NDA (II) 2023 General Ability Test · Question 66 of 146

Correct — B, Meiosis and fertilization. Meiosis is where the shuffling happens. Homologous chromosomes exchange segments by crossing over, and then each pair lines up and separates independently of every other pair, so almost no two gametes carry the same combination of alleles. Fertilization then does the second half of the job by fusing two such gametes drawn from two different parents, producing a zygote whose gene combination is new. Between them these two steps also keep the chromosome number stable — meiosis halves it, fertilization restores it.

• (a)Mitosis and fertilization — Fertilization is right but mitosis is not a source of variety. Mitosis copies the chromosomes exactly and hands each daughter cell an identical set, which is why it is the division used for growth and repair, not for making gametes.
• (c)Mitosis and binary fission — Both processes make copies rather than combinations. Binary fission is an asexual mode of division found in bacteria and some protists, and neither process involves sexual reproduction in higher organisms at all.
• (d)Meiosis and conjugation — Meiosis is correct, but conjugation is the wrong partner. Conjugation is a transfer of genetic material between two cells seen in bacteria and in some algae and ciliates — it is not the fusion of male and female gametes that higher organisms rely on.

Sexual reproduction generates variation at two separate points. Inside meiosis, crossing over in prophase I swaps segments between homologous chromosomes, and independent assortment in metaphase I sends the maternal and paternal member of each pair to gametes at random. Fertilization then adds a third layer of chance by picking one gamete from each of two genetically different parents. The result is offspring that resemble the parents but match neither.

The stem asks for two features and every option pairs a cell division with something else, so decide the division first. Only meiosis reshuffles genes, which kills options (a) and (c) immediately. Between the two meiosis options, the partner has to be the process that unites gametes in higher organisms — that is fertilization, not bacterial conjugation. Candidates lose this question by remembering that conjugation involves genetic exchange and forgetting where it occurs.

• Meiosis halves the chromosome number, turning a diploid cell into haploid gametes.
• The two shuffling events inside meiosis are crossing over in prophase I and independent assortment in metaphase I.
• Fertilization restores the diploid number by fusing a male and a female gamete.
• Mitosis yields daughter cells genetically identical to the parent cell, so it contributes no variation.

• Marking mitosis because it is the division students meet first.
• Assuming conjugation is a general sexual process rather than a bacterial and protist one.
• Crediting all the variation to meiosis and forgetting that fertilization is the second shuffle.

Asked as a pairing item like this one, as 'why does sexual reproduction produce variation', or as a mitosis-versus-meiosis comparison.

• practice — not a real PYQ

  Crossing over between homologous chromosomes takes place during which stage?

  • (a)Prophase of mitosis
  • (b)Prophase I of meiosis
  • (c)Anaphase II of meiosis
  • (d)Telophase of mitosis

  Answer(b) Prophase I of meiosis — homologues pair up and exchange segments at this stage.

• practice — not a real PYQ

  If a diploid cell of an organism has 24 chromosomes, the number of chromosomes in each of its gametes will be

  • (a)48
  • (b)24
  • (c)12
  • (d)6

  Answer(c) 12 — meiosis halves the chromosome number, so 2n = 24 gives n = 12.