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In this article we will discuss about the gene action in bacteria.
Tryptophan Biosynthesis in Salmonella:
In Salmonella, tryptophan biosynthesis was studied by Brenner (1955). Tryptophan locus is mutable at four different sites. In other words, tryptophan synthesis consists of four biochemical steps. Reaction of each step is controlled by specific enzyme. Inactivation of an enzyme blocks a specific reaction.
Tryptophan Synthesis in E. coli:
In E. coli, tryptophan locus is mutable at four different sites. Thus four different enzymes are responsible for tryptophan biosynthesis. Reaction of each step is controlled by specific enzyme. Inactivation of an enzyme locks a specific reaction.
Sickle Cell Anaemia:
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(i) Symptoms:
Sickle cell anaemia is a human disease which is characterized by sickle shape of red blood cells and anaemic condition (blood deficiency). In normal persons, the red blood cells are of round or disc shape. The red blood cells are changed into sickle shape or crescent shape in the persons suffering from this disease, due to low oxygen supply. The process of change from normal cell to sickle cell is known as sickling.
(ii) Genetic Control:
This disease is caused by single pair of recessive gene. The gene expresses in two ways. In heterozygous condition it leads to sickle cell trait which is also known as moderate sickling. In homozygous condition, it leads to sickle cell anaemia which significantly reduces the life span of the carrier.
(iii) Effects:
Sickle cells increase the viscosity of blood. Presence of sickle cells in smaller vessels causes blockage of capillaries and tissue damage. Sickle cells are easily broken in blood vessels or spleen when exposed to mechanical shock or stress.
Detection of Sickle Cell Disease:
(1) By simple blood test,
(2) Electrophoretic test, and
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(3) Amino acid sequence.
These are briefly discussed as follows:
1. Simple Test:
The sickle shape of red blood cells can be easily detected by routine blood test.
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2. Electrophoretic Test:
Sickle cell disease can be detected by electrophoretic mobility of haemoglobin. In electrophoretic test, the haemoglobin of normal persons migrates towards one pole, while that of sickle cell anaemic person migrates in opposite direction to that of normal persons.
Thus normal persons exhibit one type of haemoglobin, anaemic persons another type of haemoglobin and heterozygous individuals having sickle cell trait exhibit a mixture of both types of haemoglobin in electrophoretic test.
Thus three types of persons differ in their haemoglobin structure as follows:
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3. Amino Acid Sequence Test:
Studies on the sequence of amino acids in the polypeptide chains of haemoglobin revealed that alpha chain of sickle cell haemoglobin has the same structure as in normal haemoglobin, but the beta chain differs from the normal haemoglobin in the replacement of one amino acid by other [i.e. valine in Hbs haemoglobin and glutamic acid in the normal haemoglobin] at one particular point in the chain.
This change of one amino acid in polypeptide chain leads to sickle cell anaemia in man.
