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Control & Regulation
Genetic Control of Growth
Differences between Cells
There are a variety of different cells
within an organism.
Each type of cell looks different and
functions differently from the others.
A cell’s shape and size normally depends
on its function.
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G R Davidson
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Differences between Cells
All organisms begin their existence as a
single cell, in our case, it was a zygote
with 23 homologous pairs of
chromosomes, 23 from our mother and
23 from our father.
These 46 chromosomes contain all the
information to develop into what we are.
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G R Davidson
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Differences between Cells
These cells divided again and again by
mitosis, therefore each new cell
contained the same 46 chromosomes.
However, we are now made up of many
different types of cell and so something
must have caused this variety within us.
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Role of Genes
The specialisation of cells is brought
about by the fact that certain genes
switch on and other switch off.
In other types of cell, it is different
genes which are switched on, e.g. in
plants, the genes which produce
chlorophyll must be switched on in leaf
cells but switched off in root cells.
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Role of Genes
The structure and function of a cell
therefore depends on which genes are
switched on and off.
It is possible in some cases to switch on
genes which have been switched off.
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Role of Genes
 Expressed genes – (switched on).
 These are found in all cells and code for
the vital metabolites which are
necessary for life-giving biochemical
reactions (e.g. enzymes)
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Role of Genes
 The other type of genes are those
expressed in a cell where they code
for proteins, characteristics of this
cell and no other.
 e.g. adrenaline is made in the adrenal
glands.
 The remaining genes in these cells
remained permanently switched off or
repressed.
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How genes are controlled
 Switching genes on and off prevents a waste
of valuable resources.
 Humans contain a huge number of genes
(around 30,000) which are affected by many
internal and external factors which interact
with each other in a complex way which is
not yet fully understood.
 Cancer cells are cells which are permanently
switched on to carry out growth and cell
division, but as yet, we do not fully
understand why.
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G R Davidson
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How genes are controlled
Simpler organisms (e.g. bacteria) have a
much shorter genetic code and operate
in a less complex way than humans.
By studying how bacteria control gene
expression, we can understand some of
the principals involved.
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G R Davidson
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Enzyme Induction
A bacteria E-coli produces an enzyme
called β-galactosidase when a sugar
called lactose is present.
The enzyme digests lactose to produce
glucose and galactose.
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G R Davidson
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Enzyme Induction
However, when lactose is absent, E-coli
fails to produce the enzyme.
This would seem to indicate that it is
the lactose which switched on the gene
for the β -galactosidase.
This switching on process is called
enzyme induction.
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G R Davidson
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Enzyme Induction
To explain this process, the Operon
Hypothesis has been put forward.
An operon is one or more structural
genes and a neighbouring operator gene
which can activate the structural gene.
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Enzyme Induction
The structural gene is the DNA code
for the specific enzyme.
The operator gene switches on and off
depending on a repressor molecule which
is coded for by a regulator gene found
further along the DNA chain.
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Effect of repressor in absence of lactose
Regulator
gene
Operator
gene
OPERON
DNA chain
Repressor combines
with operator
Structural gene
‘switched off’
No β-galactosidase
produced
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Effect of repressor in presence of lactose
Regulator
gene
Operator
gene
Structural gene (which codes
for the protein)
OPERON
DNA chain
Transcription and
translation to form
repressor protein molecule
Operator Successful transcription
free
to form β-galactosidase
Some lactose
combines with
repressor
+
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G R Davidson
Enzyme digests
lactose until
supply runs
out
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Gene Products
 We have seen how genes can be controlled to
produce enzymes.
 If a gene produces an enzyme, it can have
wide ranging effects on cells and on the
organism.
 A mutation in a gene responsible for the
production of an enzyme may result in nonproduction of the enzyme so the reaction it
controls cannot take place.
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G R Davidson
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Gene Products
 A mutation in humans which results in
the non-production of the enzyme
phenylalanine hydroxylase causes a
condition known as phenylketonuria
(PKU).
 This condition results in severe mental
retardation and all new born babies are
tested to ensure they don’t suffer from
this condition.
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