NDA (II) 2023 General Ability Test · Question 94 of 146

Correct — D, Pollen formation. Halving the DNA content is the signature of meiosis, and in a flowering plant meiosis happens in only two places — inside the anther, where pollen mother cells divide to give haploid microspores that mature into pollen grains, and inside the ovule, where a megaspore mother cell does the same on the female side. Every other growth event listed here is carried out by mitosis, which copies the DNA faithfully and leaves each daughter cell with the parent's full complement. So the one moment at which the parent plant's DNA content is cut in half, among these four, is the making of pollen.

• (a)Seed germination — Germination is the embryo resuming growth. The radicle and the plumule extend by mitosis, and the seedling carries exactly the chromosome number the embryo already had, so nothing is halved.
• (b)Fruit formation — The fruit is the ripened ovary wall. It grows by mitosis from tissue that belongs to the parent plant, so the fruit wall carries the parent's full DNA content.
• (c)Flower bud formation — This is the closest trap, because the halving does eventually happen inside a flower. But the bud itself is built by mitosis at the shoot apex, and every sepal, petal, stamen and carpel it produces is ordinary parent tissue. Meiosis begins later and in one specific place — the pollen mother cells within the anther of that flower.

A flowering plant alternates between a large diploid body and a very small haploid stage. Meiosis makes the switch: within the anther, each pollen mother cell divides twice to give four haploid microspores, and each microspore develops into a pollen grain. The ovule does the equivalent, producing the embryo sac. Fertilisation then restores the full chromosome number in the zygote, so the DNA is halved once and doubled back once in every cycle.

Students often reach for seed germination or fruit formation because both are dramatic changes in the plant's appearance, and dramatic change feels like it should involve a change in the genetic material. It does not. Growth, differentiation and ripening are all mitotic, and mitosis is a copying process. The rule worth carrying into the exam is simple — the chromosome number changes only twice in the life of a plant, downwards at spore formation and upwards at fertilisation. Note also that the pollen grain is not a gamete but a whole tiny male gametophyte; the male gametes are produced inside it, and each is already haploid because the halving happened before the grain was even formed.

• Meiosis halves the chromosome number; mitosis keeps it unchanged.
• In a flowering plant, meiosis occurs in the pollen mother cells inside the anther and in the megaspore mother cell inside the ovule.
• Each pollen mother cell gives rise to four haploid microspores, which mature into pollen grains.
• Fertilisation restores the diploid number in the zygote, so the plant's DNA content is halved once and doubled once per cycle.

• Assuming any major developmental change involves a change in chromosome number; growth and ripening are mitotic.
• Choosing flower bud formation because meiosis happens inside a flower; the bud itself is built mitotically.
• Calling the pollen grain a gamete — it is the male gametophyte, and it produces the gametes.

Asked as a single-step item — identify the stage of the plant life cycle at which meiosis, and therefore halving, occurs.

• practice — not a real PYQ

  In flowering plants, meiosis takes place in the

  • (a)root tip and shoot tip
  • (b)pollen mother cells and the megaspore mother cell
  • (c)cells of the ovary wall
  • (d)cotyledons of the seed

  Answer(b) pollen mother cells and the megaspore mother cell — the only two sites at which the chromosome number is halved.

• practice — not a real PYQ

  A single pollen mother cell of a flowering plant gives rise to how many haploid microspores?

  • (a)One
  • (b)Two
  • (c)Four
  • (d)Eight

  Answer(c) Four — meiosis consists of two successive divisions, so one mother cell yields a tetrad of four microspores.