Cellular Control
You should be able to:
(a) state that genes code for polypeptides, including enzymes;
(b) explain the meaning of the term genetic code;
(c) describe, with the aid of diagrams, the way in which a nucleotide sequence codes
for the amino acid sequence in a polypeptide;
(d) describe, with the aid of diagrams, how the sequence of nucleotides within a gene
is used to construct a polypeptide, including the roles of messenger RNA, transfer
RNA and ribosomes;
(e) state that mutations cause changes to the sequence of nucleotides in DNA
molecules;
(f) explain how mutations can have beneficial, neutral or harmful effects on the way a
protein functions;
(g) state that cyclic AMP activates proteins by altering their three-dimensional
structure;
(h) explain genetic control of protein production in a prokaryote using the lac operon;
(i) explain that the genes that control development of body plans are similar in plants,
animals and fungi, with reference to homeobox sequences (HSW1);
(j) outline how apoptosis (programmed cell death) can act as a mechanism to change
body plans.
Key Definitions
A gene is a length of DNA that codes for one or more polypeptides.
A genome is the entire DNA sequence of that organism. The human genome consists of
approximately 3 million nucleotide base pairs.
A polypeptide is a polymer consisting of amino acids joined by peptide bonds.
A protein is a large polypeptide, usually 100+ amino acids. Some proteins have more than 1
polypeptide chain.
Transcription: the first stage in protein synthesis that occurs in the nucleus. It is the
creation of a single stranded mRNA copy of the DNA coding strand.
Translation: the second stage of protein synthesis which involves the assembly of
polypeptides at ribosomes in the cytoplasm. Amino acids are placed in the correct order
according to the sequence of codons on the mRNA.
Mutations are structural changes to the genetic material within a cell – either to a gene or to
a chromosome.
Lac Operon: a length of DNA containing a series of genes coding for enzymes and proteins
that allow bacteria to use lactose, plus genes regulating their transcription and translation.
Regulator Gene: DNA sequence that codes for the Lac Repressor protein
Promoter: section of DNA to which the enzyme RNA polymerase binds to begin transcription
of structural genes.
Operator: section of DNA to which the Lac Repressor protein can bind when lactose is absent
B-Galactosidase: codes for an enzyme that breaks down Lactose
Lactose Permease: codes for a carrier protein that increases the uptake of lactose into the
cell through the plasma membrane.
Homeobox genes: control the development of the body plan of an organism, including the
polarity and positioning of the organs.
Apoptosis: programmed cell death in multicellular organisms.
Genes code for
polypeptides. This
includes enzymes
like rubisco,
hormones like
adrenaline and
insulin and structural
Transcription
proteins like
collagen and keratin.
It also includes
antibodies,
haemoglobin, channel
proteins, electron
carriers and the
tubulin proteins of
the cytoskeleton.
Translation
The sequence of nucleotide
bases on a gene provide a
code, with instructions for
the construction of a
polypeptide. This genetic
code:
 is a triplet code: a
sequence of 3 nucleotide
bases codes for an amino
acid.
 is a degenerate code:
there is more than 1 triplet
code for a particular amino
acid.
 is widespread: base
sequences can code for the
same amino acid in all
organisms.
In order to make a polypeptide, a copy of the genetic code that can pass through
a pore in the nuclear envelope into the cytoplasm must be made. This happens
through transcription. Transcription creates a single stranded mRNA copy of
the DNA coding strand. This mRNA strand must then attach to a ribosome where
protein synthesis will occur – this is translation.
1. Transcription
 Takes place in the nucleus
 One gene is transcribed from DNA to mRNA
 Catalysed by enzyme RNA Polymerase
1. Portion of DNA containing the gene to be
transcribed dips into the nucleolus and the double
helix unwinds at this point only.
2. mRNA nucleotides are activated by having an
extra 2 phosphate groups added.
Hydrolysed
3. mRNA nucleotides align next to the DNA template
or ‘sense’ strand according to complementary
base pairing. The extra 2 phosphate groups are
hydrolysed to release energy used to join the
nucleotides to form a strand of mRNA.
4. mRNA leaves the nucleus through
a nuclear pore and the DNA double
helix rewinds.
DNA mRNA
Complementary
base pairing
between DNA
and mRNA:
A
U
T
A
C
G
G
C
2. Translation
 Takes place in the cytoplasm
 Assembly of polypeptides at ribosomes
 Amino acids are placed in sequence according to the sequence of codons on the mRNA.
5. mRNA molecule attaches to
a ribosome in the cytoplasm
6. The ribosome can read 2
mRNA codons at a time
7. tRNA molecules with mRNA
codons according to
complementary base pairing.
tRNA anticodons are
complementary to mRNA
codons.
8. As tRNA molecules align,
this brings amino acids into
the correct sequence to form
the protein primary structure.
9. Peptide bonds form
between the amino acids.
tRNA molecules are released
and reactivated.
10. Polypeptide chain is activated by cAMP which causes
the folding and coiling of the polypeptide chain into a
specific shape according to the protein formed.
Transfer RNA
These 3 unpaired bases are
the amino acid binding site.
The tRNA molecule will be
activated when an amino acid
binds here. The amino acid
that will bind is determined
by the anticodon code.
The anticodon is 3
bases which binds to
the complementary
codon on the mRNA
strand.
tRNA is
made in the
cytoplasm
and passes
into the
cytoplasm.
The RNA
folds into
hairpin
shapes.
Why is the amino acid
sequence so important?
The sequence of
amino acids
determines the
primary structure
of the protein.
The primary
structure
determines the
tertiary structure
– how the protein
folds up into its
three dimensional
shape.
The tertiary structure is
what allows the protein to
function. If the tertiary
structure is altered, the
protein can no longer
function. For example, the
active site of an enzyme
may have an altered shape
and the substrate molecules
will no longer fit.
Mutations
Chromosome
mutations
A large section of
DNA is lost from a
chromosome.
Sometimes the lost
section of DNA
reattaches to another
chromosome. Whole
chromosomes can be
duplicated or deleted
– an example of this is
trisomy 21 which
causes Down’s
Syndrome.
Mutations are important because they mean a change
in the nucleotide base sequence. This base sequence
determines the amino acid sequence, which in turn
determines the tertiary structure and so the
function of the protein. If the protein structure is
incorrect, it will be non functioning.
DNA Mutations
This is a change to the
sequence of nucleotides
along the DNA. DNA
mutations can be
point/substitution
which is when one base
is replaced with another
or insertion and
deletion mutations
where a base is added or
removed from the
genetic code.
Types of DNA Mutation
Silent Mutations:
The genetic code is
degenerate – there is
more than one triplet
code for each amino
acid. Some point
mutations will
therefore have no
effect on the protein
produced because
the mutated triplet
code still codes for
the same amino acid.
Missense Mutations:
Changing one base
changes one of the
amino acids in the
primary structure.
Effect on the
protein will depend
on how important the
amino acid is in its
structure and how
similar the amino
acids are to each
other.
Nonsense
Mutations:
Changing a base
has created a stop
codon in the
middle of the
mRNA. This means
that a much
shorter and
probably non
functioning protein
will be produced in
translation.
Frameshift
Mutations:
Addition or deletion
of one base means
that the correct
reading frame is
lost. Many of the
amino acids included
in the polypeptide
are likely to be
different, so the
protein will probably
be non-functioning.
What are the outcomes of mutations?
Some mutations have a neutral effect: they have no apparent
advantage or disadvantage to the organism. If a gene is altered by a
change to its base sequence, it becomes another version of the same
gene: it is an allele of the gene. Examples of this include:
 Attached ear lobes/free ear lobes
 Ability to roll the tongue
Early humans in Africa had dark skin.
Melanin pigment protected them form
the harmful effects of UV light. Any
humans that had a mutation causing
paler skin would have burned and
suffered with skin cancer.
Depending on the environment, the same mutation
for paler skin can be beneficial or harmful. When an
environment changes, individuals within a
population who have a certain characteristic may be
better adapted to the new environment. Well
adapted organisms can out-compete those who do
not have the advantageous characteristic. This is
natural selection – the mechanism for evolution.
Free Ear
Lobe
Attached
Ear Lobe
Early humans with mutations
producing paler skin due to a lack
of pigment would have an
advantage over those with dark
skin as humans moved to more
temperate climates. Sunlight there
was not intense enough to cause
vitamin D production in those with
dark skin, so they would suffer
form rickets.
The Lac Operon
Bacteria normally use glucose as their respiratory substrate because it is quick and easy to
respire in glycolysis. However, if placed in an environment with no glucose and instead with
lactose present, the bacteria will start to respire lactose.
Regulator
Gene
Control Sites
Structural Genes
I P OZ Y A
DNA
sequence
that codes
for the
Lac
Repressor
Protein
P is the promoter
site: this is the
section of DNA to
which RNA
Polymerase enzyme
binds to begin
transcription.
O is the operator
site: this is the
section of DNA to
which the Lac
repressor protein
will bind when
lactose is absent.
Z and Y are
structural genes
that code for the
enzymes BGalactosidase
and Lactose
Permease.
Lac operon: Glucose Present; No Lactose
I
3. RNA Polymerase cannot bind to
the promoter region, so the
structural genes for B-Galactosidase
and Lactose Permease cannot be
transcribed.
P O Z Y A
1. Regulator gene is
transcribed and
translated and the Lac
Repressor Protein is
synthesised.
2. Lac repressor protein
binds to operator region
4. The enzymes
cannot be
synthesised, so
lactose cannot be
respired.
The Lac Operon provides a survival advantage to the bacteria.
When glucose is present, the E. Coli should use it exclusively, as it is
much easier to respire than lactose. No amino acids or ATP is
wasted on the production of unnecessary enzymes. When glucose is
absent, but lactose is available, the Lac Operon enables the
bacterium to continue to respire to release energy.
Lac operon: Glucose absent; Lactose present
1. Regulator gene is
transcribed and
translated and the
Lac Repressor Protein
is synthesised.
I
3. RNA Polymerase can now bind to the
promoter region, so the structural genes for
B-Galactosidase and Lactose Permease can be
transcribed.
P O Z Y A
2. Lactose binds to the Lac Repressor
protein’s allosteric site. Lactose is a non
competitive inhibitor and changes the
shape of the repressor protein so that it
cannot bind to the operator region.
4. Transcription and translation
of structural genes into lac
enzymes, allowing the bacterium
to respire lactose.
Homeobox Genes
All animals possess the same 8 Hox
genes: they lie along the chromosome in
the same order as they are expressed
along the body. Each hox gene is linked
to its own set of genes responsible for
controlling 1 section of the body. One
hox gene can activate and turn on 1000s
of other genes.
How do Hox genes work?
In all of the cells that will become a
head, the ‘head’ hox gene will be
switched on. The ‘head’ hox gene is
transcribed and translated to make a
transcription factor. The transcription
factor activates all of the genes in the
genome needed to make a head. The
‘head’ genes are transcribed and
translated to form head making proteins
and enzymes.
Hox Genes code for proteins called
Transcription Factors. These
proteins are able to interact with
DNA to cause particular genes to be
transcribed into mRNA.
Duplication of hox genes drives
the evolution of complexity. All
animals have the same set of basic Hox
genes, but body plans will become more
complex when hox genes are duplicated.
For example, whereas a sponge will
have just 1 copy of each hox gene, a
mouse will have at least 4. Each
duplicate of the gene is free to take a
slightly different role to add
complexity to the organism’s body plan.
Apoptosis
1. Enzymes break down the cell cytoskeleton.
2. The plasma membrane changes and small swellings called blebs form.
3. Chromatin condenses, DNA fragments and the nuclear envelope breaks up.
4. The cell breaks into vesicles.
5. Vesicles are taken up by phagocytes, cellular debris is digested and disposed of so
it doesn’t damage other cells.
No harmful hydrolytic enzymes are released unlike with necrosis.
Apoptosis and the Hayflick Limit:
Normal body cells divide a limited
number of times. This is around 50
mitotic divisions. After this, a series of
biochemical events leads to orderly cell
death.
Apoptosis is controlled though cell
signals – either an external signal or
from within the cell itself. Nitric oxide
will make the inner mitochondria
membrane more permeable to H+ ions,
so the H+ gradient is lost. In response
to this, proteins are released which
bind to apoptosis inhibitor proteins in
the cytoplasm, allowing apoptosis to
take place.
Apoptosis is vitally important:
• In developing a complex body
structure: apoptosis allows digits
on hands and feet to separate
from each other
• In removing damaged, infected
or unneeded self cells. This is
crucial in reducing the risk of cells
becoming cancerous. Apoptosis
also removed ineffective T
Lymphocytes from the immune
system which reduced the risk of
autoimmune diseases developing.
Too much
apoptosis = cell
loss and
degradation of
organs
Too little
apoptosis =
formation of
tumours