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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