ICSE Class 10 Biology Chapter 1 Notes: Structure of Chromosomes, Cell Cycle and Cell Division

ICSE Class 10 Biology Chapter 1- Structure of Chromosomes, Cell Cycle, and Cell Division is one of the most important chapters in the Biology syllabus. This chapter helps students understand how cells grow, divide, and pass hereditary information from one generation to another.

This chapter consists of topics: cell cycle, mitosis, meiosis, chromosome structure, DNA, gene, and Mendel’s law of inheritance.

In this post, you will find detailed notes on each and every topic with explanations, definitions, and diagrams.

Structure of Chromosomes, Cell Cycle, and Cell Division Notes

 

Structure of Chromosomes

Discovery of Chromosomes

A German scientist named Walther Flemming discovered chromosomes in 1882 while studying the rapidly dividing cells of the larvae of the salamander (an amphibian)

During his observations, he noticed thread-like structures inside the nucleus that appeared to double during cell division. He named this process Mitosis.

What is a Chromosome?

Chromosomes are deeply staining, thread-like structures present inside the nucleus of every cell.

They are the carriers of hereditary material (DNA) from parents to offspring.

What is Chromatin?

When a cell is not dividing (Interphase), these threads are very long, thin, and tangled together like a ball of yarn. This network is called chromatin.

Chemical Composition of Chromatin

Chromatin is a mixture of two main components:

(i)   DNA (Deoxyribonucleic acid): - This is 40% of the chromatin. It contains the actual genetic code.

(ii)     Histones (Proteins): It makes up about 60% of the chromatin. These are special proteins that help in packaging and coiling the long DNA strands so they can fit inside the tiny nucleus.

Nucleosomes

i.     DNA is an extremely long molecule. It wraps around histones to fit inside a microscopic cell.

ii. Eight histone proteins come together to form a tiny core, like a little ball.

iii.  The DNA strand wraps around this core about two times.

iv.  This structural unit is called a nucleosome. It looks like a  “ bead on a string”. A single human chromosome contains about a million nucleosomes.

Structure of Chromosomes

When the cell enters division, the chromatin fibers condense, shorten, and thicken to form distinct chromosomes that can be observed under a microscope.

 A chromosome consists of two parts:

i.      Sister chromatids

Each chromosome is made up of two identical thread-like structures called sister chromatids.

Important features

     ·     They are formed after DNA replication during the S phase of Interphase.

    ·     Both chromatins contain identical genetic information because one chromatid is an exact copy of the other.

    ·     The chromatids lie parallel to each other.

    ·     Sister chromatids remain attached until the anaphase stage of cell division.

Role of sister chromatids

    ·     Ensure equal distribution of hereditary material into daughter cells.

    ·     Help maintain the number of chromosomes after cell division.

    ·     Carry genes responsible for inherited characteristics.

    ii.           Centromere

Features

i.The centromere is the narrow, constricted region where the two sister chromatids remain attached. It appears as a small joining point on the chromosome.

ii.It divides the chromosome into two arms. Its position differs on different chromosomes.

Functions

·     i.The centromere keeps the two chromatids attached as a single chromosome until they separate during division.

·     ii.At anaphase, the centromere splits and allows sister chromatids to move towards the opposite poles of the cell.

Genes

Genes are specific sequences of nucleotides located on a DNA molecule.

 They are the functional units of heredity. Every gene contains the instructions or “recipe” to make a specific protein. These proteins determine the body features like eye color, blood group, or height.

Structure of DNA

The molecular structure of DNA was discovered by Watson and Crick in 1953.

It looks like a twisted ladder called a double helix. DNA is a macromolecule made of thousands of repeating units called nucleotides. Each individual nucleotide is made of three components: (i) Phosphate group, (ii) Pentose sugar (Deoxyribose sugar), (iii) Nitrogenous base

The sides of the DNA ladder are made of alternative sugar and phosphate groups. The “rungs’ of the ladder are made of four types of nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine(C)

Adenine always pairs with Thymine with two hydrogen bonds, and Guanine pairs with Cytosine with three hydrogen bonds.

Cell Cycle and Cell Division

Why do cells need to divide?

Multicellular organisms like humans start life as a single cell ( the fertilized egg or zygote). Cells divide for four main reasons:

i.   Growth: To increase the size of the body by adding more cells.

ii.  Replacement:  To replace old, dead, or worn-out cells. Millions of red blood cells die every minute and must be replaced.

iii.    Repair: To heal wounds, cuts, or fractured bones by filling the damaged gap with new cells.

iv.   Reproduction: To produce special reproductive cells like sperm and eggs for counting the species.

What is the Cell Cycle?

The cell cycle is a sequence of events where a cell grows, duplicates its genetic material, and divides into two daughter cells. It is divided into three phases: (i) First growth phase, (ii) synthesis phase, and (iii) second growth phase.

i.   First Growth Phase (G₁): - The cell grows larger in volume. It synthesizes RNA and essential proteins. Cellular organelles (like mitochondria, chloroplasts) increase in number.

ii.   Synthesis phase (S phase): - DNA replication takes place. The amount of DNA doubles. Chromosomes form duplicates

iii.  Second Growth Phase: - This phase is shorter. More proteins and RNA are synthesized to prepare the cell structures needed for division.

Cell Division

Cell division is the process by which a parent cell divides to form new daughter cells. It is one of the most important biological processes because it helps in growth, repair, replacement of damaged cells, and reproduction.

In multicellular organisms, millions of cells divide continuously to maintain life processes. Every organism begins life as a single cell called a zygote, which undergoes repeated cell divisions to form a complete organism.

Importance of Cell Division

Cell division is necessary for the following reasons:

i.    Growth: An increase in the number of cells leads to growth of the body.

ii.   Repair: Damaged tissues and injured body parts are repaired through the formation of new cells.

iii. Replacement: Old, dead, and worn-out cells are continuously replaced.

iv.  Reproduction: Cell division helps in reproduction and the formation of gametes.

Types of Cell Division

There are two types of cell division:

1.    Mitosis

2.    Meiosis

    1. Mitosis

One parent cell divides to form two genetically identical daughter cells having the same chromosome number as the parent cell. It occurs in somatic or body cells.

Main features

     i.    The chromosome number remains unchanged.

     ii. A diploid(2n) parent cell produces two diploid(2n) daughter cells.

Importance of Mitosis

i.     Growth of the organism

ii.  Repair of tissues

iii. Replacement of damaged cells

iv.  Healing of wounds

v.     Asexual reproduction in some organisms

vi.  Maintains chromosome number

Phases of Mitosis

Mitosis consists of two major phases

i.     Karyokinesis (Division of the nucleus)

ii.  Cytokinesis (Division of cytoplasm)

i. Karyokinesis

It occurs in four stages: Prophase, Metaphase, Anaphase, and Telophase

(i)      Prophase: - This is the first phase of mitosis and the longest stage of mitosis. The following events occur in this stage:

    ·     The thin chromatin fibers condense, shorten, and thicken to form visible chromosomes.

    ·     Each chromosome consists of two identical sister chromatids joined at the centromere.

    ·     Nuclear membrane and nucleolus disappear.

    ·     Centrosome moves toward opposite poles and forms spindle fibers.

     ·     The cell prepares chromosomes for proper separation.

   (ii)    Metaphase: - This is the second stage of mitosis. It contains the following events:

    ·     Chromosomes arrange themselves along the equatorial plane or center of the cell.

    ·     Spindle fibers attach to chromosomes at the centromere.

    ·     Proper alignment ensures equal distribution of chromosomes.

   (iii)     Anaphase: -This is the third stage of mitosis. The following events occur in this stage:

    ·     The centromere divides into two parts.

    ·     Sister chromatids separate from each other.

    ·     Spindle fibers contract and pull daughter chromosomes toward opposite poles.

    ·     During the movement, chromosomes appear ‘V-shaped, J-shaped and L -shaped.

    ·     Equal distribution of hereditary material occurs.

   (iv)      Telophase: - This is the last stage of karyokinesis.

    ·     Daughter chromosomes reach opposite poles of the cell.

    ·     Chromosomes become thin chromatin threads again.

    ·     Nuclear membrane and nucleolus reappear.

    ·     Spindle fibres gradually disappear.

    ·     Two daughter nuclei are formed.

 

ii.      Cytokinesis

This is the division of the cytoplasm, resulting in the formation of two separate daughter cells.

(a)       In animal cells

    i.    A cleavage furrow appears at the equator.

    ii. The plasma membrane pinches inward.

    iii.     The cell divides into two daughter cells.

    iv.     The direction of division is centripetal(outside inward)

   (b)       In a plant cell

    i.     A cell plate develops at the center.

    ii.           The cell plate grows outward.

    iii.        A new cell wall forms between daughter cells.

    iv.         The direction of division is centrifugal (inside outward)

Result of Mitosis

 i.        Two genetically identical daughter cells are formed.

 ii. Chromosome number remains constant.

     2.  Meiosis

Meiosis is a type of cell division in which the chromosome number is halved. It occurs in reductive or germ cells for the formation of gametes.

Features of Meiosis

i.     Chromosome number is reduced to half.

ii.     A diploid parent cell produces four haploid daughter cells.

Importance of Meiosis

i.   It produces sperm and egg cells.

ii.   Maintains a constant chromosome number generation after generation.

iii.  Genetic variation among offspring is produced by crossing over and recombination.

Phases of Meiosis

Meiosis occurs in two successive divisions: (i) Meiosis I, (ii) Meiosis II

(i)    Meiosis I (Reduction division)

    i.    Chromosome number becomes half.

    ii. Homologous chromosomes pair together.

    iii.  Exchange of genetic material occurs between homologous chromosomes.

    iv.     Chromosomes move to opposite poles.

   (ii)      Meiosis II

This phase is the same as mitosis.

i.      Sister chromatids separate.

ii.     Four haploid daughter cells are formed.

Difference between Mitosis and Meiosis

Feature	Mitosis	Meiosis
Occurs in	Somatic cells	Germ cells
Number of divisions	One	Two
Daughter cells	2	4
Chromosome number	Same (2n)	Half (n)
Genetic makeup	Identical	Different
Crossing over	Absent	Present
Function	Growth and repair	Gamete formation

 

 

Frequently Asked Questions

Q1. What is a chromosome in Biology?

Answer

A chromosome is a thread-like structure in the nucleus of a cell. It contains DNA and proteins and carries hereditary information from parents to offspring.

Q2. What is the difference between chromatin and chromosomes?

Answer

Chromatin is a thin, thread-like network present in the nucleus during the resting stage of a cell, and chromosomes are condensed and visible structures formed during cell division.

Q3.  What are sister chromatids?

Answer

Sister chromatids are two identical copies of a chromosome that are joined together at the centromere. They are formed after DNA replication during the S phase of interphase.

Q4. What is the function of the centromere?

Answer

The centromere holds sister chromatids together and provides the attachment site for spindle fibres during cell division, ensuring proper chromosome separation.

Q5.  What is DNA, and why is it important?

Answer

DNA is the genetic material present in cells. It contains the instructions for the development, functioning, and inheritance of traits in living organisms.