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
Genetic Information Variation and Relationship
Between Organisms
DNA
● Prokaryotes
- Short.
- Circular.
- Chromosomes
coil around
themselves via
supercoiling
which condense
the circular
chromosome to
fit inside the cell.
- Located in the
cytoplasm (no
nucleus).
- Not associated
with histone
proteins.
● Eukaryotes
- Long.
- Linear.
- DNA is tightly wrapped around histones, coils and condenses into chromosomes.
- Located in the nucleus.
- Diploid eukaryotes have homologous pairs of chromosomes meaning the
chromosomes from the mother and father have the same genes at the same loci but
alleles may be different there.
● Mitochondria and Chloroplasts
- Have their own DNA (in the form of prokaryotes).
- DNA in mitochondria encodes enzymes that are used in respiration.
- DNA in chloroplasts encodes enzymes that are used in photosynthesis.
- The endosymbiotic theory states that bacterial cells were engulfed by a larger cell
during evolution (mitochondria and chloroplasts are bacterial cells).
- In eukaryotic cells, DNA and RNA synthesis occur in a separate compartment from
protein synthesis. In prokaryotic cells, both processes occur together.
Page 1
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
Genetic Information Variation and Relationship
Between Organisms
DNA
● Prokaryotes
- Short.
- Circular.
- Chromosomes
coil around
themselves via
supercoiling
which condense
the circular
chromosome to
fit inside the cell.
- Located in the
cytoplasm (no
nucleus).
- Not associated
with histone
proteins.
● Eukaryotes
- Long.
- Linear.
- DNA is tightly wrapped around histones, coils and condenses into chromosomes.
- Located in the nucleus.
- Diploid eukaryotes have homologous pairs of chromosomes meaning the
chromosomes from the mother and father have the same genes at the same loci but
alleles may be different there.
● Mitochondria and Chloroplasts
- Have their own DNA (in the form of prokaryotes).
- DNA in mitochondria encodes enzymes that are used in respiration.
- DNA in chloroplasts encodes enzymes that are used in photosynthesis.
- The endosymbiotic theory states that bacterial cells were engulfed by a larger cell
during evolution (mitochondria and chloroplasts are bacterial cells).
- In eukaryotic cells, DNA and RNA synthesis occur in a separate compartment from
protein synthesis. In prokaryotic cells, both processes occur together.
Page 1
