disorders.
- These can differentiate into
most cell types, but cannot form
whole organisms.
● Multipotent Stem Cells
- Multipotent cells are found in
mature mammals and can only
differentiate into a few different
cell types.
- For example, multipotent cells
in the bone marrow can produce a range of blood cells.
● Unipotent Stem Cells
- Unipotent cells are also found in mature mammals and can only differentiate into
one type of cell.
- For example, cardiomyocytes are unipotent cells that can produce new muscle cells.
● Proteins in Differentiation
- When a gene is expressed, it is transcribed into mRNA to be translated into a protein.
- Differentiation occurs as certain proteins are made and the presence of certain
proteins means the cell has become specialised.
Page 2
