NCERT Class 11 Biology Chapter 10 Notes Cell Cycle And Cell Division- Download PDF

Download NCERT Notes for Class 11 Biology Chapter 10 PDF

This chapter explains how cells grow, divide, and multiply to form new cells. Important stages like interphase, mitosis, and meiosis are also discussed. Students can download the NCERT notes for class 11 Biology chapter 10 PDF from the link given below. Studying from a downloaded PDF file helps in focusing on just the important points without wasting time. The NCERT Class 11 Biology Notes are important for quick and smart revision.

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Cell Cycle and Cell Division Class 11 Notes

Students will understand why cell division matters to keep life going and pass on genes. The chapter Cell Cycle and Cell Division talks about how cells grow, copy their DNA, and then divide into new cells. The Cell Cycle and Cell Division Class 11 Notes help them understand tough words and diagrams better.

What is the Cell Cycle?

The cell cycle refers to the chain of linear events that the cell goes through during its lifespan, from initiation to division into two daughter cells. It is a vital process in the growth, development, repair, and reproduction of all organisms. The cell cycle helps genetic material be accurately duplicated and transferred to ensure there is genetic stability. Accurate control of the cell cycle is necessary to avoid a process of unchecked cell growth that may result in conditions such as cancer.

Phases of the Cell Cycle

The cell cycle is broadly divided into two major phases:

1. Interphase (Preparation Phase)
   Interphase is the longest phase of the cell cycle, where the cell grows, replicates its DNA, and prepares for division. It is further divided into three sub-phases:

G₁ Phase (First Gap Phase):

• The cell grows in size and synthesises proteins and organelles.

• It prepares for DNA replication.

• The G₁ checkpoint ensures conditions are favourable for DNA synthesis.

S Phase (Synthesis Phase):

• DNA replication occurs, doubling the genetic material.

• Each chromosome duplicates to form two identical sister chromatids.

• The number of chromosomes remains the same, but the amount of DNA doubles.

G₂ Phase (Second Gap Phase):

• The cell continues to grow and prepares for mitosis.

• Proteins required for cell division are synthesised.

• The G₂ checkpoint ensures DNA replication is error-free before proceeding to mitosis.

2. M Phase (Mitotic Phase – Cell Division Phase)
The M phase is the division phase, where the duplicated genetic material is evenly distributed into two daughter cells. It consists of:

• Karyokinesis (Nuclear Division): Includes mitosis (for somatic cells) or meiosis (for gametes).

• Cytokinesis (Cytoplasmic Division): Division of the cytoplasm to form two separate daughter cells.

Mitosis: The Process of Somatic Cell Division

Mitosis is a form of cell division in which one parent cell divides to give rise to two genetically equivalent daughter cells, each having the same number of chromosomes as the parent cell. It takes place in somatic (body) cells and plays a critical role in growth, repair, and asexual reproduction.

Significance of Mitosis

• Retains the number of chromosomes (diploid to diploid).

• Preserves genetic stability by forming the same cells.

• Assists in tissue development and repair by replacing old or broken cells.

• Is very important in asexual reproduction in unicellular organisms such as Amoeba.

Stages of Mitosis

Mitosis consists of four primary stages:

1. Prophase (Initiation of Cell Division)

• Chromatin condenses into observable chromosomes, each made up of two sister chromatids united by a centromere.

• The nucleolus and nuclear membrane dissolve.

• Centrosomes migrate towards opposite poles, and the mitotic spindle (formed of microtubules) forms.

2. Metaphase (Chromosome Alignment)

• Chromosomes align on the equatorial plane (metaphase plate).

• Spindle fibres bind to the chromosomes' centromeres.

• This step is significant to ensure proper chromosome separation.

3. Anaphase (Chromatid Separation)

• Sister chromatids move apart as spindle fibres draw them toward opposite poles.

• After separation, each chromatid is a daughter chromosome.

• This guarantees each daughter cell has an equal number of chromosomes.

4. Telophase (Mitosis End)

• Chromosomes decondense into chromatin.

• The nucleolus and nuclear membrane reappear to form two distinct nuclei.

• The spindle fibres break down.

Cytokinesis: Final Step of Cell Division

After mitosis, the cytoplasm divides in a process called cytokinesis, forming two identical daughter cells.

• In animal cells, a cleavage furrow forms and pinches the cell into two.

• In plant cells, a cell plate forms, developing into a new cell wall.

Role of Mitosis in Growth, Repair, and Asexual Reproduction

• Mitosis increases the number of cells, helping in the growth of an organism from a single fertilised egg to a multicellular body.

• Essential for embryonic development and organ formation.

• Damaged or dead cells are replaced through mitosis (e.g., skin, blood, and liver cells).

• Helps in wound healing and tissue regeneration (e.g., starfish regrowing limbs).

• In unicellular organisms like bacteria, yeast, and amoebas, mitosis enables asexual reproduction (binary fission or budding).

• Allows plants to propagate through vegetative reproduction (e.g., potato tubers, strawberry runners).

Meiosis

Meiosis is a reproductive cell division that takes place in germ cells (sperm and egg cells) by halving the number of chromosomes. It produces a haploid (n) gamete from a diploid (2n) parent cell. This way, when a fertilisation process happens, the zygote resulting from it re-establishes a diploid number, thus ensuring stability in the species' chromosomes.

Meiosis consists of two successive divisions:

Meiosis I (Reductional Division)

The stages of meiosis I include

Prophase I:

• Chromatin condenses into chromosomes.

• Homologous chromosomes pair up (synapsis) to form tetrads.

• Crossing over (genetic exchange) occurs at chiasmata, ensuring variation.

• Nuclear membrane dissolves, and spindle fibres form.

Metaphase I:

• Homologous chromosome pairs align at the metaphase plate.

• Independent assortment of homologous chromosomes occurs, increasing variation.

Anaphase I:

• Homologous chromosomes separate and move to opposite poles, but sister chromatids remain together.

Telophase I and Cytokinesis:

• Chromosomes decondense, and two haploid daughter cells are formed.

Meiosis II (Equational Division)

The stages of meiosis II include:

Prophase II:

• Chromosomes condense again. Spindle fibres reappear.

Metaphase II:

• Chromosomes align at the metaphase plate, similar to mitosis.

Anaphase II:

• Sister chromatids separate and move to opposite poles.

Telophase II and Cytokinesis:

• Nuclear membranes reform, and four genetically unique haploid cells are produced.

How Meiosis Provides Genetic Variation

Meiosis adds genetic diversity by:

• Crossing Over (Prophase I): Homologous chromosomes swap genetic information, forming new combinations of alleles.

• Independent Assortment (Metaphase I): Random allocation of pairs of homologous chromosomes results in various genetic combinations.

Significance of Cell Cycle and Cell Division

The importance of the cell cycle and cell division is as follows:

Growth, Development, and Tissue Repair

• Mitosis facilitates organism growth by adding to the number of cells.

• Critical for embryonic development and organogenesis.

• Replaces dead or damaged cells, contributing to wound healing and tissue regeneration.

Function in Preserving Chromosome Number in Organisms

• Mitosis maintains daughter cells that receive the same genetic material, retaining the chromosome number.

• Meiosis halves the chromosome number in gametes, avoiding doubling in every generation and species stability.

Genetic Stability and Variations

• Mitosis ensures genetic stability by resulting in identical cells.

• Meiosis enhances genetic variety, which is essential for evolution and adaptation.

Differences Between Mitosis and Meiosis

The difference between Mitosis and Meiosis is given in the table below:

Also Read

• NCERT Notes for Class 11 Physics

• NCERT Notes for Class 11 Chemistry

• NCERT Notes for Class 11 Mathematics

Chapter 10 Previous Year Questions and Answers

Some of the questions which have come up in past years from the chapter are given below. To solve these questions effectively, students can once read the Class 11 Biology chapter 10 notes.

Question 1. Meiosis occurs in organisms during

Option 1. Sexual reproduction

Option 2. Vegetative reproduction

Option 3. Both sexual and vegetative reproduction

Option 4. None of the above

Answer :

In sexually reproducing animals, meiosis is a sort of cell division that lowers the number of chromosomes in gametes, also known as sex cells, or egg and sperm. Human body (or somatic) cells have two sets of chromosomes, one from each parent, making them diploid.

Hence, the correct answer is option (1) Sexual reproduction

Question 2. Which of the events listed below is not observed during mitosis?

Option 1. Crossing over

Option 2. Movement of centrioles to opposite poles

Option 3. The appearance of chromosomes with two chromatids joined together at the centromere.

Option 4. Chromatin condensation

Answer :

During prophase I of meiosis, homologous chromosomes exchange genetic material, a process known as crossing over.
Although it does not happen during mitosis, this procedure guarantees genetic variety in gametes.
No genetic material is exchanged during mitosis; instead, it is primarily concerned with the equal division of genetic material to create two daughter cells that are genetically identical.
During mitosis, other processes are seen, including spindle formation, sister chromatid separation, and chromosomal condensation.

Hence, the correct answer is option 1)Crossing over.

Question 3. A bivalent of meiosis I consists of

Option 1. Four chromatids and two centromeres

Option 2. Two chromatids and two centromeres

Option 3. Two chromatids and one centromere

Option 4. Four chromatids and four centromeres

Answer :

During meiosis, two homologous chromosomes couple up to produce a bivalent. When two homologous chromosomes pair, a bivalent comprises a total of four chromatids, since each chromosome is made up of two sister chromatids.
A bivalent, on the other hand, has two centromeres rather than four. This is because every chromosome has a single centromere, and the bivalent homologous chromosomes both maintain their own.

The answer is the option (1) Four chromatids and two centromeres

Features of the Class 11 Biology Chapter 10 Notes

The notes are designed to provide a clear understanding of the concepts covered in the chapter. These notes have several useful features, some of which are listed below:

• Explanations of each topic are provided in detail.

• Students can access the NCERT notes for class 11 Biology chapter 10 PDF to score well.

• In the Class 11 Biology chapter 10 notes, well-labelled and colourful diagrams are added to make learning enjoyable.

• The notes are prepared by subject experts according to the latest syllabus.

• The format of the Class 11 Biology chapter 10 notes is student-friendly.

Also Read:

• NCERT Solution
• NCERT Solutions for Class 11
• NCERT Solutions for Class 11 Biology

Chapter-Wise NCERT Class 11 Notes Biology

Below are all the chapter-wise links presented in a simple and easy-to-understand table format to help with quick revision and learning.