NEET Biology Cell Cycle and Cell Division MCQs Set 04

Answer: (d) 7

Explanation:
1. Each mitotic division doubles the cell number, so the number is 2^n.
2. 128 = 2^7, so 7 divisions are needed.

Teacher's Note:
1. Cells double each division.
2. Do not confuse 127 new cells with 7 divisions.

Answer: (c) maintains the constant chromosome number of a particular species

Explanation:
1. Meiosis halves the chromosome number to form gametes.
2. Fertilisation restores it, so the number stays constant over generations.

Teacher's Note:
1. Reduction and restoration keep the species number constant.
2. Growth is the job of mitosis.

Answer: (b) two

Explanation:
1. S phase has replicated the DNA.
2. Each chromosome now has two chromatids, each with one DNA double helix.

Teacher's Note:
1. Two chromatids, two DNA molecules.
2. G1 chromosomes have one DNA molecule each.

Answer: (a) non-sister chromatids of a bivalent

Explanation:
1. Exchange of segments takes place between non-sister chromatids of homologous chromosomes.
2. Sister chromatids are identical, so exchange between them gives no new combination.

Teacher's Note:
1. Non-sister means from different homologues.
2. This produces recombinant chromatids.

Answer: (a) DNA replication takes place in the S phase

Explanation:
1. Interphase is long and M phase is short, and G2 comes before M.
2. DNA content is unchanged in G1 and doubles only in the S phase.

Teacher's Note:
1. S phase equals DNA synthesis.
2. Interphase is more than 95 percent of the cycle.

Answer: (b) pachytene

Explanation:
1. Synapsis begins in zygotene and is completed in pachytene.
2. The paired structure is called a bivalent or tetrad.

Teacher's Note:
1. Zygotene starts pairing, pachytene completes it.
2. Crossing over follows.

Answer: (d) plastids

Explanation:
1. Plastids are absent in animal cells, which divide normally.
2. The plasma membrane, cytoskeleton and mitochondria are needed for division and energy.

Teacher's Note:
1. Animal cells have no plastids.
2. Cytoskeleton gives spindle microtubules.

Answer: (a) movement of chromosomes toward the poles

Explanation:
1. The kinetochore forms on the centromere and attaches to spindle fibres.
2. Without it chromosomes cannot be pulled to the poles.

Teacher's Note:
1. Acentric fragments get lost in division.
2. Centromere equals attachment site.

Answer: (b) During S phase

Explanation:
1. Histones are needed to package newly made DNA into chromatin.
2. They are therefore made during the S phase together with DNA replication.

Teacher's Note:
1. DNA and histones are made together.
2. Tubulin is made in G2.

Answer: (c) 16 bivalents

Explanation:
1. n = 16 means 2n = 32 chromosomes, forming 16 homologous pairs.
2. Each pair is a bivalent at metaphase I.

Teacher's Note:
1. Number of bivalents equals n.
2. Bivalents are tetrads of four chromatids.

Answer: (b) karyokinesis

Explanation:
1. Karyokinesis means nuclear division.
2. Prophase, metaphase, anaphase and telophase are its four stages.

Teacher's Note:
1. Karyokinesis is nuclear division.
2. Cytokinesis is division of cytoplasm.

Answer: (a) the mitotic apparatus

Explanation:
1. Asters and spindle fibres together make the apparatus that moves chromosomes.
2. This is called the mitotic apparatus.

Teacher's Note:
1. Mitotic apparatus equals asters plus spindle.
2. Higher plants have a spindle but no asters.

Answer: (b) opposite poles of the cell

Explanation:
1. The two centrosomes separate and migrate to opposite poles.
2. From there they organise the spindle.

Teacher's Note:
1. Opposite poles set up a bipolar spindle.
2. Centrioles duplicate in S phase.

Answer: (c) asters

Explanation:
1. Microtubules radiating from each centrosome form a star-shaped structure.
2. This is the aster.

Teacher's Note:
1. Aster means star.
2. Asters are absent in plant cells.

Answer: (c) each pole

Explanation:
1. A nuclear envelope reforms around each cluster at telophase.
2. Hence each pole gets its own nucleus.

Teacher's Note:
1. Two nuclei form at the two poles.
2. The cell then divides by cytokinesis.

Answer: (d) separation of cytoplasm

Explanation:
1. Cytokinesis is the division of the cytoplasm into two daughter cells.
2. Division of the nucleus is karyokinesis.

Teacher's Note:
1. Karyokinesis is nuclear division.
2. Cytokinesis is cytoplasmic division.

Answer: (b) pairing of homologous chromosomes and recombination between non-sister chromatids of homologous chromosomes

Explanation:
1. Synapsis pairs homologous chromosomes.
2. Crossing over exchanges segments between their non-sister chromatids.

Teacher's Note:
1. Pairing and exchange both involve homologues.
2. Sister exchange gives no new combinations.

Answer: (c) Pachytene

Explanation:
1. In pachytene the paired homologues are fully synapsed and each bivalent shows four chromatids.
2. Such a structure is called a tetrad.

Teacher's Note:
1. Tetrad stage equals pachytene.
2. Chiasmata appear later in diplotene.

Answer: (c) DNA replicates but the chromosome number remains the same

Explanation:
1. Interkinesis separates meiosis I from meiosis II.
2. It is short and has no DNA replication, so the statement that DNA replicates is incorrect.

Teacher's Note:
1. No S phase in interkinesis.
2. The cell goes straight into prophase II.

Answer: (a) the G1 phase is longer than the S phase

Explanation:
1. Cells that divide slowly spend most of their time growing and preparing in G1.
2. The S phase has a roughly fixed length.

Teacher's Note:
1. Cell-cycle length differences are mostly due to G1.
2. G0 cells leave G1 altogether.

Answer: (a) two homologous chromosomes

Explanation:
1. Synapsis is the pairing of homologous chromosomes in zygotene.
2. The other pairs are not chromosomal pairings.

Teacher's Note:
1. Synapsis is chromosome pairing only.
2. It produces bivalents.