Q.Write a brief note on chromosomal mutations and gene mutations.

Types of Mutations

DNA is copied billions of times, and though the machinery is remarkably accurate, mistakes and changes do happen. A permanent, heritable change in the genetic material is called a mutation — and mutations are important twice over: they are the ultimate source of the variation that drives evolution, yet they can also cause disease. To make sense of them, biologists sort mutations by how much of the genetic material is affected.

The two broad categories

Mutations are classified into gene (point) mutations and chromosomal mutations, depending on the scale of the change.

1. Gene (point) mutations affect a single gene — a change at the level of the DNA base sequence, often just one or a few nucleotides. These include:

• Substitution — one base is replaced by another.
• Insertion — an extra base (or bases) is added.
• Deletion — a base (or bases) is lost.

Even a tiny change like this can alter the protein a gene codes for. A classic example is sickle-cell anaemia, caused by a single base substitution in the gene for haemoglobin.

2. Chromosomal mutations affect whole chromosomes and are of two kinds:

• Structural changes — a piece of a chromosome is altered, for example by deletion (loss of a segment), duplication (a segment repeated), inversion (a segment reversed) or translocation (a segment moved to another chromosome).
• Numerical changes — the number of chromosomes changes. This includes aneuploidy (gain or loss of one chromosome, as in the extra chromosome 21 that causes Down's syndrome) and polyploidy (whole extra sets of chromosomes, common and important in plants).