Mutation

Mutation is an unpredictable, random and spontaneously changes in genetic material. Different alleles of a gene contain slightly different sequence of base. It is caused by several environmental factors:

1. Ionizing radiation (α, β and γ radiation)
2. Ultraviolet radiation
3. Chemical (mustard gas, DDT, CCl4 etc)

Mutation is classified into

1. Gene mutation
2. Chromosome mutation

Gene Mutation

Sickle cell anemia

• Base substitution
• Inherited blood disorder sickle cell anemia.
• Gene codes for the β chain has amino acid sequence produced as

Val-His-Leu-Thr-Pro-Glu-Glu-Lys compared with

Val-His-Leu-Thr-Pro-Val-Glu-Lys in sickle cell anemic.

• Base CTT is replaced by CAT.
• This unusual β chain cause hemoglobin become less soluble and stick to each other forming long fibre
• Red blood cells’ shaped are distorted and become half moon shaped or sickle shape.
• It affects the oxygen transporting capability of red blood cells. Thus, red blood cells stuck in small capillary and prevent normal red blood cells from passing through.
• Patient may suffer from severe anemia and death.

Phenylketonuria

• Base substitution
• Affects gene codes for enzyme phenylalanine hydroxylase.
• Functions of phenylalanine hydroxylase: catalyse the conversion of amino acid phenylalanine to tyrosine and then melanin.
• Melanin is brown pigment in skin and hair.
• If no phenylalanine hydroxylase, no tyrosine and no melanin. — > lighter skin and hair colour.
• Accumulation of phenylalanine in blood and tissues causes brain damage in young children.
• Mentally retarded if untreated.

Chromosome mutation

• Two types of chromosome mutation:

1. Changes of chromosome structure
2. Changes of chromosome number

Down’s syndrome

• Example of autosomal chromosome disorder.
• Mutation in the changes of chromosome number is caused by non-disjunction in meiosis where the chromosomes are not separate normally during anaphase I or anaphase II. It causes the abnormal number of chromosomes formed in the gametes.