Biology chapter no 7: Cell Division

 

Cell Cycle and Cell Division – Grade 7

Life of all multicellular organisms starts from a single cell called a zygote. A zygote is formed after fertilization when male and female gametes fuse. From this single cell, the whole body of an organism develops.

Growth of every living organism depends on cell division. New cells are formed when old cells divide. According to cell theory, every new cell arises from a pre-existing cell.

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7.1 Cell Cycle

The sequence of events that occurs in the life of a cell from one division to the next is called the cell cycle.

The cell cycle has two main phases:

• Interphase – preparation phase
• M-phase – division phase

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Interphase

Interphase is the stage between two successive cell divisions. It is the longest phase of the cell cycle. During this phase, the cell is very active and prepares itself for division.

Interphase is divided into three sub-phases:

• G₁ phase
• S phase
• G₂ phase

G₁ Phase (First Growth Phase)

This phase begins immediately after cell division. The cell grows in size and carries out normal life activities.

• RNA synthesis (mRNA, rRNA and tRNA)
• Protein synthesis
• Synthesis of cell membranes

S Phase (Synthesis Phase)

This phase is called synthesis phase because DNA is synthesized or replicated. As a result, the amount of DNA in the cell becomes double.

• DNA replication occurs
• Histone proteins are synthesized

G₂ Phase (Second Growth Phase)

In this phase, the cell prepares for division. The nucleus increases in size and important metabolic activities occur.

• Proteins required for cell division are formed
• RNA synthesis continues
• In animal cells, centrioles duplicate

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M-Phase (Period of Division)

The M-phase is the division phase. The letter “M” stands for mitosis or meiosis.

It consists of two processes:

• Karyokinesis – division of nucleus
• Cytokinesis – division of cytoplasm

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7.2 Types of Cell Division

There are three types of cell division found in animal cells:

• Amitosis
• Mitosis
• Meiosis

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A. Amitosis (Direct Division)

Amitosis is the simplest type of cell division. The nucleus elongates and forms a constriction. This constriction deepens and divides the nucleus into two unequal parts.

After nuclear division, cytoplasm also divides, forming two daughter cells.

Amitosis occurs in:

• Unicellular organisms
• Old cells
• Abnormal cells
• Foetal membrane cells

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B. Mitosis (Indirect Division)

Mitosis is a type of cell division in which one cell divides to form two identical daughter cells. The daughter cells are exactly similar to the parent cell.

Mitosis occurs in two steps:

• Karyokinesis
• Cytokinesis

Stages of Karyokinesis

1. Prophase

• Chromatin condenses into chromosomes
• Each chromosome has two sister chromatids joined by centromere
• Nucleolus disappears
• Nuclear membrane breaks down
• Centrioles move to opposite poles
• Spindle fibres begin to form

2. Metaphase

• Chromosomes become short and thick
• Chromosomes arrange at the equatorial plane
• This plane is called metaphase plate
• Spindle fibres attach to centromeres

3. Anaphase

• Centromere divides
• Sister chromatids separate
• Daughter chromosomes move to opposite poles
• Chromosomes appear like banana-shaped structures

4. Telophase

• Chromosomes uncoil and lose identity
• Nuclear membrane reappears
• Nucleolus reforms
• Spindle fibres disappear
• Two daughter nuclei are formed

Cytokinesis

Cytokinesis is the division of cytoplasm. In animal cells, a cleavage furrow forms at the equator. In plant cells, a cell plate is formed due to the presence of a rigid cell wall.

Organelles like mitochondria and plastids are equally distributed.

Significance of Mitosis

• Maintains constant chromosome number
• Helps in growth and development
• Replaces old and damaged cells
• Helps in asexual reproduction
• Maintains genetic stability

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Death of Cell

Cell death can be of two types:

• Necrosis – uncontrolled cell death due to injury
• Apoptosis – programmed cell death

Apoptosis is beneficial. For example, cells between fingers die during embryonic development to form separate fingers.

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C. Meiosis (Reduction Division)

Meiosis occurs in reproductive cells during formation of gametes. The chromosome number is reduced to half. Four haploid daughter cells are formed.

Meiosis I (Heterotypic Division)

This division reduces chromosome number from diploid to haploid.

Prophase I

• Leptotene – chromosomes become visible
• Zygotene – homologous chromosomes pair
• Pachytene – crossing over occurs
• Diplotene – chiasmata become visible
• Diakinesis – terminalization of chiasmata

Metaphase I

Homologous chromosomes arrange at equator.

Anaphase I

Homologous chromosomes separate and move to opposite poles.

Telophase I and Cytokinesis I

Two haploid cells are formed.

Meiosis II (Homotypic Division)

This division is similar to mitosis. Each haploid cell divides again.

Four haploid daughter cells are formed.

Significance of Meiosis

• Formation of gametes
• Maintains chromosome number in species
• Produces genetic variations
• Helps in evolution