Gene mutation is defined as a sudden discrete change in the genetic material of gene which is heritable. Mutations when they arise suddenly in nature are called spontaneous mutations.

Mutations which arise suddenly in nature are called spontaneous mutations.

Spontaneous mutations are genetic changes that occur naturally without any external factors or influences. They can arise randomly during DNA replication or as a result of errors in cellular processes. These mutations can lead to variations in the genetic material of organisms, and they play a significant role in evolution.

Types of spontaneous mutations:

1. Point mutations: These are the most common type of spontaneous mutation and involve changes in a single nucleotide base pair. Point mutations can be further classified into three categories:

- Substitution: One nucleotide is replaced by another, which can result in a different amino acid being incorporated into a protein during translation.
- Insertion: An extra nucleotide is inserted into the DNA sequence, causing a shift in the reading frame and potentially altering the entire protein sequence.
- Deletion: One nucleotide is removed from the DNA sequence, leading to a shift in the reading frame and potential changes in the resulting protein.

2. Frameshift mutations: These occur when the number of nucleotides inserted or deleted is not a multiple of three, causing a shift in the reading frame. Frameshift mutations can have severe consequences as they disrupt the normal protein sequence and often result in non-functional or truncated proteins.

3. Expanding nucleotide repeats: In certain regions of the genome, there are repetitive sequences of nucleotides. Spontaneous mutations can cause the expansion of these repeat sequences, leading to genetic disorders such as Huntington's disease or fragile X syndrome.

4. Transposon movement: Transposons, or "jumping genes," are DNA sequences that can move from one location to another within the genome. Spontaneous mutations can cause transposons to jump to new positions, potentially disrupting gene function or causing genetic rearrangements.

5. Chromosomal rearrangements: Spontaneous mutations can also involve large-scale changes in the structure of chromosomes, such as inversions, translocations, or duplications. These chromosomal rearrangements can have significant impacts on gene expression and can lead to genetic disorders.

Overall, spontaneous mutations are crucial for generating genetic diversity in populations and driving evolution. While they can have negative consequences, such as causing genetic disorders, they also provide the raw material for natural selection to act upon, allowing organisms to adapt and survive in changing environments.

(a)Spontaneous mutation: a mutation that arises naturally and not as a result of exposure to mutagens.