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NCERT Solutions Class 11 Biology Chapter 10 - Cell Cycle and Cell Division
NCERT Solutions for Class 11 Biology Chapter 10 (Cell Cycle and Cell Division) explore the process by which a single cell grows and divides to form a complex multicellular organism. Growth and reproduction—the hallmarks of living things—are entirely dependent on the cell cycle. This chapter details the precise orchestration of DNA replication and chromosome segregation.
NCERT Solutions for Class 11 Biology Chapter 10 by ALLEN are designed by expert faculty to simplify the stages of nuclear and cytoplasmic division. The solutions focus on the "checkpoints" of the cell cycle and the intricate dance of chromosomes during Mitosis and Meiosis, making these abstract microscopic processes easy to visualize and understand. For NEET aspirants, this chapter is highly scoring but requires conceptual clarity, especially regarding the reduction of chromosome numbers in Meiosis. These solutions provide the technical precision needed to master the phases of Prophase I (Pachynema, Diplonema, etc.), which are frequent targets for high-level medical entrance questions.
1.0NCERT Solutions for Class 11 Biology Chapter 10 Cell Cycle and Cell Division: Key Concepts
This chapter examines the sequence of events by which a cell duplicates its genome and eventually divides into two daughter cells. The key lessons include:
- Cell Cycle: Understanding the Interphase (G1, S, and G2 phases) and the M Phase (Mitosis).
- M Phase (Mitosis): Studying the stages of nuclear division (Karyokinesis):
- Prophase: Chromosomal material condenses.
- Metaphase: Chromosomes align at the equatorial plate.
- Anaphase: Centromeres split and chromatids move to opposite poles.
- Telophase: Nuclear envelope assembles around chromosome clusters.
- Cytokinesis: Understanding how the cytoplasm divides in plant cells (cell plate formation) vs. animal cells (furrowing).
- Significance of Mitosis: Role in growth, cell repair, and maintaining the nucleo-cytoplasmic ratio.
- Meiosis: Detailed study of the specialized division that reduces chromosome number by half.
- Meiosis I: Understanding Prophase I and its five sub-stages: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
- Meiosis II: Similar to mitosis, resulting in four haploid daughter cells.
- Significance of Meiosis: Understanding how it conserves chromosome number across generations and introduces genetic variation through crossing over.
2.0NCERT Solutions Class 11 Biology Chapter 10 -Cell Cycle and Cell Division: Detailed Solutions
- What is the average cell cycle span for a mammalian cell?
Ans. 24 hours.
- Distinguish cytokinesis from karyokinesis.
Ans. Differences between cytokinesis and karyokinesis are:
- Describe the events taking place during the interphase.
Ans. The interphase, though called the resting phase, is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner. The interphase is further divided into three phases:
- (Gap 1) phase
- S (Synthesis) phase
- (Gap 2) phase
phase corresponds to the interval between mitosis and initiation of DNA replication. During phase the cell is metabolically active and grows continuously but does not replicate its DNA. S or synthesis phase marks the period during which DNA synthesis or replication takes place. During this time the amount of DNA doubles per cell. In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm.
During the phase synthesis of DNA stops while cell growth continues with synthesis of protein and RNA in preparation for mitosis.
- What is (quiescent phase) of cell cycle?
Ans. phase is the phase of inactivation of cell cycle due to non-availability of mitogens and energy rich compounds. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.
- Why is mitosis called equational division?
Ans. The number of chromosomes in the parent and progeny cells is the same, so mitosis is called as equational division.
- Name the stage of cell cycle at which each one of the following events occur: (i) Chromosomes are moved to spindle equator. (ii) Centromere splits and chromatids separate. (iii) Pairing between homologous chromosomes takes place. (iv) Crossing over between homologous chromosomes takes place.
Ans.
(i) Metaphase (ii) Anaphase (iii) Zygotene of prophase I of meiosis 1 (iv) Pachytene of prophase I of meiosis I
- Describe the following: (a) Synapsis (b) Bivalent (c) Chiasmata Draw a diagram to illustrate your answer.
Ans.
(a) Synapsis: During zygotene of prophase I stage homologou s chromosomes start pairing together and this process of association is called synapsis. Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of complex structure called synaptonemal complex. (b) Bivalent: The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad i.e., 4 chromatids or a pair of chromosomes. (c) Chiasmata: The beginning of diplotene is recognized by the dissolution of the synaptonemal complex and the tendency of the synapsed homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers. These X-shaped structures are called chiasmata.
- How does cytokinesis in plant cells differ from that in animal cells? Ans. Plant cytokinesis and animal cytokinesis differ in following respects:
- Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size. Ans. During formation of male gametes (i.e., spermatozoa) in a typical mammal (i.e., human being), the four daughter cells formed from meiosis are equal in size. On the other hand, during formation of female gamete (i.e., ovum) in a typical mammal (i.e., human being), the daughter cells are unequal in size.
- Distinguish anaphase of mitosis from anaphase I of meiosis. Ans. Anaphase of mitosis : The chromosomes move towards the spindle poles with the centromeres projecting towards the poles and the limbs trailing behind. There is corresponding shortening of chromosome fibres. The two pole-ward moving chromosomes of each type remain attached to each other by interzonal fibres. Ultimately, two groups of chromosomes come to lie at the spindle poles.
Anaphase I of meiosis : Chiasmata disappear completely and the homologous chromosomes separate. The process is called disjunction. The separated chromosomes (univalents) show divergent chromatids and are called dyads. They move towards the spindle poles and ultimately form two groups of haploid chromosomes.
- List the main differences between mitosis and meiosis.
Ans.
- What is the significance of meiosis?
Ans. Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process, per se, paradoxically, results in reduction of chromosome number by half. It also increases the genetic variability in the population of organisms from one generation to the next. Variations are very important for the process of evolution.
- Discuss with your teacher about
(i) haploid insects and lower plants where cell division occurs, and (ii) some haploid cells in higher plants where cell division does not occur.
Ans.
(i) Cell division occurs in haploid insect, such as drones of honey bee and lower plant like gametophyte of algae, bryophytes, and pteridophytes.
(ii) Synergids and antipodals in embryo sac of ovule are haploid cells where cell division does not occur.
- Can there be mitosis without DNA replication in S-phase? Ans. No there cannot be any mitotic division without-DNA replication in 'S' phase.
- Can there be DNA replication without cell division? Sol. Yes. polyploidy is the multiplication of chromosomes present in a set in nucleus without karyokinesis and cytokinesis result-ing in numerous copies of set within cell. This increase in sets of chromosomes is called polyploidy.
- Analyse the events during every stage of cell cycle and notice how the following two parameters change. (i) number of chromosomes (N) per cell-B (ii) amount of DNA content (C) per cell-B Ans. Number of chromosomes and amount of DNA change during S-phase and anaphase of cell cycle. S or synthesis phase marks the period during which DNA synthesis or replication takes place. During this time the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C then it increases to 4C. However, there is no increase in the chromosome number; if the cell had diploid or 2N number of chromosomes at , even after S phase the number of chromosomes remains the same, i.e., 2N. In mitotic anaphase, number of chromosomes remains the same. It is only sister chromatids which move towards their respective poles. DNA content remains unchanged. In anaphase I of meiosis, number of chromosomes are reduced to half, i.e., from 2N to IN and also DNA content decrease to one half i.e., from 4C to 2C. In anaphase II of meiosis II DNA content decreases to one half from 2C to 1C but chromosome number remain same.
3.0Key Features of NCERT Solutions for Class 11 Biology Chapter 10
- Ploidy and DNA Content Clarity:
The solutions provide clear explanations of how DNA content ($C$) and chromosome number ($n$) change during the S phase and Meiosis, a common point of confusion for students. - Prophase I Breakdown:
Expertly detailed descriptions of the sub-stages of Prophase I, highlighting key events like Synapsis (Zygotene) and Crossing over (Pachytene). - Mitosis vs. Meiosis Comparison:
Detailed tables contrasting the two types of division, focusing on the site of occurrence, number of daughter cells, and genetic outcome. - Cytokinesis Mechanism:
Step-by-step logic of how plant cells form a cell plate due to the presence of a rigid cell wall, compared to the centrifugal furrowing in animal cells. - Prepared by Expert Faculty:
Prepared by ALLEN subject experts, ensuring that every biological term and stage description is accurate and aligned with the latest NCERT/NEET syllabus. - Diagram-Oriented Precision:
The solutions emphasize identifying phases through chromosomal alignment (e.g., Metaphase plate), aiding in practical exams and MCQ-based identification. - Complete Coverage of all NCERT questions:
Every question is answered with the depth required to understand the continuity of life at the cellular level.