Mutation and Polyploidy

A gene is a segment of DNA, which is a sequence of nucleotides. The term mutation was coined by Hugo De Vries, and is defined as a sudden change in DNA or chromosome which is heritable.

The factors which cause mutation are called mutagens. These may be physical or chemical mutagens.

• A physical mutagen is any physical factor, like prolonged exposure to sunlight, UV rays, IR rays, etc. The high energy of these changes the sequence of nucleotides in the DNA, or the size or number of the chromosome.
• A chemical mutagen is any chemical factor that changes the sequence of nucleotides in DNA or chromosomes.

Types of Mutation (Based on the Cell Involved)

Based on the cell involved, mutation is of two types: somatic mutation and germinal mutation.

• Somatic mutation: when mutation occurs in somatic cells, it is called somatic mutation, which cannot be transmitted (to offspring). When a proto-oncogene is changed and becomes an oncogene, it causes cancer.
• Germinal mutation: when mutation occurs in reproductive cells, like sperm and ova, it is called germinal mutation, which is heritable.

Types of Mutation (Based on Origin)

Based on origin, mutation is of two types: gene mutation and chromosomal mutation.

1) Gene Mutation

A segment of DNA that determines characters is called a gene, which occupies a specific position in the chromosome; this position is called the locus. The segment of DNA is a sequence of deoxyribonucleotides.

When a deoxyribonucleotide is added or lost, it changes the whole sequence of deoxyribonucleotides downstream. Sometimes a deoxyribonucleotide is replaced with another deoxyribonucleotide. Based on this, gene mutation has two sub-types:

a) Frame Shift Mutation When a deoxyribonucleotide is added to, or lost from, the segment of DNA, it is called frame shift mutation. When the addition or loss occurs near the beginning of the sequence, it brings about a greater change (since the whole downstream reading frame shifts).

b) Substitution Mutation When a deoxyribonucleotide is replaced with a deoxyribonucleotide of the same kind or of a different kind, it is called substitution mutation. It is of two sub-types:

1. Transition: the substitution mutation in which a purine is replaced with a purine, and a pyrimidine is replaced with a pyrimidine, is called transition.
2. Transversion: sometimes a purine is replaced with a pyrimidine, and vice versa; this kind of substitution is called transversion.