Class 11 Biology Chapter 10 Cell Cycle and Cell Division

Key Concept: Chromosome Visibility

c) Leptotene

[Solution Description]

Chromosomes become gradually visible during the Leptotene stage of Prophase I.

Key Concept: Genetic Variability

c) Homologous chromosomes undergo crossing over.

[Solution Description] During meiosis, crossing over occurs between non-sister chromatids of homologous chromosomes in prophase I, resulting in new combinations of alleles which contribute to genetic variability.

Key Concept: Organelle Duplication

d) G1 phase

[Solution Description]

Organelle duplication mainly occurs during the G1 phase of Interphase. This phase follows cell division and involves growth and synthesis necessary for further DNA synthesis.

Key Concept: Basic Definition

c) Cell repair and replacement

[Solution Description] The primary purpose of mitosis in multicellular organisms is to assist in the repair and replacement of cells. This ensures that damaged or old cells are replaced with new ones, maintaining tissue health and function.

Key Concept: Mitosis Definition

c) Growth and repair by producing identical cells

[Solution Description] Mitosis is crucial for growth and development in multicellular organisms as it produces diploid daughter cells with identical genetic material, contributing to the organism’s growth and repair.

Key Concept: Cell Cycle Coordination

c) To ensure identical genetic information in each daughter cell

[Solution Description] Ensuring DNA replication occurs before mitosis is crucial because it allows each daughter cell to inherit an exact copy of the genetic material. Without this coordination, cells might end up with incomplete or erroneous sets of chromosomes, leading to malfunction or disease.

Key Concept: Role in Evolution

d) By increasing genetic diversity among offspring

[Solution Description] Meiosis contributes significantly to evolution by increasing genetic diversity. Through mechanisms like crossing over and independent assortment, meiosis reshuffles alleles, creating unique assortments in each gamete. This genetic variation is essential for natural selection, enabling populations to adapt to changing environments over generations.

Key Concept: Stage Identification

c) Anaphase

[Solution Description] During anaphase, the sister chromatids are separated and move towards opposite poles of the cell as spindle fibers shorten.

Key Concept: Key Features

b) Nuclear envelope disintegrates

[Solution Description] The beginning of metaphase is characterized by the complete disintegration of the nuclear envelope, allowing chromosomes to become fully visible and aligned at the cell equator.

Key Concept: Basic Definition

c) Production of diploid daughter cells with identical genetic material

[Solution Description] Mitosis is primarily responsible for producing diploid daughter cells with identical genetic material, which is essential for growth and development in multicellular organisms.

Key Concept: Basic Definition

b) Production of diploid cells with identical genetic complement

[Solution Description] The primary purpose of mitosis is to ensure that each daughter cell receives an identical genetic complement as the parent cell. This means that the resulting daughter cells are diploid and have the same genetic material, maintaining genetic stability across generations.

Key Concept: Hypothetical Scenarios, Real-world Application

b) Increased chance of chromosomal abnormalities or polyploidy.

[Solution Description]

If interkinesis were altered to include partial DNA replication, this could lead to duplications or anomalies in chromosome numbers once meiosis II concludes. Normally, interkinesis does not involve DNA replication to ensure halving of the genome content in gametes.

Partial replication during interkinesis could lead to polyploidy or unexpected combinations of alleles, potentially affecting genetic stability adversely. Such changes may disrupt normal gamete function, causing issues in offspring viability or fertility. Additionally, it might introduce complex anomalies not typically seen in natural conditions, impacting organism development negatively.

Key Concept: Stage Identification

c) Prophase

[Solution Description] Chromatin condensation begins during prophase, where the chromosomal material becomes visibly compacted to form mitotic chromosomes.

Key Concept: Meiotic Spindle Assembly, Homologous Chromosome Behavior

d) Diakinesis

[Solution Description] Spindle fibers play a crucial role in chromosome movement during cell division. In Prophase I, spindle assembly begins in the Diakinesis stage when they start attaching to kinetochores located at centromeres of bivalents. This marks the final preparation step preceding chromosome alignment during Metaphase I, ensuring the proper segregation of chromosomes in Anaphase I.

Key Concept: Comparison with Mitosis

b) Meiosis results in four genetically diverse haploid cells; mitosis results in two identical diploid cells

[Solution Description] Meiosis results in four haploid cells that are genetically different due to recombination and independent assortment, whereas mitosis produces two diploid daughter cells that are identical to the parent cell.

Key Concept: Definition Recall

b) Cytokinesis

[Solution Description] The process that divides the cytoplasm at the end of cell division is known as cytokinesis.

Key Concept: Interkinesis Function, Telophase I vs Telophase II

c) It provides time for reorganization without DNA replication.

[Solution Description] Interkinesis is a short phase between Telophase I and Prophase II, where typically no DNA replication occurs. The primary purpose of interkinesis is to provide a brief period for the cells to reorganize and prepare for the subsequent cell division events without undergoing DNA synthesis. This ensures the proper transition to Prophase II without altering the chromosome number set after reductional division.

Key Concept: Homologous Chromosomes

b) They undergo synapsis and crossing over

[Solution Description] In prophase I of meiosis, homologous chromosomes undergo synapsis, where they pair closely along their length, aligning gene by gene. This pairing allows for crossing over, which is the exchange of genetic material between non-sister chromatids of homologous chromosomes. This exchange increases genetic diversity among gametes.

Key Concept: Advanced Phase Dynamics, Complex Process Analysis

c) It stabilizes bivalent alignment on the metaphase plate

[Solution Description] Terminalisation of chiasmata during diakinesis ensures proper separation of homologous chromosomes during subsequent phases. It marks the end of crossover events allowing homologs to align correctly on the metaphase plate, facilitating accurate chromosome segregation.

Key Concept: Definition Recall

b) Cytokinesis

[Solution Description] The division of cytoplasm following nuclear division is known as cytokinesis.

Key Concept: Advanced Chromosome Dynamics, Mechanism Analysis

c) Aneuploidy resulting from unequal separation

[Solution Description] When microtubules from opposite poles do not correctly attach to kinetochores on homologous chromosomes, this leads to incorrect pulling forces during anaphase. The most plausible result is aneuploidy, where cells end up with abnormal numbers of chromosomes.

Key Concept: DNA Content Analysis

c) 4C

[Solution Description] During the S phase, the DNA content doubles from 2C to 4C. The DNA content remains at 4C throughout the G2 phase as no further replication occurs until mitosis.

Key Concept: Advanced Application, Critical Analysis

c) The resultant gametes will exhibit double the chromosome number.

[Solution Description] During normal meiosis, DNA replication occurs once before meiosis I, ensuring that each resulting gamete has half the original chromosome number. If replication occurs during interkinesis, the chromosome number would not be halved properly, resulting in gametes with twice the expected chromosome number. This happens because the chromosomes, already replicated before meiosis I, undergo another replication leading to diploid instead of haploid cells.

Key Concept: Real-World Application, Hypothetical Scenario

a) Formation of polyploid gametes

[Solution Description] If DNA replication occurred during interkinesis, instead of having sister chromatids entering prophase II, cells would contain replicated chromatids, doubling the expected DNA content entering meiosis II. This error could lead to the production of gametes with twice the normal number of chromosomes, causing polyploidy if fertilization occurs. Thus, the primary consequence would be the failure to maintain haploid chromosome numbers in daughter cells.

Key Concept: Basic Definition

c) Centromeres split and chromatids move to opposite poles

[Solution Description] Anaphase is marked by the splitting of centromeres and the movement of chromatids to opposite poles.

Key Concept: DNA Replication Timing

b) S phase

[Solution Description] The cell cycle is composed of several phases, including interphase and the M phase. Interphase includes the G1, S, and G2 phases. During the S phase, DNA replication occurs, making it crucial for the duplication of genetic material before mitosis.

Key Concept: Nuclear Envelope Formation

d) Nuclear envelope

[Solution Description]

During telophase, the nuclear envelope reforms around the chromosomes that have reached the spindle poles. This process marks the beginning of the formation of two separate daughter nuclei.

Key Concept: Chromosome Pairing

c) It allows for genetic recombination between chromatids.

[Solution Description] The pairing of homologous chromosomes during meiosis is crucial for the process known as crossing over or recombination. This leads to genetic variation in offspring by exchanging segments between non-sister chromatids.

Key Concept: Phase Transition

c) When they do not divide further

[Solution Description] Cells typically transition from G1 to G$_0$ when they no longer need to divide or when they are fully differentiated to perform specific functions without further cell cycle progression.

Key Concept: DNA Replication

b) S

[Solution Description] DNA replication occurs during the S phase of interphase.

Key Concept: Real-world Application, Enzyme Mechanism, Structural Changes

c) Increases genetic diversity, enhancing adaptability and survival

[Solution Description] Crossing over allows for exchange of genetic material between homologous chromosomes, creating new allele combinations. This increases genetic diversity within populations. Greater genetic variation improves the ability of species to adapt to changing environments, as it provides raw material for natural selection to act upon, potentially increasing survival and reproduction under environmental stresses.

Key Concept: Mechanism Analysis, Anaphase I vs Anaphase II

a) Anaphase I; Spindle fiber shortening

[Solution Description] In Anaphase I of meiosis, homologous chromosomes are separated and move to opposite poles. This movement is primarily facilitated by the shortening of spindle fibers attached to kinetochores on the chromosomes. The separation ensures that each pole receives one chromosome from each homologous pair, crucial for reducing the diploid chromosome number to haploid.