Biology · Principles of Inheritance and Variation · NEET
A mutation is a change or alteration in the DNA sequence. NCERT says mutation is 'a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism.' So DNA normally passes from parent to child without change, but sometimes a change happens. That change is the mutation. It matters for NEET because mutation (along with recombination) is a source of variation in DNA, and many disorders come from mutated genes.
NCERT describes two levels. (1) Point mutation: a change in a single base pair of DNA. Its classic example is sickle cell anaemia. (2) Chromosomal-level mutation: loss (deletion) or gain (insertion or duplication) of a whole segment of DNA. This changes the chromosome and gives chromosomal aberrations, which are commonly seen in cancer cells. For NEET, remember: point mutation = one base pair; chromosomal aberration = a whole segment.
Yes. Sickle cell anaemia is the classical NCERT example of a point mutation. A single base pair changes in the beta globin gene: the sixth codon changes from GAG to GUG. This replaces the amino acid glutamic acid (Glu) with valine (Val) at the sixth position of the beta globin chain. Because only one base pair changes, it is a point mutation (a single base substitution), not a chromosomal aberration.
Deletion means a base pair or a DNA segment is LOST. Insertion means a base pair or segment is GAINED (added). At the chromosome level, deletions and insertions/duplications change the chromosome itself. Inside a gene, insertion or deletion of one or two bases causes a frameshift mutation, which shifts the reading frame and changes every codon after that point. This is why insertions and deletions can be very damaging.
A frameshift mutation happens when one or two bases are inserted into or deleted from a gene. This shifts the reading frame from the point of insertion or deletion, so every codon after it is read wrongly and codes for different amino acids. Important NEET point: if the inserted or deleted bases are three (or a multiple of three), one or more whole codons are added or removed, but the reading frame stays the same after that point, so it is NOT a frameshift.
Not always. A mutation always changes the DNA sequence (the genotype), but it does not always change the phenotype. Some changes fall in non-coding regions or do not change the final protein, so the visible trait stays the same. But when the mutation changes an important protein (like beta globin in sickle cell anaemia), the phenotype clearly changes. For NEET, the safe statement is: mutation alters genotype, and it can alter phenotype.
Mutations can happen on their own (spontaneous) during DNA copying, or they can be caused by mutagens. Physical mutagens like UV rays and other radiation, and chemical mutagens, can induce mutations. NCERT links UV radiation and other agents to mutation. For NEET, just remember that agents which cause mutation are called mutagens.
Which one of the following disorders is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule?
Select the correct statements about sickle cell anaemia. A. There is a change in gene for beta globin. B. In the beta globin, there is valine in the place of Lysine. C. It is an example of point mutation. D. In the normal gene U is replaced by A.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Mutation is any change or alteration in the DNA sequence, which changes the genotype and can change the phenotype of an organism.
Two levels: point mutation (change in a single base pair, e.g. sickle cell anaemia) and chromosomal mutation/aberration (loss or gain of a DNA segment by deletion, insertion or duplication).
A point mutation. The sixth codon of the beta globin gene changes from GAG to GUG, replacing glutamic acid with valine.
A point mutation changes a single base pair (often a substitution). A frameshift mutation is caused by inserting or deleting one or two bases, which shifts the reading frame and changes all codons after that point. See the next concept for a full comparison.
No. Some mutations are harmful, some are neutral (no change in phenotype), and a few can be helpful. But many genetic disorders in humans are linked to inherited changed or altered genes.
