CBSE Class 11 Biology Chapter 10: Cell Cycle and Cell Division NCERT Solutions

Solution: The typical duration of the cell cycle for a mammalian cell is approximately 24 hours. This period can vary depending on the specific cell type and environmental conditions.

Solution: Cytokinesis and karyokinesis are distinct processes occurring during cell division:

1. Cytokinesis: This is the biological process involving the division of the cell's cytoplasm. It typically follows karyokinesis (nuclear division) and results in the formation of two separate daughter cells.
2. Karyokinesis: This is the biological process involving the division of the cell's nucleus. It encompasses the stages of mitosis (prophase, metaphase, anaphase, telophase) where the chromosomes are duplicated and segregated.

In essence, karyokinesis deals with the nucleus, while cytokinesis deals with the cytoplasm.

Solution: Interphase is the preparatory phase of the cell cycle, during which the cell grows and duplicates its genetic material in preparation for division. It is divided into three sub-phases:

1. G₁ Phase (First Gap Phase): This is the initial growth phase where the cell is metabolically active. It grows in size and synthesizes proteins and RNA necessary for the subsequent S phase and overall cell function. The cell prepares its DNA for replication during this stage.
2. S Phase (Synthesis Phase): During this crucial phase, the cell replicates its DNA. The amount of DNA in the cell doubles, but the chromosome number remains the same as each chromosome now consists of two sister chromatids.
3. G₂ Phase (Second Gap Phase): In this phase, the cell continues to grow and synthesizes additional proteins and RNA required for mitosis. It ensures that all cellular components are ready for the upcoming division.

Interphase is a dynamic period where the cell undergoes significant growth and prepares its genetic material for accurate distribution into daughter cells.

Solution: The G₀ phase, also known as the quiescent phase, is a stage in the cell cycle where cells exit the active cycle of division. In this phase, cells are metabolically active and perform their specialized functions, but they do not proliferate or divide. These cells can remain in G₀ for extended periods and may re-enter the cell cycle to divide if stimulated, or they may differentiate terminally. Such cells are vital for tissue repair and replacement, compensating for cells lost due to injury or normal turnover.

Solution: Mitosis is termed 'equational division' because the process ensures that the daughter cells produced are genetically identical to the parent cell. During mitosis, the chromosomes replicate, and then these replicated chromosomes are equally distributed into the two resulting daughter cells. Consequently, the chromosome number in each daughter cell is precisely the same as that in the parent cell (e.g., a diploid parent cell produces two diploid daughter cells).

Solution: The stages of the cell cycle at which these specific events occur are:

1. Chromosomes are moved to spindle equator: This event occurs during Metaphase of mitosis or meiosis. The chromosomes align along the metaphase plate, which is the equatorial plane of the spindle.
2. Centromere splits and chromatids separate: This critical event happens during Anaphase of mitosis or meiosis II. The sister chromatids separate and are pulled towards opposite poles of the cell.
3. Pairing between homologous chromosomes takes place: This process, known as synapsis, occurs during the Zygotene stage of Prophase I in meiosis.
4. Crossing over between homologous chromosomes takes place: This exchange of genetic material between non-sister chromatids of homologous chromosomes occurs during the Pachytene stage of Prophase I in meiosis.

Solution:

(a) Synapsis: Synapsis is the process of intimate pairing between homologous chromosomes. This pairing occurs during the zygotene stage of Prophase I of meiosis. Homologous chromosomes, one inherited from each parent, align side-by-side along their entire length.

(b) Bivalent: A bivalent is a structure formed during synapsis when a pair of homologous chromosomes (each consisting of two sister chromatids) come together. Therefore, a bivalent consists of four chromatids and two homologous chromosomes. These bivalents are visible during the pachytene stage of Prophase I.

(c) Chiasmata: Chiasmata (singular: chiasma) are the points of contact between homologous chromosomes where crossing over has occurred. They represent the physical manifestation of genetic recombination between non-sister chromatids. Chiasmata become visible during the diplotene stage of Prophase I, holding the homologous chromosomes together until they separate during anaphase I. They ensure the proper segregation of homologous chromosomes.

(Note: A diagram illustrating synapsis, bivalents, and chiasmata would typically show two homologous chromosomes paired, with X-shaped structures (chiasmata) at points where segments have been exchanged between chromatids.)