NDA (II) 2023 General Ability Test · Question 70 of 146

Correct — B, Both chromosome number and DNA content remains constant. Sexual reproduction is built around two steps that cancel each other out. Meiosis halves the chromosome number so that a diploid parent makes haploid gametes, and fertilisation then fuses two of those gametes and puts the number straight back. Because the DNA content of a cell tracks its chromosome set, it follows the same halving and doubling. Compare a body cell of the parent with a body cell of the offspring and both counts read the same — in humans, 46 chromosomes in the parent, 23 in the gamete, 46 again in the child.

• (a)Chromosome number increases but DNA content remains constant — The two halves of the statement contradict each other, since more chromosomes would mean more DNA. It also describes what would happen if gametes were made by mitosis instead of meiosis — the number would double every generation, which it plainly does not.
• (c)Chromosome number decreases but DNA content remains constant — A halving does occur, but only in the gametes and only for one step. Fertilisation immediately restores the number, so the offspring does not carry fewer chromosomes than its parent. And a genuine drop in chromosome number would take the DNA content down with it.
• (d)Both chromosome number and DNA content decreases — If every generation lost chromosomes and DNA, the genome would shrink away within a few generations and the species could not survive. The halving at meiosis is always matched by the doubling at fertilisation.

Every sexually reproducing species has a fixed chromosome number, and it is meiosis that keeps it fixed. A diploid (2n) germ cell divides twice to give haploid (n) gametes, so the set is halved exactly once. Fertilisation then joins a male and a female gamete, n plus n, and the zygote is back at 2n. This halve-and-restore cycle is why the offspring's cells hold the same amount of nuclear DNA as the parent's.

Read the stem as asking for the statement that is true when you compare parent with offspring — not parent with gamete. That distinction decides the question. Options (a), (c) and (d) all describe a one-way drift, up or down, and no species could tolerate that across generations. Option (c) is the sharpest trap because a halving really does happen, but it happens at the gamete stage and is undone before the offspring exists. One fair caveat is that DNA content does double inside any cell during the S phase before it divides; the constancy this question is testing is between the ordinary body cells of the two generations.

• Meiosis halves the chromosome number, turning a 2n cell into n gametes.
• Fertilisation fuses two haploid gametes and restores the 2n number in the zygote.
• In humans the sequence runs 46 chromosomes in a body cell, 23 in a gamete and 46 again in the zygote.
• Because halving and restoration cancel, the chromosome number and nuclear DNA content of a species stay constant across generations.

• Comparing the parent with the gamete instead of with the offspring.
• Splitting chromosome number and DNA content apart when the two move together.
• Assuming that because meiosis halves the set, something must be permanently lost.

Asked as a constancy statement like this one, as a numerical item (if 2n is given, find n), or as 'what is the significance of meiosis'.

No directly related past PYQ was found.

• practice — not a real PYQ

  The chromosome number of a species is kept constant from one generation to the next chiefly by

  • (a)mitosis alone
  • (b)meiosis followed by fertilisation
  • (c)binary fission
  • (d)DNA replication in the S phase

  Answer(b) meiosis followed by fertilisation — the halving and the restoring balance each other exactly.

• practice — not a real PYQ

  If the gametes of an organism carry 14 chromosomes, the chromosome number in its body cells is

  • (a)7
  • (b)14
  • (c)28
  • (d)56

  Answer(c) 28 — gametes are haploid, so the diploid body cell has twice the gamete number.