Structure of Chromosomes, Cell Cycle and Cell Division — ICSE Class 10 Biology Important Questions

Answer: (b) Centromere.

The centromere is the constricted region that holds the two sister chromatids together and to which spindle fibres attach.

Answer: (b) S phase.

DNA is duplicated during the synthesis (S) phase of interphase, so each chromosome then has two chromatids.

Answer: (b) Metaphase.

In metaphase the chromosomes arrange themselves at the equatorial plate with their centromeres attached to spindle fibres.

Answer: (b) 4.

Meiosis is a reductional division that halves the chromosome number, so a $2n=8$2n=8 cell forms haploid cells with $n=4$n=4 chromosomes each.

Answer: (a) Both statements are true and R correctly explains A: crossing over during prophase-I exchanges genetic material between homologous chromosomes, producing new gene combinations and hence variation.

(i) Homologous chromosomes: A pair of chromosomes, one from each parent, that are similar in size, shape and the genes they carry (though the alleles may differ).

(ii) Gene: a segment of DNA on a chromosome that carries the information for one characteristic; it is the unit of heredity passed from parents to offspring.

(i) It brings about growth of an organism by increasing the number of body cells. (ii) It repairs and replaces worn-out or damaged cells and helps in healing of wounds, and produces genetically identical cells (also the basis of asexual reproduction).

At metaphase a chromosome consists of two identical sister chromatids joined at the centromere.

Chromatin is the thread-like network of DNA and proteins in the nucleus of a non-dividing cell. During cell division it coils and condenses to form the visible chromosomes. Each chromatid holds one long DNA molecule, and genes are segments of that DNA.

(Any three differences are acceptable.)

The cell cycle is the sequence of events from the formation of a cell to its own division into two daughter cells. It has interphase (G1, S and G2, the longest part) followed by the M phase.

G1 phase: the cell grows and makes proteins and organelles.

S phase: DNA replicates, so each chromosome comes to have two chromatids.

G2 phase: the cell grows further and prepares for division (for example, proteins for the spindle are made).

M phase: the nucleus divides by mitosis, followed by cytokinesis, which divides the cytoplasm.

Prophase: The chromatin condenses into distinct chromosomes, each seen as two chromatids joined at the centromere. The nuclear membrane and nucleolus begin to disappear, and the centrioles (in animal cells) move to opposite poles and start forming spindle fibres.

Metaphase: The chromosomes arrange themselves at the equator of the cell (equatorial plate). The spindle fibres attach to the centromere of each chromosome. This is the stage where chromosomes are most clearly visible.

Anaphase: The centromeres split and the two chromatids of each chromosome separate. The spindle fibres pull the daughter chromosomes to opposite poles, so each pole gets an equal and identical set of chromosomes.

Telophase: The chromosomes at each pole uncoil into chromatin, the nuclear membrane and nucleolus reappear, and two daughter nuclei are formed. The spindle disappears.

Mitosis is followed by cytokinesis, in which the cytoplasm divides to give two genetically identical diploid daughter cells.

Significance of meiosis:

(i) Maintains chromosome number: Meiosis halves the chromosome number so that gametes are haploid $(n)$(n). When two gametes fuse at fertilisation, the diploid $(2n)$(2n) number is restored, keeping the chromosome number of the species constant generation after generation.

(ii) Introduces variation: During prophase-I, crossing over between homologous chromosomes and the independent assortment of chromosomes produce new combinations of genes. This genetic variation is the raw material for evolution and adaptation.

(iii) Forms gametes/spores: It produces the reproductive cells needed for sexual reproduction.

If gametes were formed by mitosis: Gametes would be diploid $(2n)$(2n). At fertilisation the chromosome number would double in every generation ($2n \rightarrow 4n \rightarrow 8n$2n → 4n → 8n...). This would disturb the characteristic chromosome number of the species and disrupt normal development, so sexual reproduction could not continue normally. Also, there would be no crossing over, hence far less variation.

(i) Cell A shows mitosis (chromosome number unchanged, two daughter cells) and cell B shows meiosis (chromosome number halved, four daughter cells).

(ii) During metaphase each chromosome has two chromatids (as DNA has already replicated in the S phase).

(iii) The division of cell B (meiosis) normally occurs in reproductive cells (germ cells) to form gametes.

(iv) Meiosis maintains the chromosome number of the species across generations (and introduces variation through crossing over) — any one point is acceptable.