CHAPTER 10 : CELL CYCLE AND CELL DIVISION

K C MEENA

PRINCIPAL KV JAMALPUR

1

12/31/2021

Cell cycle

• It is a series of events that takes place in a cell, leading to the formation of two daughter cells from a single mother cell.
• Cell cycle is divided into two basic phases: Interphase and M phase Phases of cell cycle.
• Interphase
• M phase (mitosis phase) karyokinesis and cytokinesis

​

• Interphase
• G1 phase
• S phase
• G2 phase
• Go phase-quiescent stage

​

• Mitotic phase
• Karyokinesis (nuclear division): – Prophase, Metaphase, Anaphase and Telophase.

​

• Cytokinesis (division of cytoplasm)

2

12/31/2021

3

12/31/2021

Interphase

• Interphase involves a series of changes that prepares the cell for division. It involves the period of cell growth and cell division in an orderly manner.
• It is divided into three phases:

​

• G1 phase – It involves growth of cell and preparation of DNA for replication.

​

• S phase – It involves DNA synthesis. The amount of DNA doubles, but the chromosome number remains the same.

​

• G2 phase – It involves protein synthesis and further growth of cell, which prepares it for division.

​

• G0 phase or Quiescent phase – In adult animals, some cells do not divide or may divide occasionally. These cells do not divide further and exits the G1 phase to enter an inactive stage called Quiescent Stage (G0) of cell cycle.

​

4

12/31/2021

Mitosis

• It is a process of cell division where chromosomes replicate and get equally distributed into two daughter cells. Hence, it is also called equational division.
• The process of mitosis keeps the chromosome number equal in daughter as well as parental cell.
• Mitosis usually takes place in somatic cells.

5

12/31/2021

Phases of Mitosis

• Mitosis involves four stages:

Prophase

• It involves initiation and condensation of chromosomes.
• Nucleolus and nuclear membrane disappears.

​

Metaphase

• hromosomal material condenses to form compact chromosomes that get aligned in the middle of nucleus at equatorial plate.

6

12/31/2021

Anaphase

• Centromere splits and chromosomes move apart towards two opposite poles due to shortening of spindle fibres.

​

Telophase

• Chromosomes finally reach their respective poles.
• Nuclear envelope assembles around each chromosome clusters.
• Nucleolus and other organelles reform.

​

7

12/31/2021

8

12/31/2021

9

12/31/2021

CELL CYCLE AND MITOSIS VIDEO

Karyokinesis and Cytokinesis

• Karyokinesis is the division of nucleus during mitosis or meiosis which is followed by cytokinesis.

​

• Cytokinesis involves the division of cytoplasm of a cell.

​

• Cytokinesis is achieved in animal cell by cleavage, which deepens and divides the cell into two.

​

• It is achieved in plant cell by cell plate formation.

​

• When karyokinesis is not followed by cytokinesis, a multinucleated condition arises. This is called Syncytium.

10

12/31/2021

11

12/31/2021

Significance of mitosis

• Results in formation of diploid genetically identical daughter cells
• Growth of the body takes place by mitosis.
• Cell repair and replacement of worn out tissues
• Maintenance of nucleo-cytoplasmic ratio
• Vegetative reproduction in plants takes place by mitosis.

12

12/31/2021

II Meiosis

• It is the process which involves the reduction in the amount of genetic material.
• It mainly occurs in germ cells.
• At the end of meiosis II, four haploid cells are formed.
• It is comprised of two successive nuclear and cell division with a single cycle of DNA replication.
• The phases of meiosis are as shown below-

13

12/31/2021

14

12/31/2021

15

12/31/2021

Meiosis I

1. Prophase I – It comprises of 5 stages:

i. Leptotene

• Chromosomes start condensing.

ii. Zygotene

• Pairing of chromosomes called synapsis occurs.
• A pair of synapsed homologous chromosomes is called bivalent or tetrad.

iii. Pachytene

• Exchange of genetic material (crossing over) between non-sister chromatids occurs.
• Chiasmata formation

iv. Diplotene

• Bivalents formed during pachytene separate from each other (except at chiasmata) due to dissolution of synaptonemal complex.

v. Diakinesis

• Terminalisation of chiasmata can be observed.
• By the end of this stage, the nucleolus disappears and the nuclear envelope breaks.

16

12/31/2021

2. Metaphase I

• Bivalents (tetrad) get aligned along metaphase plate through spindle fibres.

3. Anaphase I

• Homologous chromosomes separate while chromatids remain attached at their centromere.

4. Telophase

• Nucleolus and nuclear membrane reappear around chromosome clusters at each pole.
• Inter-kinesis – It is the stage between two meiotic divisions.

17

12/31/2021

Meiosis II�

1. Prophase II

• Chromosomes become compact.
• Nuclear membrane disappears.

2. Metaphase II

• Chromosomes align at the equator.
• Kinetochores of sister chromatids attach to spindle fibres at each pole.

3. Anaphase II

• Chromatids separate by splitting of centromere.
• As a result, chromatids move towards their respective poles in the cell.

4. Telophase II

• Nuclear envelope and nucleolus reform around the chromosome clusters.

​

Cytokinesis:

• After meiosis II, the process of cytokinesis results in the formation of four haploid cells.

​

​

18

12/31/2021

Significance of meiosis:

• It results in reduction of chromosome number by half in gametes, which again doubles during fertilization. Therefore, it helps to conserve the chromosome number of species from generation to generation.
• Crossing-over, occurring in pachytene stage of meiosis I, is a source of genetic variation in sexually reproducing organisms.
• The variation thus formed helps in evolution.

19

12/31/2021

Differences between Mitosis and Meiosis:

MITOSIS

MEIOSIS

• It occurs in Somatic cells.
• It gives 2 daughter cells
• Cells formed are diploid (2n)
• During Metaphase, chromosomes are not lined in pair.
• During Prophase no recombination takes place
• During Anaphase, centromere splits and chromatids move towards opposite poles
• It takes very short duration.
• Only one division occurs and forms two diploid daughter cells

• It occurs in Sex cells.
• It gives 4 gametes
• Cells formed are haploid (n)
• During Metaphase I, homologous chromosomes are lined up in pairs.
• During Prophase I, recombination / crossing over occur between homologous chromosomes.
• During Anaphase I, homologous chromosomes separate and Anaphase II, centromere splits and chromatids move towards opposite poles.
• It takes very long duration.
• Two divisions occur and form 4 haploid cells.

20

12/31/2021

21

12/31/2021

Thanking you