CELL DIVISION By Fulden Donangil

Figure 1

Types of Cell Division:

1-Mitosis

2-Meiosis

MITOSIS Mitosis is a kind of cell divison which occurs in somatic

cells and results in two daughter cells.

These daughter cells are identical to each other and to the parent cell.

Daughter cells are diploid which means that they they have 2 sets of chromosomes.

One set of chromosomes is donated from the mother and the other set of chromosome donated from father.

There are 5 stages of Mitosis

1-Interphase

2-Prophase

3-Metaphase

4-Anaphase

5-Telophase Figure 2

Interphase

There are 3 stages in interphase;

-G1: The cells grow and get bigger for the division.

-S : DNA is synthesized by the process of DNA Replication.

-G2 : Mitochondria and other organelles divide. Figure 3

Prophase Chromosomes become

shorter and fatter.

They move to opposite poles of the cell and spindle fibres are formed.

Nucleolus disappears.

Nuclear membrane breaks down

Figure 4

Metaphase

Chromosomes line up at the equator individually.

Spindle fibers attach to chromosomes at the centromere.

Figure 5

Anaphase Spindle fibres pull

chromosomes apart.

Microtubules shortening.

Centromere of each chromosomes breaks down.

Daughter chromosomes travels to opposite poles.

Figure 6

Telophase

All chromosomes are at opposite poles.

Spindle fibres break down.

Nuclear envelope reformed.

Chromosomes become invisible. Figure 7

Animation of Mitosis

http://www.cellsalive.com/mitosis.htm

MEIOSIS Meiosis is a kind of cell division which occurs in

gametes(sperms and eggs) and results in four daughter cells.

Daughter cells are genetically different from each other and from parent cells.

Daughter cells are haploid which means that they have one complete set of chromosomes.

During meiosis DNA replicates once but the nucleus divides twice. Therefore two division occur in meiosis.

Importance of Meiosis

It halves the chromosome number.

It is important to have a diploid cell with correct number of chromosome(46 chromosome).

It increases variation.

First Division of Meiosis

Prophase 1

Metaphase 1

Anaphase 1

Telophase 1

Prophase 1

Most of the steps are same as in the mitosis except;

Homologous chromosomes pair up to from bivalent consisting of 4 chromatids.

Two chromosomes can overlap with each other which is known as CROSSING OVER.

Metaphase 1

Most of the steps are same as mitosis except;

Spindle fibres move bivalent chromosomes to line up at the equator instead of single chromosomes.

Anaphase 1

The centromeres do not divide

One chromosome from each homologous pair moves to each end of the cell.

As a result, the chromosome number in each cell is half of the original.

Telophase 1

All of the steps are same as mitosis except;

Whole chromosome is produced instead of one sister chromatid.

Second Division of Meiosis

The second division of meiosis is completely the same as mitosis.

However at the end of Meiosis, 4 daughter cells each with half of the chromosome number of the original diploid cell are formed.

Stages of Second Division

Prophase 2Metaphase 2 Anaphase 2

Telophase 2

Animation of Meiosishttp://www.cellsalive.com/meiosis.htm

Questions 5 differences between mitosis and

meiosisMITOSIS MEIOSIS

2 daughter cells formed 4 daughter cells formed

Occurs in body cells Occurs in gametes

Daughter cells are diploid Daughter cells are haploid

Daughter cells are identical to each other

Daughter cells are different from each other

No crossing over of chromosomes

Crossing over of chromosomes in prophase 1

Questions(a) (i) The diagrams show some of the stages of mitosis. Arrange the letters A - D to give the correct sequence of stages.

Sequence D A C B

(ii) Describe the role of the spindle in mitosis:

(b) Sexual reproduction involves the fusion of gametes. Explain the importance of meiosis in the life cycle of a sexually reproducing organism.

It attaches centromeres and seperates the daughter chromatids.

Meiosis halves the chromosome number and increase the variation.

References

Edexcel AS Biology, A Pearson Company, 2008, page 148,149,150,158,159

Richard Llopis Garcia’s CD

THE END