Consider the following statements: 1. DNA replication takes place when chromatin is opened up. 2. Chromatin organises itself into rod-shaped chromosomes before cell division. 3. Both prokaryotes and eukaryotes have the same process for cell division. Which of the statements given above is/are correct?
- (a)1 only
- (b)1 and 2 only
- (c)1, 2 and 3
- (d)3 only
Correct — B, 1 and 2 only. Statement 1 is true: DNA is copied during the S-phase when the chromatin is decondensed ('opened up'), because replication enzymes need access to the loosely packed DNA. Statement 2 is true: as a eukaryotic cell prepares to divide, its diffuse chromatin coils and condenses into the compact, rod-shaped chromosomes seen at metaphase, so the copies can be shared cleanly. Statement 3 is false, which is why the answer stops at 1 and 2 — prokaryotes and eukaryotes do not divide the same way.
- (a)1 only — This drops statement 2, but the condensation of chromatin into rod-shaped chromosomes before division is a correct fact. Both 1 and 2 are true, so '1 only' undercounts.
- (c)1, 2 and 3 — This wrongly accepts statement 3. Prokaryotes divide by binary fission, whereas eukaryotes divide by mitosis (or meiosis for gametes) with a spindle apparatus, so their division processes are not the same.
- (d)3 only — This keeps the one false statement and rejects the two true ones — the reverse of the correct reading. Statement 3 is false and statements 1 and 2 are true.
In a eukaryotic cell, DNA is wound around histone proteins to form chromatin. Chromatin exists in a loose (open) form when genes are being read or copied and in a highly condensed rod-shaped form — the chromosome — when the cell divides. DNA is duplicated during the S-phase of interphase, before the chromatin condenses for mitosis.
Statements 1 and 2 describe two different states of the same material: open chromatin for replication, condensed chromosomes for division. Both are standard and true. Statement 3 is the classic trap — it invites you to overgeneralise 'cell division' across all life. In reality prokaryotes lack a nucleus and spindle and split by binary fission, while eukaryotes use mitosis or meiosis, so the processes differ and the answer is 1 and 2 only.
- DNA replication occurs in the S-phase of interphase, when chromatin is decondensed and accessible.
- Before mitosis, chromatin condenses into compact rod-shaped chromosomes so duplicated DNA can be divided evenly.
- Prokaryotes (bacteria) divide by binary fission — no nucleus, no spindle.
- Eukaryotes divide by mitosis (body cells) or meiosis (gamete formation), using a spindle apparatus.
Only statement 3 is false, so the correct combination is 1 and 2 only.
- Assuming 'cell division' works the same way in bacteria and in human cells (statement 3).
- Confusing when DNA is copied (S-phase, chromatin open) with when it condenses (before mitosis).
Asked as statement-based 'which is/are correct', usually pairing a true fact about the cell cycle with a false over-generalisation.
With reference to the recent developments in science, which one of the following statements is not correct?
- (a) Functional chromosomes can be created by joining segments of DNA taken from cells of different species.
- (b) Pieces of artificial functional DNA can be created in laboratories.
- (c) A piece of DNA taken out from an animal cell can be made to replicate outside a living cell in a laboratory.
- (d) Cells taken out from plants and animals can be made to undergo cell division in laboratory petri dishes.
Answer(a) Functional chromosomes can be created by joining segments of DNA taken from cells of different species.
Same concepts — DNA replication and cell division. UPSC's options touch replicating isolated DNA and culturing cells to divide in vitro, mirroring this NDA question's focus on replication and division.
- practice — not a real PYQ
In which phase of the cell cycle is DNA replicated?
- (a)G1 phase
- (b)S phase
- (c)G2 phase
- (d)M phase
Answer(b) S phase — DNA synthesis (replication) occurs in the S-phase of interphase.
- practice — not a real PYQ
Bacteria (prokaryotes) usually reproduce by:
- (a)Mitosis
- (b)Meiosis
- (c)Binary fission
- (d)Budding of a spindle
Answer(c) Binary fission — prokaryotes have no nucleus or spindle, so they split by binary fission.