Chapter 29
The Jacob-Monod Hypothesis of Gene Action in Bacteria
The bacteria E. coli synthesises several hundred enzymes for metabolism and synthesis, some
continuously, others only in the presence of an inducer substance in its environment. One
such enzyme is -galactosidase.
The bacteria will grow rapidly in a culture medium containing glucose (as the energy source)
and inorganic salts. If, however, lactose is substituted for glucose, E. coli will not grow
immediately but after a short time lag it begins to show growth rate as for a glucose medium.
Lactose is a sugar found in milk. Each molecule of lactose is made up of a molecule of
glucose and a molecule of galactose :
1) Give an explanation for this time lag.
Experimental evidence shows that -galactosidase is produced only when it is needed
although the corresponding gene is always present on the bacterial chromosome. The gene
for the enzyme is said to be induced by the presence of a specific substrate in the
environment. In this case lactose is the inducer of its own breakdown.
Operon
An operon is made up of one or more structural genes (containing the DNA code for the
enzymes in question) and a neighbouring operator gene which controls the structural
gene(s). The operator gene is, in turn, affected by a repressor molecule coded for by a
regulator gene situated further along the DNA chain.
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Chapter 29
Diagram 1. Gene Repressed – Lactose Absent : complete
Diagram 2
Enzyme Induction – Lactose Present: Complete
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Chapter 29
Genetic Control of the Enzyme – Phenylalanine hydroxylase
Phenylalanine and tyrosine are 2 amino acids obtained in a human’s diet. During normal
metabolism, excess phenylalanine undergoes the following pathway.
Complete:
Phenylketonuria (PKU) is a hereditary disorder caused by a blockage in this metabolic
pathway.
Describe how this disease arises.
How is this condition detected in new born babies?
How is this condition controlled?
Why is the sufferer not completely albino
Describe with the aid of a diagram how the condition of albinism arises.
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