Q.In sickle cell anaemia glutamic acid is replaced by valine. Which one of the following triplets codes for valine?

Concept and Intuition

A mutation is any change in the nucleotide sequence of DNA. This can range from a single base-pair substitution (point mutation) to insertions, deletions, or larger chromosomal changes. Because genes are made of DNA, any "alteration of genes" is inherently a "variation in DNA." When such a change occurs in germ-line cells, it becomes heritable and shows up as an altered genotype, which in turn can (though not always) alter the phenotype.

Step-by-Step Solution

1. Statement I — "Alteration of genes": true, this is the basic definition of mutation.
2. Statement II — "Variation in DNA": true, since genes are DNA sequences, altering a gene means altering the underlying DNA sequence.
3. Statement III — "Heritable change in genotype and phenotype": true when the mutation occurs in germ cells and is passed to offspring, changing both the genetic makeup (genotype) and often the observable traits (phenotype). …

Concept and Intuition

Mutations are classified by their scale and mechanism:

• A point mutation involves a change at a single base pair (substitution).
• Frame shift mutations arise from insertion or deletion of one or a few base pairs, which shifts the triplet reading frame for every codon downstream of the change, usually scrambling the resulting protein.
• Chromosomal aberrations are large-scale changes in chromosome number or structure (deletions, duplications, translocations, inversions, aneuploidy) — these are a hallmark of cancer cells.
• A mutagen is any physical, chemical or biological agent that induces mutations; ultraviolet radiation is a classic physical mutagen (it causes pyrimidine dimers in DNA).

Step-by-Step Solution

1. "Change in a single base" → point mutation → III. …

Concept and Intuition

A mutation is defined as a sudden, heritable change in the genetic material (DNA sequence) of an organism that is not the result of segregation or recombination. Since the genotype is the blueprint that determines the phenotype, a change at the DNA level typically manifests as an observable change in an organism's traits. This is exactly how mutations were historically discovered — de Vries first noticed phenotypic variants (like unusual flower forms) in Oenothera and traced them to underlying genetic (genotypic) changes.

Step-by-Step Solution

1. Option (A) restricts mutation to phenotype only — incorrect, because the phenotype change is a consequence of an underlying genetic change.
2. Option (B) restricts mutation to genotype only — incomplete, because it ignores that this genetic change is usually expressed as a phenotypic difference. …

Concept and Intuition

Sickle cell anaemia arises from a single base-pair substitution in the gene coding for the beta-globin chain of haemoglobin. This is classified as a point mutation because only one nucleotide (and hence one codon) is altered, changing the sixth amino acid of the beta chain from glutamic acid (glutamate) to valine. This substitution changes haemoglobin's surface property, causing it to polymerise under low oxygen tension and deform red blood cells into a sickle shape.

Step-by-Step Solution

1. Type of mutation: a single nucleotide substitution altering one codon — this is a point mutation, not a deletion, frameshift, or somatic-only mutation (it's a germline point mutation). …

Concept and Intuition

Mutagens are physical or chemical agents that increase the rate of mutation in DNA. Ionizing radiations — such as X-rays and gamma rays — carry enough energy to knock electrons out of atoms/molecules, directly breaking DNA strands or altering bases, and are classic, well-documented physical mutagens used both in nature (background radiation) and deliberately in mutation breeding programs. Infrared radiation, by contrast, is non-ionizing and much lower energy (mainly causing heating effects), so it is not considered a significant mutagen. Ethylene and IAA (indole-3-acetic acid) are plant growth hormones involved in signaling processes like fruit ripening and cell elongation respectively — they are not DNA-damaging agents.

Step-by-Step Solution

1. Identify what a mutagen must do: damage or alter DNA structure/sequence.
2. Recognize gamma radiation as high-energy, ionizing radiation capable of breaking DNA strands — a classic mutagen. …

Concept and Intuition

Mutations are permanent changes in the nucleotide sequence of DNA. They range in scale from point mutations (a change at a single base pair, via substitution, insertion, or deletion) to large-scale chromosomal aberrations (changes in chromosome structure or number, such as deletions, duplications, inversions, translocations, aneuploidy). The textbook's flagship example connecting a point mutation directly to a human genetic disease is sickle-cell anaemia, where a single base substitution changes one codon (GAG to GTG), causing one amino acid substitution (glutamic acid to valine) at the sixth position of the β-globin chain — this single change alters haemoglobin's shape and behaviour under low oxygen. Phenylketonuria, by contrast, is introduced in textbooks as an inborn error of metabolism caused by deficiency of the enzyme that converts phenylalanine to tyrosine, without being showcased as the point-mutation example.

Step-by-Step Solution

1. (A) Mutation results in alteration of genes — this is essentially the definition of mutation at the gene/nucleotide level. TRUE.
2. (B) Mutation leads to variation in DNA — again, this is the basic premise: mutation is a source of genetic variation. TRUE.
3. (C) Alteration in chromosomes results in aberrations — larger structural or numerical changes in chromosomes are indeed termed chromosomal aberrations, distinct from point/gene mutations. TRUE. …

Concept and Intuition

Hugo de Vries, working with Oenothera lamarckiana (evening primrose) in the early 1900s, observed that new, distinct forms suddenly appeared in his breeding plots without any obvious intermediate stages — a sharp contrast to the gradual, continuous variation Darwin had emphasised. He termed this discontinuous variation "mutation," and proposed the Mutation Theory of evolution, arguing that evolution proceeds via sudden, large-effect jumps rather than only slow, continuous accumulation of small changes.

Step-by-Step Solution

1. Recall which plant is historically linked with the discovery of mutation — this is a standard fact tied to de Vries's name.
2. Pisum sativum (garden pea) is linked to Mendel's laws of inheritance, not mutation.
3. Snapdragon (Antirrhinum) is associated with incomplete dominance. …