Grade 11 Biology Botany CELL DIVISION

 CELL DIVISION

CELL DIVISION:

The process of formation of more than one daughter cells from pre-existing cells is known as Cell

division.

CELL CYCLE:

The total changes that occurs between one cell division to other cell division is known as cell cycle.

Period of cell cycle vary in different cells. E.g. Bacterial cell cycle is of 20 minutes, epithelial cell has 8-10

minutes, etc. Cell cycle is divided into following phases

i) Interphase: It is the longest phase of cell cycle and physiologically most active stage. All

necessary materials are synthesized in this phase so is also called as Preparatory phase. It is

divided into 3 sub-phases:

a) Gap one (G1) phase: In this phase following changes occurs:

➢ Cell size increases

➢ Carbohydrates, lipids and proteins except histone and spindle fibre proteins are

synthesized.

➢ RNAs are formed.

b) Synthetic (S) phase: In this phase following changes occurs:

➢ DNA replicates.

➢ Histone protein is formed.

➢ Each chromosome has two chromatids joined by centromere.

c) Gap two (G2) phase: In this phase following changes occurs:

➢ Cell organelles replicates

➢ Protein for spindle fiber are synthesized.

➢ Cell stores energy.

ii) Mitotic phase: It is the final phase of cell cycle which consists of two sub-phases i.e.

Karyokinesis (division of nucleus) and Cytokinesis (division of cytoplasm). Karyokinesis

completes in four phases i.e. Prophase, Metaphase, Anaphase and Telophase. Cytokinesis

occurs by cell plate method in Plant cells while it occurs via cell furrow method in Animal

cells.

METHODS OF CELL DIVISION:

Cell division occurs by three ways i.e. amitosis, mitosis and meiosis.

I) AMITOSIS:

It is a type of cell division in which a cell divides directly without differentiation of

chromosomes and spindle fibres. It is the process of cellular division which majorly takes in

the lower organisms like bacteria. This type of cellular division is a primitive type of divisionin which the nucleus of the cell divides unequally and then the cytoplasm divides. That is,

the karyokinesis is followed by cytokinesis.

II) MITOSIS (EQUATIONAL CELL DIVISION):

It is a type of cell division in which a mother cell divides into two daughter cells having equal

number of chromosomes to that in mother cell. This type of cell division occurs in somatic

cells of organisms so is also known as Somatic cell division. Mitosis cell division completes in

following 3 stages:

1. Interphase:

During interphase cell prepares itself for upcoming division by

synthesizing all required materials such as biomolecules,

duplicating DNAs, RNAs and other cell organelles.

2. Karyokinesis:

The nuclear division completes in four sub-phases: Prophase, Metaphase, Anaphase and

Telophase.

a) Prophase:

It is the first and longest phase of karyokinesis and following

changes occurs during prophase:

➢ Shortening and thickening of chromosome.

➢ Each chromosome appears in rod shape with two

chromatids.

➢ Dissapearnace of nucleus and nucleolus.

➢ Start of formation of spindle fibre.b) Metaphase:

It is the second phase of karyokinesis and following changes

occurs during Metaphase:

➢ Formation of spindle fibre completes.

➢ All chromosomes arranges at equatorial plane and

centromere attaches to spindle fibre with help of

tractile fibril.

➢ Chromosomes lie so close that an apparent plate i.e.

Metaphasic plate is formed.

c) Anaphase:

It is the third phase of karyokinesis and following changes

occurs during Anaphase:

➢ Centromere of each chromosome divides so that each

sister chromatids becomes chromosome.

➢ Chromosomes moves toward opposite poles due to

contraction of spindle fibre. They take ‘U’, ‘V’, ‘L’, ‘J’,

and ‘I’ shapes during movement.

➢ At the end of anaphase, two groups of chromosomes

are formed in two poles.

d) Telophase:

It is the last phase of Karyokinesis and following changes occurs

during Telophase:

➢ The nuclear membrane and nucleolus re-appears.

➢ Two groups of chromosomes are organized into two

nuclei.

➢ The chromosome elongates and forms chromatin.

3. CYTOKINESIS:

It is the division of cytoplasm to form two daughter cells. Cell organelles are

distributed during cytokinesis. It takes place by cell plate method in plant cells

while in animal cell it occurs by cell furrow method.SIGNIFICANCES OF MITOSIS

➢ Mitosis is responsible for growth and development of multicellular organisms.

➢ It is a means of multiplication in unicellular organisms.

➢ It replaces old and worn out cells.

➢ Regeneration of lost part is due to mitosis cell division.

➢ Would or injury is healed by repeated mitosis cell division.

➢ It produces genetically identical cells.

III) MEIOSIS (REDUCTIONAL DIVISION):

It is a type of cell division in which a diploid mother cell divides into 4 haploid daughter cells having half

number of chromosomes as that in mother cell. This type of cell division occurs in reproductive cells at

the time of gamete formation. Meiosis cell division completes in following 3 stages:

1. Interphase:

During interphase cell prepares itself for upcoming division by

synthesizing all required materials such as biomolecules,

duplicating DNAs, RNAs and other cell organelles.

2. Karyokinesis:

In meiosis, nucleus divides twice and first nuclear division is Meiosis I while second is Meiosis II.

a) Meiosis I

This is the first nuclear division of meiosis. It is reductional division in which homologous

chromosomes are separated into two nuclei. Meiosis I completes in four phases:

I) PROPHASE I

It is complicated and longest phage of Karyokinesis. It is further divided into five

subphases-Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.

i) Leptotene: Following changes occurs during Leptotene:

➢ Size of nucleus increases.

➢ Shortening and thickening of chromosome.

➢ Chromosome appears in single chromatid form.

ii) Zygotene: Following changes occurs during Zygotene.

➢ Homologous chromosomes pairs up by process called Synapsis.

➢ Bivalents (paired homologous chromosome) is formed.

➢ Shortening and thickening of chromosomePachytene: Following changes occurs during Pachytene

➢ Nucleoprotein between sister chromatid dissolves and Chromosome

appears in two chromatid form (i.e. Tetrad formation).

➢ Non-sister chromatid overlap or coil each other.

➢ Crossing over takes place (i.e. exchange of short segment of

chromatid).

iv) Diplotene: Following changes occurs during Diplotene:

➢ Non-sister chromatids starts separating.

➢ They separate except in region of chaismata.

➢ Nuclear membrane and nucleolus start disappearing.

v) Diakinesis: Following changes occurs during Diakinesis:

➢ Chaismata moves towards ends of chromosome due to condensation

of chromosome (Terminalisation).

➢ Nuclear membrane and nucleolus completely disappear.

➢ Spindle fibre starts appearing.

II) METAPHASE I:

Following changes occurs during Metaphase I.

➢ Formation of spindle fibre is completed.

➢ Bivalents arrange themselves in equator in two planes forming two

metaphasic plates.

➢ Centromere attaches with spindle fibre.

III) ANAPHASE I: Following changes occurs during Anaphase I.

➢ Homologous chromosome separate from one another and pass to opposite

poles.

➢ Centromeres of chromosomes do not break so each chromosomes bears Ptwo

chromatids and are called Dyads.

➢ At end of anaphase I two haploid group of chromosomes are formed.

IV) TELOPHASE I: Following changes occurs during Telophase I.

➢ Nuclear membrane and nucleolus reappear.

➢ Spindle fibre disappear.

➢ Chromosomes elongates and and are organized into two haploid nuclei.

➢ In some cases Telophase I is absent.Meiosis II: In meiosis II number of chromosome remains same as after meiosis I. It is similar

to mitosis so is also called Meiotic Mitosis or homotypic division. It is divided into Prophase

II, Metaphase II, Anaphase II and Telophase II.

I) PROPHASE II:

➢ Shortening and thickening of dyad chromosomes.

➢ Disappearance of nuclear membrane and nucleolus.

➢ Spindle fibre starts to appear.

II) METAPHASE II:

➢ Spindle fibre is completely formed.

➢ Dyad chromosomes arrange themselves in equatorial plane.

➢ Centromere attaches with spindle fibres.

III) ANAPHASE II:

➢ Centromere of dyad chromosomes break so sister chromatids becomes

chromosomes.

➢ Sister chromosomes move towards opposite poles.

➢ At end of Anaphase II, four groups of chromosomes are formed each having

haploid number of chromosomes.

IV) TELOPHASE II:

➢ Four groups of chromosomes arranges into four haploid nuclei.

➢ Chromosme elongates into Chromatin.

➢ Spindle fibre disappears.

➢ Nuclear membrane and nucleolus reappears.

3. Cytokinesis:

Cytokinesis occurs either by Successive method or Simultaneous method. In successive method,

each karyokinesis is followed by cytokinesis s oafter Meiosis I two haploid cells are formed whilein Simultaneous method, cytokinesis takes place only after meiosis II and four haploid daughter

cells are formed.

SIGNIFICANCES OF MEIOSIS:

➢ It is responsible for formation of gametes.

➢ Meiosis maintains fix number of chromosome in sexually reproducing organisms.

➢ Crossing over produces new combination of genes.

➢ Mutation may occur due to irregularities in Meiosis.

➢ It is necessary for production of spores.

➢ It brings variation in organisms.