1. AQA Unit 5 Biology

2. DNA
RNA
tRNA
Shape
Double stranded, twisted
into a double helix and
held together by hydrogen
bonds.
Single-stranded.
Single-stranded, folded into a
clover shape and held
together by hydrogen bonds.
Sugar
Deoxyribose sugar.
Ribose sugar.
Ribose sugar.
Bases
Adenine, Guanine,
Thymine, Cytosine.
Adenine, Guanine, Uracil,
Cytosine.
Adenine, Guanine, Uracil,
Cytosine.
Other features
• 3 adjacent bases known
as a codon.
• Phosphate-sugar
backbone for protection.
• Hydrogen bonds which
can be broken for
replication.
• Double-helix allows for
protection.
• DNA is semiconservative.
• DNA is the “code” for
particular proteins to be
present.
• Three adjacent bases
are a codon.
• DNA is copied into RNA
(transcription) and then
joins with a ribosome in
the cytoplasm for protein
synthesis (translation).
• Each tRNA molecule has a
specific sequence of 3 bases
called an anticodon and an
amino acid binding site.
• Hydrogen bonds between
base pairs hold the molecule
in shape.
• tRNA is found in the
cytoplasm where it is involved
in translation by carrying
amino acids used to make
proteins to the ribosomes.

3. RNA polymerase
attaches to DNA at
the beginning of a
gene.
DNA
(double
stranded)
The RNA polymerase
lines up free RNA
nucleotides (U
replaces T)
The RNA is a
complimentary copy
of the DNA strand.
The bases are joined
at the
backbone, forming
an mRNA molecule.
mRNA moves out of
the nucleus
through a pore and
attaches to a
ribosome in the
cytoplasm.
The hydrogen bonds
between the two DNA
strands in the gene
break, DNA uncoils.
The RNA polymerase
moves
along, separating the
DNA and forming the
RNA.
One strand is used to
make a mRNA copy.
The hydrogen bonds
between the DNA
strands reform back
to a double helix.
Splicing occurs to
remove non-coding
“introns”.
The RNA polymerase
reaches a “stop”
signal.
mRNA stops being
produced and RNA
polymerase detaches
from DNA.

4. mRNA attaches
itself to a ribosome
and tRNA carry
amino acids to the
ribosome.
tRNA with a complimentary
anticodon attaches itself to the first
codon on the mRNA by specific base
pairing.
A third tRNA molecule binds to the
mRNA, its amino acid binds to the
second amino acid, and the second
tRNA molecule moves away.
A second tRNA molecule attaches
itself to the second codon on the
mRNA in the same way.
The amino acids which are attached to
these tRNA moleculesare joined by a
peptide bond. The first tRNA molecule
moves, leaving its amino acid behind.
The process continues in the same
way, producing a chain of linked
amino acids (polypeptide chain)
The molecules reach a stop
codon, and translation stops.
The polypeptide chain moves
away from the ribosome as
translation is complete.

5. The code is non-overlapping, each base triplet is read in sequence and is separate to the one
before it.
The code is degenerate, there is more than one possible combination for most amino acids.
The code is universal, the same triplets code for the same amino acids in all living things.
Amino Acid
DNA codon mRNA codon tRNA anticodon
Serine
AGA
UCU
AGA
Leucine
GAT
CUA
GAU
Tyrosine
ATA
UAU
AUA
Valine
CAC
GUG
CAC
Alanine
CGT
GCA
CGU
Hint: The tRNA anticodon always
matches the DNA triplet but with U
instead of T, mRNA is always
complimentary to the DNA code.

6. 
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Transcription factors are molecules that control the expression of genes.
They are specific.
In the nucleus they bind to specific DNA sites near the start of their target
genes.
They control expression of genes by controlling the rate of transcription.
Some transcription factors, called Activators, work by increasing the rate of
transcription. They help RNA polymerase join to the DNA strand.
Other transcription factors, called Repressors, work by decreasing the rate of
transcription. They bind to the start of the target gene, preventing RNA
polymerase from binding. This stops transcription.
Oestrogen can bind to a transcription factor called an oestrogen receptor,
forming an oestrogen-oestrogen receptor complex. The complex can act
either as an activator or a repressor, this depends on the type of cell and
target gene.
Small interfering RNA (siRNA) cuts up target mRNA into sections so that it
can no longer be translated. This is called translation interference.
Hint: “expression” just
means transcription &
translation!

7. Type of Mutation
Effect
Deletion
• E.g. ATGCCT becomes ATCCT – there
is a frame shift and every amino acid
after the point of the deletion changes
as every triplet changes. This could
result in a different protein being
formed, or a non-functioning enzyme.
Substitution
• E.g. ATGCCT becomes ATACCT – the
one particular amino acid that this
base belongs to can change
(sometimes the substitution can still
lead to the same amino acid and the
same protein). The protein formed in
the end could be altered.
DNA mutations can be caused by errors in
DNA replication OR mutagenic agents such
as UV radiation, ionising radiation and
some chemicals and viruses.

8. Tumour Suppressor
Genes
Proto-oncogenes.
Tumor suppressor genes can be
inactivated if a mutation occurs
in the DNA sequence.
The effect of a proto-oncogene
can be increased if a mutation
occurs in the DNA sequence. A
mutated proto-oncogene is
called an oncogene.
When functioning normally,
tumour suppressor genes slow
cell division by producing
proteins that stop cells dividing
or cause them to self-destruct.
When functioning normally,
proto-oncogenes stimulate cell
division by producing proteins
that make cells divide.
If a mutation occurs in the
tumour suppressant gene, the
protein isn’t produced and the
cells divide uncontrollably,
resulting in a tumour.
If a mutation occurs, the gene
can become overactive. This
stimulates cells to divide
uncontrollably resulting in a
tumour.

9. Diagnosis
Treatment
Prevention
Cancer – Acquired
Mutations (i.e. Those due
to lifestyle choices)
• Normally diagnosed after
symptoms have appeared.
High-risk individuals can
be screened for particular
mutations.
• Treatment is different for
different mutations e.g.
Some cancers are treated
with drugs, others with
surgery to remove chunks
of the tumour, and
radiotherapy.
• Protective clothing for
those who work with
mutagenic agents.
• Sunscreen for when the
skin is exposed to UV
radiation.
• Vaccination against
some viruses with links to
certain cancers.
Cancer – Hereditary
Mutations
• Routine screening for
certain mutations, more
screenings if the patient is
a high risk.
• Depends on the
mutation.
• Removing the organ that
the mutation affects
before the cancer
develops.
Genetic Disorders
(Hereditary Mutations)
• DNA analysis to look for
mutations. In parents,
DNA analysis can be done
to determine if they are a
carrier of a mutant gene.
• Gene therapy – it is
possible to treat
symptoms of cystic
fibrosis by inserting a
normal copy of the
mutated gene.
• Parents can undergo
pre-implantation genetic
diagnosis before IVF
treatment to prevent any
offspring having the
disease. Embryos are
screened.

10. 
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Multi-cellular organisms (such as humans) are made up of many
different cell types that are specialised for function.
These specialised cells originated from stem cells.
Stem cells are found in the embryo where they divide to become new
cells and then become specialised. They are also found in some
adult tissues where they can become cells that need to be replaced
e.g. Red blood cells.
Stem cells that can divide into any type of cell are called totipotent
cells.
Totipotent stem cells are only present in the embryos of humans, in
adult life any stem cells can only differentiate into a few kinds of
specialised cells.
All stem cells in plants are totipotent.

11. Stem Cell Therapies
Treating Diseases
(possibilities so far!)
Ethical Issues
• Stem cells can divide into any cell
type, so could be used to replace
cells damaged by illness or injury.
• Spinal cord injuries – to replace
damaged nerve tissue.
• Involves destruction of an embryo
which could become a fetus if placed
in a womb.
• Some stem cell therapies exist for
some diseases affecting the blood
and immune system.
• Heart disease and damage caused
by heart attacks – to replace
damaged heart tissue.
• Some believe that from the
moment of fertilisation, an
individual has the right to life.
• Bone marrow contains some stem
cells that can differentiate into any
type of blood cell.
• Bladder conditions – used to grow
whole bladders that are implanted to
replace faulty ones.
• Some have fewer objections to
stem cells obtained from unfertilised
embryos.
• Bone marrow transplants can be
used to replace faulty bone marrow
in some patients that produce
abnormal blood cells.
• Respiratory diseases – donated
windpipes can be stripped down to
their simple collagen structure, then
covered with stem cells to generate
tissue.
• Some believe stem cells should
only be obtained from adult cells –
however these stem cells are not
totipotent.
This technique has been used to
treat leukaemia, and sickle cell
anaemia.
• Organ transplants – organs could
be grown to provide new organs for
people on waiting lists.
• The decision makes in society must
take into account everyone’s views
when making descisions about
scientific research.