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Genetic Control
Chapter 29
Control of gene action
• Genes code for the synthesis of proteins
• Some genes can be ‘switched off’ or ‘on’
• E.g. B-galactosidase
lactose
glucose + galactose
• In E.coli, enzyme only produced if lactose is
present
• Gene for B-galactosidase is switched on if lactose is
present
• Switched off in absence of lactose
• This is enzyme induction
• Glucose can then be used to aid respiration
Jacob-Monod hypothesis
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Operon
area of gene activity on a DNA strand
contains structural genes
- these code for the protein (e.g. enzyme)
& an operator
- controls the structural gene
Operator is affected by a repressor molecule
If the repressor is joined to the operator, the operon is switched off
The repressor is coded for by a regulator gene
Repressor will not bind to the operator gene if it is already bound to
an inducer
• This control prevents waste of cell resources
• E.g. ATP & amino acids
Role of genes in cell metabolism
• Metabolism – all the chemical processes that occur in an
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organism
A metabolic pathway (e.g. respiration) – several stage
process
Each stage controlled by an enzyme
One metabolite converted to another
However, if mutation occurs, enzymes not produced
Pathway not complete, intermediate metabolites can
accumulate
This is an inborn error of metabolism
Sufferers are usually homozygous recessive
Inborn errors of metabolism
• Phenylalanine & tyrosine – amino acids obtained from protein in the
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diet
An enzyme converts phenylalanine to tyrosine
Phenylketonuria (PKU)
Affected person lacks allele of gene to make enzyme 1
Phenylalanine cant be converted to tyrosine
Undergoes alternative metabolic pathway – produces toxins (affect
brain cells)
Occurs in 1 in 10,000
All babies now screened – blood tested for excess phenylalanine
If disorder is present, can be managed by diet control
Possible genetic cure in the future
Albinism
Lack of enzyme 3
No melanin produced
Pink skin, pale iris, white hair
Genetic control of differentiation
• Every cell has all the genes for constructing the whole
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organism
After differentiation, groups of cells specialise
Only certain types of genes continue to operate
Reversibility
Amphibian:
-Fully differentiated nucleus transplanted into a fertilised egg
(nucleus removed)
-Egg can grow into a fertile adult
Dolly the sheep:
- Nucleus from udder cell transferred into an egg (no nucleus)
- Egg transplanted into surrogate mother
- ‘Dolly’ identical to the ewe where the original udder cell
came from
Illustrate that cell genes can be switched on again
Two categories of genes
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First group
– switched on in all cells
- code for vital metabolites
Second group
- only switched on in a particular type of cell
- code for proteins characteristic of that cell type
E.g. insulin from pancreas cells
- many proteins may cause specific modifications to
occur
• All other genes permanently switched off
Genetic control of cell
differentiation
• Blood
• - red and white cells come from an original
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undifferentiated ‘stem’ cell
Parenchyma
- large, thin walled, vacuolated cells
- contain plastids (formed from proplastids)
- many types – features depend on which genes
are switched on
- relatively unspecialised (potentially
meristematic)
Genetic control of leaf shape
• Leaf starts as a primordium
• Produced by an apical meristem
• Leaf blade develops from meristematic
cells at edge of primoridum
• Equal activity of these cells produces
regular shaped leaves e.g. privet
• Unequal activity – irregular shape e.g oak