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

The average duration of the cell cycle for a mammalian cell is approximately 24 hours. However, this can vary significantly depending on the cell type and organism. For example, a yeast cell can complete its cell cycle in about 90 minutes.

The key differences between cytokinesis and karyokinesis are:

Interphase is the longest phase of the cell cycle and is a period of intense cellular activity and growth in preparation for division. It is divided into three sub-phases:

1. G1 Phase (Gap 1):
   • This is the interval between mitosis and the initiation of DNA replication.
   • The cell is metabolically active and grows continuously.
   • It synthesizes proteins, RNA, and other molecules necessary for cell growth.
   • The cell decides whether to continue with the division or enter a quiescent stage (G0).
2. S Phase (Synthesis Phase):
   • This is the phase where DNA synthesis or replication takes place.
   • The amount of DNA per cell doubles (from 2C to 4C), but the chromosome number remains the same (2n).
   • In animal cells, centriole duplication also occurs during this phase.
3. G2 Phase (Gap 2):
   • This is the phase after DNA replication and before the start of mitosis.
   • The cell continues to grow and synthesize proteins, particularly tubulin, which is required for the formation of spindle fibres.
   • The cell prepares itself for the M-phase.

The G0 phase, or quiescent phase, is an inactive stage of the cell cycle where cells remain metabolically active but do not proliferate or divide.

• Cells enter the G0 phase when they do not need to divide further. They exit the G1 phase to enter this stage.
• These cells can remain in the G0 phase for long periods, even indefinitely (e.g., nerve cells, mature heart muscle cells).
• However, some cells in the G0 phase can re-enter the G1 phase and resume the cell cycle if they receive the appropriate signals (e.g., liver cells regenerating after an injury).

Mitosis is called equational division because the number of chromosomes in the daughter cells is equal to the number of chromosomes in the parent cell.

• During mitosis, a parent cell divides to form two genetically identical daughter cells.
• If a parent cell is diploid (2n), the two daughter cells produced will also be diploid (2n).
• There is no change or reduction in the chromosome number. This is why it is termed "equational."

(i) Chromosomes are moved to the spindle equator.

→ Metaphase: During metaphase, the chromosomes align themselves at the metaphase plate, which is the equator of the spindle.

(ii) Centromere splits and chromatids separate.

→ Anaphase: During anaphase (of mitosis) or Anaphase II (of meiosis), the centromeres split, and the sister chromatids separate and move towards opposite poles.

(iii) Pairing between homologous chromosomes takes place.

→ Zygotene stage of Prophase I (Meiosis): The pairing of homologous chromosomes (synapsis) occurs during this stage.

(iv) Crossing over between homologous chromosomes takes place.

→ Pachytene stage of Prophase I (Meiosis): The exchange of genetic material, or crossing over, occurs between non-sister chromatids of homologous chromosomes.

(a) Synapsis:
Synapsis is the process of pairing of homologous chromosomes during the Zygotene stage of Prophase I of meiosis. The paired chromosomes are held together by a protein complex called the synaptonemal complex. This pairing is essential for crossing over to occur.

(b) Bivalent:
A bivalent, or a tetrad, is the structure formed by a pair of synapsed homologous chromosomes. Each chromosome in the pair consists of two sister chromatids, so a bivalent contains a total of four chromatids. These bivalents are clearly visible during the Pachytene stage.

(c) Chiasmata:
Chiasmata (singular: chiasma) are the X-shaped structures that become visible during the Diplotene stage of Prophase I. These points represent the locations where crossing over has occurred between non-sister chromatids of homologous chromosomes. Chiasmata hold the homologous chromosomes together after the synaptonemal complex dissolves.

Cytokinesis differs significantly in plant and animal cells due to the presence of a rigid cell wall in plants.

• Equal Size: In males, during spermatogenesis, the meiotic divisions are equal. A primary spermatocyte divides to form two equal-sized secondary spermatocytes, which then divide to form four equal-sized spermatids. This ensures that all four resulting gametes (sperm) are viable and functional.
• Unequal Size: In females, during oogenesis, the meiotic divisions are highly unequal. A primary oocyte divides to form one large secondary oocyte (which receives most of the cytoplasm) and a very small first polar body. The secondary oocyte then divides to form a large ovum (egg) and a tiny second polar body. This unequal division ensures that the mature egg has enough cytoplasm and stored nutrients to support the early development of the embryo.

Meiosis is a fundamentally important process for sexually reproducing organisms. Its significance lies in:

1. Conservation of Chromosome Number: Meiosis is a reductional division, meaning it halves the chromosome number (from diploid 2n to haploid n). This is crucial because when two gametes (sperm and egg) fuse during fertilization, the diploid chromosome number is restored in the zygote. Without meiosis, the chromosome number would double in each generation.
2. Introduction of Genetic Variation: Meiosis introduces genetic variation in the offspring in two key ways:
   • Crossing Over: The exchange of genetic material between homologous chromosomes during Prophase I creates new combinations of alleles on the chromosomes.
   • Independent Assortment: The random orientation and separation of homologous chromosomes during Anaphase I lead to a vast number of different chromosome combinations in the gametes.
3. Basis of Evolution: The genetic variation produced by meiosis provides the raw material upon which natural selection acts. This is a driving force of evolution, allowing populations to adapt to changing environments.

Kinetochores are disc-shaped protein structures that assemble on the centromere of a chromosome. Their function is to attach the chromosome to the spindle fibres (microtubules) during cell division, allowing the chromosomes to be pulled towards the poles.

• Meiosis I is called reductional division because the number of chromosomes is reduced to half. Homologous chromosomes separate, but sister chromatids remain together. A diploid (2n) cell becomes two haploid (n) cells.
• Meiosis II is called equational division because it is similar to mitosis. The sister chromatids separate, but the chromosome number of the cells remains the same (n).

The synaptonemal complex is a protein structure that forms between homologous chromosomes during the Zygotene stage of meiotic Prophase I. It acts like a zipper, holding the paired chromosomes together precisely, which is essential for synapsis and crossing over.

The terminalisation of chiasmata (the movement of chiasmata towards the ends of the chromosomes) occurs during the Diakinesis stage, which is the final stage of Prophase I in meiosis.

• In G2 phase: After DNA replication in the S phase, each of the 20 chromosomes will have two sister chromatids. So, the total number of chromatids will be 20 chromosomes × 2 chromatids/chromosome = 40 chromatids.
• At Anaphase of mitosis: The centromeres split, and the sister chromatids separate. Each chromatid is now considered an individual chromosome. Therefore, there will be 40 chromosomes moving to the poles (but 0 chromatids, as they have separated).

• Prophase: Chromatin condenses to form visible chromosomes. The nuclear envelope and nucleolus disappear. Centrioles move to opposite poles, and spindle fibres begin to form.
• Metaphase: Chromosomes, each with two sister chromatids, align at the cell's equator, forming the metaphase plate. Each chromosome is attached to spindle fibres from opposite poles via its kinetochore.
• Anaphase: The centromeres split, and sister chromatids separate. The separated chromatids (now individual chromosomes) are pulled towards opposite poles by the shortening of spindle fibres.
• Telophase: Chromosomes arrive at the poles and decondense back into chromatin. The nuclear envelope and nucleolus reappear, forming two distinct nuclei. Spindle fibres disappear. Cytokinesis usually begins during late anaphase or telophase.

Prophase I is the longest and most complex phase of meiosis, divided into five sub-stages:

1. Leptotene: Chromatin condenses to form long, thread-like chromosomes.
2. Zygotene: Homologous chromosomes pair up (synapsis) to form bivalents. The synaptonemal complex begins to form.
3. Pachytene: Crossing over (exchange of genetic material) occurs between non-sister chromatids of homologous chromosomes. Bivalents are clearly visible as tetrads.
4. Diplotene: The synaptonemal complex dissolves. Homologous chromosomes start to separate but remain attached at points of crossing over, called chiasmata.
5. Diakinesis: Chromosomes fully condense. Terminalisation of chiasmata occurs. The nuclear envelope breaks down, and the meiotic spindle begins to form.

(a) Identify the stages of the second and third cells observed.
→ The second cell, with 16 structures (chromosomes) aligned at the center, is in Metaphase. The third cell, with 32 structures moving towards opposite ends, is in Anaphase.

(b) What is the 'n' (haploid) number of chromosomes for an onion?
→ The somatic cell (root tip) has 16 chromosomes, which is the diploid (2n) number. Therefore, the haploid (n) number of chromosomes for an onion is 2n/2 = 16/2 = 8.

(c) Explain why the number of structures doubled in the third cell.
→ The number of structures doubled in the third cell (Anaphase) because the centromere of each of the 16 chromosomes splits, and the two sister chromatids separate. Each separated chromatid is now considered an independent chromosome. So, for a brief period during Anaphase, the chromosome count is temporarily doubled to 32 (16 moving to one pole, 16 to the other).

• DNA content of a haploid gamete (n) = C = 4 pg.
• A somatic cell is diploid (2n), so its DNA content in the G1 phase = 2C.
• G1 Phase: 2C = 2 × 4 pg = 8 pg.
• In the S phase, DNA replicates, so the content doubles.
• G2 Phase: 4C = 4 × 4 pg = 16 pg.
• After the M phase (mitosis), the cell divides into two daughter cells, each with the same DNA content as the parent cell in G1.
• After M Phase: Each daughter cell will have 2C = 8 pg.