Class 10 | Science 2 | Easy notes on chapter 2 Life processes in living organisms part -1

Class 10 | Easy notes on chapter 2 Life processes in living organisms part -1 

CELL DIVISION

• Cell division is one of the very important properties of cells and living organisms.

Only because of this property:

• new organisms are formed from existing ones

• growth of organisms happens

• damaged body can be repaired (e.g. wound)

Types of cell division :

1. Mitosis and 

2. Meiosis.

• Mitosis occurs in somatic 

cells and stem cells.

• Meiosis occurs in germ cells. 

MITOSIS (Mitotic cell division)

• Somatic cells and stem cells  divide by mitosis. 

• Mitosis is completed through two main steps :

1. Karyokinesis (nuclear division) and 

2. Cytokinesis (cytoplasmic division).

1. Karyokinesis :

Karyokinesis is completed through four steps :

A. Prophase

B. Metaphase

C. Anaphase

D. Telophase

A. Prophase : 

• In prophase, Due to condensation of basically thin thread-like chromosomes, they become short and thick and they start to appear along with their pairs of sister chromatids. 

• Centrioles duplicate and each centriole moves to opposite poles of the cells.

• Nuclear membrane and 

nucleolus start to disappear.

B. Metaphase :

• Nuclear membrane completely disappears in metaphase.

• Chromosomes complete their condensation and become clearly visible along with their sister chromatids.

• All chromosomes are arranged parallel to equatorial plane (central plane) of the cell.

• Special type of flexible protein fibers (spindle fibers) are formed between centromere of each chromosome and both centrioles.

C. Anaphase :

• In anaphase, centromeres split and thereby sister chromatids of each  chromosomes separate.

They are pulled apart in opposite directions with the help of spindle fibers.

• Separated sister chromatids are called as daughter chromosomes.

• Chromosomes being pulled appear like bunch of bananas. In this way, each set of 

chromosomes reach at two opposite poles of the cell.

D. Telophase :

• The chromosomes which have reached at opposite poles of the cell start to decondense due to which they again become thread-like thin and invisible.

• Nuclear membrane is formed around each set of chromosomes reached at poles.

• Thus, two daughter nuclei are formed in a cell.

• Nucleolus also appears in each daughter nucleus.

• Spindle fibers completely disappear.

In this way, karyokinesis completes and cytokinesis

begins.

2. Cytokinesis :

• The cytoplasm divides by cytokinesis and two new cells are formed which are called as daughter cells.

• In this process, a notch is formed at the equatorial plane of the cell which deepens gradually and thereby

two new cells are formed.

• In case of plant cells, instead of the notch, a cell plate is formed exactly along midline of the cell and thus cytokinesis is

completed.

• Mitosis is essential for growth of the body. Besides, it is necessary for restoration of

emaciated body, wound healing, formation of blood cells, etc.

Some terms

• Somatic cells : Cells other than reproductive cells

• Haploid cells : Cells having single set of chromosomes.

• Gametes (or reproductive cells) are haploid cells.

• Diploid cells : They have 2 copies of each chromosome. One set comes from father while the other comes from mother.

• Somatic cells – i.e. non-

reproductive cells of the body – are diploid cells.

For example, our skin cells.

• Homologous chromosomes : One chromosome of each homologous pair comes from the mother and one comes from the father. Homologous chromosomes are similar but not identical.

• Centriole : The organelle that duplicates during cell

division and forms spindle fibers (protein fibers)

• Centromere : Region near the middle of a chromosome where 2 sister chromosomes connect.

MEIOSIS :

• Meiosis is completed through two stages.

1. meiosis-I

2. meiosis-II

1. meiosis-I :

• In meiosis-I, recombination / crossing over occur between homologous chromosomes

and these homologous chromosomes divided into two groups and thus two haploid cells are formed.

2. Meiosis-II :

Meiosis-II is just like mitosis. In this stage, the two haploid daughter cells formed in

meiosis-I undergo division by separation of recombined sister chromatids and four haploid daughter cells are formed.

• Process of gamete production and spore formation occurs by

meiosis.

• In this type of cell division, four haploid (n) daughter cells are formed from one diploid (2n) cell.

• During this cell division, crossing over occurs between the homologous chromosomes and thereby genetic recombination occurs.

• Due to this, all the four daughter cells are genetically different from parent cell and from each other too.