11. Mutations – characteristics and types. Gene rearrangements and point mutations.

What Are Gene Mutations?

• Gene mutations are changes in the DNA sequence that occur without the cell's intentional control.
• These changes typically involve small-scale alterations in one or a few nucleotides.
• Mutations play a crucial role in evolution as they create new genetic variants, which natural selection can act upon.
• Many mutations appear after the cell tries to repair chemical injuries to the DNA. While the DNA may survive the repair with a changed sequence, it cannot survive the initial damage itself.

Classification of Gene Mutations

A. Based on the Cell Type Affected and Inheritance

• Somatic mutations occur in body cells and are not passed on to offspring unless reproduction is asexual.
• Germ line mutations occur in reproductive cells and can be inherited by the next generation.

B. Based on Impact on an Organism’s Fitness

• Harmful mutations reduce an organism’s chances of survival or reproduction.
• Beneficial mutations improve fitness and offer a survival advantage.
• Neutral mutations do not affect fitness but may still be passed on. They form the basis of the molecular clock due to their steady rate of occurrence.
• Whether a mutation is helpful or harmful can depend on environmental conditions. The same mutation can be neutral in one setting and beneficial or harmful in another.

C. Based on the Structural Change in DNA

• Substitution involves replacing one nucleotide with another.
• Insertion adds an extra nucleotide to the sequence.
• Deletion removes a nucleotide from the sequence.

D. Based on the Cause of Mutation

• Spontaneous mutations occur naturally due to internal factors.
• Induced mutations are caused by external agents called mutagens.