CBSE Class 11 Biology Chapter 12: Cell Cycle and Cell Division NCERT Solutions

The typical duration of the cell cycle for a mammalian cell is approximately 24 hours. This period includes all the stages from the beginning of one cell division to the completion of the next.

Cytokinesis and karyokinesis are two distinct processes involved in cell division:

Interphase is the preparatory phase of the cell cycle where the cell grows and duplicates its genetic material before division. It is divided into three sub-phases:

1. G₁ Phase (First Gap Phase): During this phase, the cell grows physically larger, copies its organelles, and makes the molecular building blocks for DNA replication. The cell is metabolically active and synthesizes proteins and RNA necessary for growth.
2. S Phase (Synthesis Phase): This is the crucial phase where the cell synthesizes a complete copy of the DNA in its nucleus. The amount of DNA per cell doubles, but the chromosome number remains the same as each chromosome now consists of two sister chromatids.
3. G₂ Phase (Second Gap Phase): The cell continues to grow, synthesizes proteins and RNA, and prepares for mitosis. It ensures that DNA replication is complete and checks for any errors before entering the M phase.

The G₀ phase, also known as the quiescent phase, is a stage of the cell cycle where cells are metabolically active but have exited the active cycle of division. These cells do not proliferate or divide unless stimulated by external signals. Many differentiated cells, such as mature nerve cells or muscle cells, remain in the G₀ phase indefinitely. Cells in G₀ can be used to replace cells lost due to injury or normal turnover.

Mitosis is referred to as equational division because the process results in the formation of two daughter cells that are genetically identical to the parent cell. Specifically, the chromosome number remains the same in the daughter cells as it was in the parent cell. For example, if a diploid parent cell (2n) undergoes mitosis, it produces two diploid daughter cells (2n).

The stages of the cell cycle where these events occur are:

1. Metaphase: Chromosomes align at the metaphase plate (spindle equator).
2. Anaphase: The centromere splits, and sister chromatids separate, moving towards opposite poles.
3. Zygotene (of Prophase I in Meiosis): Pairing between homologous chromosomes (synapsis) occurs.
4. Pachytene (of Prophase I in Meiosis): Crossing over, the exchange of genetic material between non-sister chromatids of homologous chromosomes, takes place.

These terms are related to the process of meiosis, specifically during Prophase I:

1. Synapsis: This is the process where homologous chromosomes pair up along their entire length. It begins during the zygotene stage of Prophase I and is facilitated by the synaptonemal complex.
2. Bivalent: When two homologous chromosomes pair up during synapsis, the resulting structure is called a bivalent (or a tetrad, as it consists of four chromatids – two from each homologous chromosome). Bivalents are visible during the pachytene and diplotene stages of Prophase I.
3. Chiasmata: These are the points of contact, or cross-shaped structures, where crossing over has occurred between homologous chromosomes. Chiasmata become visible during the diplotene stage of Prophase I and hold the homologous chromosomes together until anaphase I. They represent the physical manifestation of genetic recombination.

(Note: A diagram illustrating these concepts would typically show paired homologous chromosomes with crossing over points (chiasmata) during Prophase I of meiosis.)