Cell Division – Mitosis and its significance: NEET MCQs with Answers

Explanation: The stage shown is metaphase. In it the chromosomes have moved to the spindle equator and become aligned along the metaphase plate through spindle fibres attached to both poles, and each chromosome is still made up of two sister chromatids because the centromere has not yet split.

Explanation: The centromere splits at the onset of anaphase in mitosis and at the onset of anaphase II in meiosis, separating sister chromatids. In anaphase I only homologous chromosomes separate and centromeres stay intact; metaphase is only the alignment stage.

Explanation: A metaphase chromosome consists of two sister chromatids held together by the centromere, not four, so that statement is incorrect. The nuclear envelope has completely disintegrated, chromosomes are maximally condensed and they align at the equatorial (metaphase) plate.

Explanation: Both statements are true. A multicellular organism grows because mitosis increases its cell number. Mitosis is an equational division: daughter cells receive the same chromosome number and genetic make-up as the parent, so the new cells formed during growth are genetically alike, which keeps the organism genetically stable as it grows. In this sense the Reason accounts for the Assertion.

Explanation: The figure shows chromatids being pulled towards opposite poles by the spindle fibres, with their centromeres leading and the arms trailing — the characteristic feature of anaphase. In metaphase the chromosomes would be aligned at the equator, and in telophase they would be clustered at the poles with nuclear envelopes reforming.

Explanation: A bacterium has no organised cell cycle with a distinct S phase; its circular DNA replicates immediately before the cell divides by binary fission.

Explanation: Mitosis proceeds by chromosome condensation and breakdown of the nuclear envelope in prophase, alignment at the equator in metaphase, centromere division and poleward segregation in anaphase, and reorganisation in telophase. Crossing over never occurs in mitosis.

Explanation: The APC triggers the destruction of the proteins that hold sister chromatids together; if it is defective the chromatids remain joined and segregation at anaphase fails.

Explanation: The spindle microtubules bind to the kinetochores, the disc-shaped protein complexes assembled on the surface of the centromere.

Explanation: Pairing of homologous chromosomes, or synapsis, is confined to zygotene of prophase I of meiosis; the other three occur in mitosis.

Explanation: At metaphase the centromeres of all the chromosomes lie at the equator with their arms directed towards the poles, each chromosome carrying two chromatids. In anaphase the centromere of each chromosome divides so that each sister chromatid acquires its own centromere, and on the book's reckoning each anaphase chromosome therefore carries half the number of chromatids.

Explanation: Polytene or salivary gland chromosomes were reported by Balbiani in 1881 from the salivary glands of the larva of Chironomus. They become giant through endoduplication, that is, repeated replication of chromatids without their separation, and each polytene chromosome is formed by the pairing of two homologous chromosomes.

Explanation: At metaphase chromosomes are maximally condensed and aligned at the equatorial plate, so their morphology, size and number are most easily studied. Karyotypes are prepared from metaphase chromosomes.

Explanation: Polytene chromosomes are formed by the pairing of two somatic homologous chromosomes which undergo repeated endomitosis, forming a number of strands. These strands remain attached to a common large chromocentre and are rich in heterochromatin.

Explanation: Colchicine is a mitotic poison derived from the bulb-like corms of Colchicum autumnale, the autumn crocus. It binds to tubulin and thereby inhibits the formation of the spindle fibres, so the duplicated chromosomes are not separated and the haploid cell becomes diploid.