Mutations

1. Meaning and biological basis

DNA stores hereditary information in its nucleotide sequence. Before a cell divides, DNA replication copies this information. Although copying is highly accurate, occasional errors occur. A lasting sequence change can arise when an error escapes proofreading and repair, or when DNA damage is processed incorrectly. In RNA viruses, changes in the RNA genome are also mutations. A lesion such as a damaged base is not automatically a mutation: it may be repaired before a permanent sequence change becomes established.

A gene is a functional stretch of DNA, but mutations also occur outside genes. Changes in regulatory regions can alter when, where or how strongly a gene is expressed. Changes affecting splice sites can disrupt the processing of precursor messenger RNA. Thus, a mutation need not change a protein-coding sequence directly to have an important biological effect.

Mutation and recombination are distinct. Mutation can create a new allele, whereas crossing over and independent assortment mainly produce new combinations of existing alleles. Together with gene flow, genetic drift and natural selection, these processes shape population evolution. Mutations are generally random with respect to an organism’s adaptive needs, although mutation rates vary across organisms and genomic regions.

2. Major types of mutations

A substitution replaces one nucleotide with another. Within a protein-coding region, a synonymous substitution leaves the encoded amino acid unchanged because the genetic code is degenerate. A missense substitution changes an amino acid, while a nonsense substitution creates a premature stop codon. Synonymous changes are often called silent, but some affect splicing, RNA stability or translation and are therefore not always functionally neutral.

Insertions and deletions add or remove nucleotides. Within a coding sequence, an insertion or deletion whose length is not a multiple of three generally causes a frameshift, changing the downstream reading frame. Changes involving multiples of three can add or remove amino acids without shifting that frame. Repeat expansions, such as expansion of a CAG repeat in Huntington disease, constitute another important category.

Larger alterations include deletions, duplications, inversions and translocations of chromosome segments. Numerical chromosome abnormalities are commonly studied alongside mutations: aneuploidy involves gain or loss of individual chromosomes, while polyploidy involves additional complete chromosome sets. Down syndrome usually results from trisomy 21 caused by chromosome segregation failure, rather than a single-gene mutation.

• Loss-of-function variants reduce or abolish a gene product’s activity; gain-of-function variants increase activity or create a new function.
• A mutation’s molecular category does not alone determine its severity: location, gene function and genetic background also matter.

3. Causes, mutagens and DNA repair

Spontaneous mutations arise from replication errors, spontaneous chemical changes in bases and damage associated with normal metabolism, including reactive oxygen species. Induced mutations follow exposure to mutagens. Ultraviolet radiation can produce pyrimidine dimers, commonly involving adjacent thymine bases. Ionising radiation, including X-rays and gamma rays, can damage bases and cause DNA strand breaks. Chemical mutagens include alkylating agents and some compounds present in tobacco smoke.

Cells possess several safeguards. DNA polymerase proofreading corrects many copying errors; mismatch repair removes errors missed during replication. Base-excision repair addresses certain small base lesions, while nucleotide-excision repair removes bulky lesions such as those produced by ultraviolet exposure. Double-strand breaks can be repaired through homologous recombination or end joining. These mechanisms reduce mutation frequency but do not eliminate it.

Mutagenicity and carcinogenicity are related but not identical. A mutagen increases genetic changes, whereas a carcinogen promotes cancer; some carcinogens act through non-mutagenic mechanisms. The Ames test uses specially selected bacterial strains to screen substances for mutagenic potential. A positive result is a warning signal requiring further assessment, not conclusive evidence that a substance causes human cancer.

4. Inheritance, disease and evolution

Germline mutations occur in the lineage producing eggs or sperm and can be inherited. Somatic mutations arise in other cells and may generate a genetically distinct cell population within an individual, producing mosaicism. They generally are not inherited through sexual reproduction. However, the distinction requires care in plants and organisms reproducing vegetatively, where a mutated tissue can contribute to a new individual.

Inherited genetic disorders are not necessarily present in earlier generations. A pathogenic variant may arise de novo in a parental gamete or early embryo. Conversely, congenital means present at birth and does not necessarily mean genetic. Environmental exposures or developmental disturbances can also cause congenital conditions. Modern clinical genetics often uses the neutral term variant because a sequence difference is not automatically disease-causing.

Cancer commonly develops through accumulated somatic changes affecting cell growth, DNA repair and programmed cell death. Activation of oncogenes or loss of tumour-suppressor function can contribute. An inherited predisposition, such as a pathogenic BRCA1 or BRCA2 variant, increases risk but does not guarantee cancer. In evolution, a variant’s effect depends on context: heterozygosity for the sickle-cell allele provides protection against severe falciparum malaria, although inheriting two sickle-cell alleles causes sickle-cell anaemia.

5. Applications and contemporary relevance

In mutation breeding, breeders expose plant material to radiation or chemicals and screen the resulting population for useful traits. Selected plants then undergo evaluation and breeding. Induced mutations are generally not targeted to a chosen sequence, unlike targeted genome-editing approaches such as CRISPR-based methods. Mutation breeding does not necessarily introduce foreign DNA and should not be treated as synonymous with transgenic modification.

Mutation analysis supports carrier screening, diagnosis of inherited disorders, cancer treatment selection and pathogen surveillance. Genome sequencing identifies variants, but clinical interpretation also requires evidence about their biological effects and disease associations. India’s National Sickle Cell Anaemia Elimination Mission, launched in 2023, combines awareness, screening and management, particularly in affected tribal populations. Genetic counselling should support informed decisions without discrimination or stigma.

Bacterial antibiotic resistance can emerge through spontaneous mutations or acquisition of resistance genes by horizontal gene transfer. Antibiotic use selects resistant organisms; it does not simply teach bacteria to mutate in a useful direction. Similarly, viral mutations generate variants, but most do not necessarily increase transmissibility or severity. RNA viruses generally mutate faster than DNA organisms, although coronaviruses possess proofreading machinery. Public-health significance must therefore be established through genomic, laboratory and epidemiological evidence.

Previous year questions

No source-linked PYQ is available here yet. Use the practice questions below.

Practice questions