Which of the following statements is not true ?

Question

UnlockIAS · Accepted Answer

Correct — B, 'Nucleus of a cell does not play a role in cellular reproduction.' This is the one false statement in the set, and it is false in the strongest possible way: the nucleus is not merely involved in cell reproduction, it is the part of the cell whose division defines the event. In a eukaryotic cell the hereditary material sits inside the nucleus as chromatin, and when the cell divides that chromatin condenses into visible chromosomes, the nuclear envelope breaks down, the spindle pulls one copy of every chromosome to each pole, and two daughter nuclei re-form. Biologists give this nuclear division its own name — karyokinesis — and it precedes the division of the cytoplasm, cytokinesis. Remove the nucleus and the cell stops dividing: an enucleated Amoeba survives for a while but cannot reproduce, and a mammalian mature red blood cell, which extrudes its nucleus as it matures, can never divide again. Statement (b) therefore inverts the truth, and inverting a true statement is exactly how a 'which is NOT true' item is built.

Why the other options are wrong — (a) True, so it cannot be the answer. A chromosome is chromatin in its condensed, transport-ready form, and chromatin is a DNA–protein complex: the DNA double helix is wound roughly twice around an octamer of histone proteins to make a nucleosome, the 'beads-on-a-string' unit, and further non-histone (acidic) proteins organise the fibre into the compact rod seen at metaphase. The name itself records how they were found — Wilhelm Waldeyer coined 'chromosome' in 1888 from the Greek chroma (colour) and soma (body), because these bodies took up basic dyes strongly during staining.  (c) True, and it is the central claim of the chromosome theory of inheritance advanced by Walter Sutton and Theodor Boveri in 1902–03: Mendel's factors behave exactly as chromosomes do during gamete formation, so the factors must be carried on the chromosomes. Human somatic cells carry 46 chromosomes in 23 pairs — one of each pair from each parent — and it is this DNA, packaged in chromosomes, that is copied and handed on. Note that a small amount of DNA also sits outside the nucleus, in mitochondria and in plant plastids, but that is an addition to the statement, not a contradiction of it.  (d) True, and it is the standard school-textbook definition: a gene is the functional segment or functional unit of DNA — the stretch that carries the instruction for one polypeptide or one functional RNA. Long tracts of a chromosome are non-coding (regulatory sequences, introns, repeats), which is precisely why the definition has to specify a *functional* segment rather than 'any' segment. The wording of the option is loose English ('is called genes'), but the biology it states is correct, so it is not the statement the question is hunting for.

Concept: In a eukaryotic cell the genetic material is DNA, and DNA does not float free — it is wrapped around histone proteins to form chromatin, which is housed inside the nucleus behind a double nuclear envelope. When the cell prepares to divide, chromatin coils tightly into discrete chromosomes, one for each DNA molecule, each already duplicated into two sister chromatids joined at the centromere. A gene is a functional stretch of that DNA. So the hierarchy a candidate must hold in the head is: nucleus contains chromosomes → a chromosome is one long DNA molecule plus its proteins → a gene is a functional segment of that DNA. Every one of the four options in this question is a sentence about one link in that chain, and only option (b) breaks it.

State PCS 2024 Q107 — Science & Technology PYQ with Answer

State PCS 2024 Q107 — Science & Technology PYQ with Answer