Mutations – Class 12 | Chapter – 5 | Biology Short Notes Series PDF
Mutations: A mutation is a change in the DNA sequence of an organism. Mutation can result from errors in DNA replication during cell division, exposure to mutagens or a viral infection. Germline mutations (that occur in eggs and sperm) can be passed on to offspring, while somatic mutations (that occur in body cells) are not passed on.
The central dogma of molecular genetics involves the conversion of DNA into RNA (through transcription) and protein synthesis (through translation). Mutation make the protein synthesis go wrong during translation or mistakes in DNA are present that result in abnormalities in bodies in the form of some diseases like sickle cell anemia.
Types of Mutations
Point Mutation
| Substitution | During replication, one base is inserted incorrectly, replacing the pair at the appropriate location on the complementary strand. | Sickle-cell anemia |
| Insertion | In replicating DNA, one or more additional nucleotides are added, frequently causing a frameshift. | One form of beta-thalassemia |
| Deletion | During replication, one or more nucleotides may be “skipped” or removed, which usually causes a frameshift. | Cystic fibrosis |
Chromosomal Mutation
| Inversion | The flipping and reinserting of a single chromosomal region. | Opitz-Kaveggia syndrome |
| Deletion | When a chromosome segment is lost, all the genes in that segment are also gone. | Cri du chat syndrome |
| Duplication | A chromosomal segment is repeated, increasing the concentration of the genes in that area. | Some cancers |
| Translocation | A section of one chromosome is inappropriately joined to another chromosome. | One form of leukemia |
Copy Number Variation
| Gene amplification | An increase is made in the tandem copies of a locus. | Some breast cancers |
| Expanding trinucleotide repeat | There are more repeating trinucleotide sequences than usual. | Fragile X syndrome, Huntington’s disease |
Causes of Mutations
- Internal Causes: Mutations occur when the DNA fails to copy accurately. All these mutations lead to evolution. During cell division, the DNA makes a copy of its own.
- External Causes: When the DNA is exposed to certain chemicals or radiations, it causes the DNA to break down. The ultraviolet radiations cause the thymine dimers to break resulting in a mutated DNA.
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