Unit 4: Cell Cycle — Short Questions

Ans. During G1 phase of interphase following events occur:
i. Cell makes proteins and organelles and so grow in size.
ii. Cell also makes enzymes required in S-phase for replication of DNA.

Ans. During this phase, the DNA of each chromosome is replicated (copied). It results in the duplication of chromosomes (each chromosome consists of two sister chromatids). The total number of chromosomes in cell remains the same.

Ans. During anaphase spindle fibres attached with chromosomes pull towards the poles, making the sister chromatids of chromosomes separated.

Ans. During crossing over, exchange of segments of non-sister chromatids of homologous chromosomes take place. This process leads to recombination of genetic material and increase genetic diversity.

Ans. During mitosis, spindle fibres ensure the accurate separation of sister chromatids of chromosomes attached with spindle fibres in telophase.

Ans. In animal cells, a furrow develops at the equator. At this furrow, the cytoplasm has a ring of microfilaments. The ring contracts and the furrow moves inward. In this way parent cell is pinched into two. While in plant cells, Golgi apparatus makes vesicles. These vesicles move to the middle and fuse to form a plate called phragmoplast. The plate grows outward and its membranes fuse with the cell membrane. The result is two daughter cells.

Ans.

Prophase of mitosis

• Pairing of homologous chromosomes does not take place.
• Crossing over is absent.

Prophase-1 of meiosis-I

• During prophase-1, pairing of homologous chromosomes takes place.
• Crossing over leads to genetic recombination.

Ans. The number of chromosomes remains the same during mitosis.

Example

A diploid (2n) parent cell will produce two diploid (2n) daughter cells. While during meiosis the number of chromosomes remain half as compare to parent cell.

Example

A diploid (2n) parent cell produces four haploid (n) daughter cells.

Ans.
i. Constriction of spindle fibres towards their respective poles.
ii. Equal separation of sister chromatids of chromosomes.

Ans. During cell division, the centrosome of cell duplicates into two. Thses two centrosomes migrate to the opposite side of the nucleus and make network of microtubules called spindle fibres. These spindle fibres ensure accurate separation of chromosomes.

Ans. Sister chromatids are identical copies of a single chromosomes. These sister chromatids are separated during anaphase-II of meiosis.

Ans. Some animals can regenerate parts of the body. For this purpose, they form new cells by carrying out mitosis in the cells of remaining parts.

Ans. Cells that have stopped dividing are in G₀ phase. In multicellular eukaryotes, cells enter G₀ phase from G1.

Examples

i. Some cells of liver and kidneys enter the G₀ phase temporarily.
ii. Neurons remain in G₀ phase for indefinite period.
iii. Other cells, such as epithelial cells do not enter into G₀ phase.

Ans. It is last sub-phase before cell division. In this phase increased protein synthesis occurs for the production of spindle fibres.

Ans. In this phase cell is divided into two daughter cells. This phase is called M-phase (mitotic phase).

Ans.
i. Benign Tumours: Not all tumours are cancerous however, a benign tumour is an abnormal mass of essential normal cells. They always remain at their original site in the body.
ii. Malignant Tumour: A malignant tumour is cancerous. It is capable of spreading into neighbouring
iii. Due to uncontrolled division, masses of cells are formed. These masses are called tumors.

Ans. (a) Mitosis and meiosis.

Mitosis

• It takes place in somatic cells.
• It consists of one division.
• Crossing over does not take place during prophase.
• Chromatids divides at anaphase.
• Individual duplicated chromosomes align at the metaphase plate during metaphase.
• Daughter chromosome move to opposite poles during anaphase.
• Two diploid daughter cells are formed.
• The daughter cells are genetically identical to each other and to the parent cell.
• The number of chromosomes remains constant.
• Mitotic products are usually capable of undergoing additional mitotic divisions.

Meiosis

• It takes place in germ line cells of sex organs.
• It consists of two divisions.
• Crossing over takes place during prophase-I.
• Chromatids does not divide at anaphase-I.
• Paired homologous chromosomes align at the metaphase plate during metaphase-I.
• Homologous chromosomes with two sister chromatids, separate and move to opposite poles during anaphase-I.
• Four haploid daughter cells are formed.
• The daughter cells are not genetically identical to each other and to the parent cell.
• The number of chromosomes becomes half in meiosis.
• Meiotic products cannot undergo further divisions.

(b) Chromatin and Chromosome

Chromatin

In the beginning of prophase, the chromosomes are not visible as they are in the form of fine thread like structures called chromatin.

Chromosome

During prophase, the chromatin begins to shorten, thicken and coil by a process called condensation. It results in the appearance of chromosomes.

(c) Chromosome and Chromatids

Chromosome

The chromatin begins to shorten, thicken and coil by a process called condensation. It results in the appearance of chromosomes. It consists of two chromatids. Two chromatids of a chromosome is joined by centromere.

Chromatids

Unseparated replica of a chromosome is called chromatids.

(d) Centromeres and Centrioles

Centromeres

The centromere is a constriction in chromosome, where chromatids are joined with each other.

Centrioles

A pair of centrioles located near the anterior surface of the nucleus. It consist of triplet of microtubules arranged to form a hollow cylinder.

(e) Cytokinesis and Karyokinesis

Cytokinesis

The process of cytoplasmic division is called cytokinesis.

Karyokinesis

The process of nuclear division is called karyokinesis.

(f) Centromeres and Kinetochores

Centromeres

The centromere is a constriction in chromosome.

Kinetochores

Kinetochores are proteins that act as attachment site for spindle fibres.

(g) Haploid and Diploid Cells

Diploid Cells

• Diploid means cells have full number of chromosomes.
• Somatic cells are diploid (2n) cells.
• These are responsible for growth and maintenance of the organisms.
• These are produced by meiosis.
• Diploid number of chromosomes in man is 46.

Haploid Cells

• Haploid means cells having half number of chromosomes.
• Germ line cells are haploid (n) cells.
• These are involved in sexual reproduction.
• These are produced by mitosis.
• The haploid number of chromosomes in man is 23.

Ans Mistakes in the cell cycle checkpoints can lead to uncontrolled division of abnormal cells. Checkpoints stop damaged cells from dividing, they ensure chromosomes are correctly copied and distributed during cell division. If they fail, abnormal cellular mass will keep on growing and form tumors, which ultimately may lead to cancer. Checkpoints also detect and repair DNA damage before cell division, failure will increase the risk of mutations that can cause cancer.

Ans Skin cells divide continuously to replace old or damaged cells because skin is constantly exposed to wear and tear. While the nerve and muscle cells exist in a resting state called the G₀ phase, where they stop dividing permanently because they are highly specialized and do not need frequent replacement.