Cell Division

Purposes of cell division Increase the number of cells for growth and repair of worn out tissues Transmit genetic information to later generations Reproduction and heredity

What's inside the nucleus? chromosome nucleus chromatins

What is a chromosome? Genetic materials found inside the nucleus of a cell Made of protein and DNA DNA controls protein synthesis  Control the appearance of characters and metabolic activities of an organism

A chromosome carries a lot of genes A gene is a short length of DNA on chromosome which controls an inherited character of the organism gene controlling skin colour gene controlling tongue rolling chromosome gene controlling eye colour A chromosome carries a lot of genes gene controlling blood group

Homologous chromosomes Each species has a fixed number of chromosomes in the nucleus of each of its cells Chromosomes always exist in pairs in the body (somatic) cells Homologous chromosomes Each human somatic cell has 46 chromosomes 23 pairs of homologous chromosomes

Members of homologous chromosomes carry same genes But the genes on the members of homologous chromosomes may be of different forms  Alleles allele for white skin colour allele for white skin colour

Alleles Alternative forms of genes on the same position of the homologous chromosomes which control the same character but have different expressions

22 pairs are identical in both sexes autosomes The 23rd pair is different in male and female sex chromosomes

Steps of cell division Nuclear division Cytoplasmic cleavage

Types of nuclear division Mitosis Meiosis

Mitosis Produces two identical daughter cells Each daughter cell has the same kind and number of chromosomes as the original parent cell

Interphase (Resting stage) Chromosomes cannot be seen Chromosomes duplicate and double in number

nuclear membrane disappearing Prophase nuclear membrane disappearing chromatids centromere Chromosomes become visible Each chromosome consists of two identical chromatids homologous chromosomes

Metaphase Chromosomes line up at the equator (centre of the cell) Spindles are formed to attach to the centromere of each chromosome

Anaphase Sister chromatids separate as individual chromosomes They move apart towards the opposite poles

nuclear membrane forming Telophase nuclear membrane forming Chromosomes gradually disappear Nuclear membrane is form around each set of chromosomes

Interphase Animal cell Plant cell

Prophase Animal cell Plant cell

Metaphase Animal cell Plant cell

Anaphase Animal cell Plant cell

Telophase Animal cell Plant cell

Cytoplasmic division Division of cytoplasm cleavage furrow cell plate Division of cytoplasm Animal cells : by formation of cleavage furrow Plant cells : by formation of cell plate

Can you identify the different stages of mitosis from the diagram below? anaphase interphase telophase metaphase prophase

Self Learning Exercise Click the button for a photomicrograph showing the cells of a plant at different stages of mitosis Q: Arrange the above cells labelled 1 to 4 in the correct sequence of mitosis. A. 1 --> 2 --> 4 --> 3 B. 3 --> 2 --> 4 --> 1 C. 3 --> 4 --> 2 --> 1 D. 2 --> 3 --> 4 --> 1

Here is a photomicrograph showing the cells of a plant at different stages of mitosis : Back

This is not a correct sequence of mitosis. Sorry, you’ve got the wrong answer!!! This is not a correct sequence of mitosis. Try again...

This is not a correct sequence of mitosis. Sorry, you’ve got the wrong answer!!! This is not a correct sequence of mitosis. Try again...

Congratulation!!! You’ve got the correct answer!!! Click here

This is not a correct sequence of mitosis. Sorry, you’ve got the wrong answer!!! This is not a correct sequence of mitosis. Try again...

Self Learning Exercise Click the button for a photomicrograph showing the cells of a plant at different stages of mitosis Q: What is the DNA content of cell 1 as compared with that of cell 3? A. DNA content of cell 1 and cell 3 are the same. B. DNA content of cell 1 is doubled that in cell 3. C. DNA content of cell 1 is only half of that in cell 3. D. DNA content of cell 1 is only quarter of that in cell 3.

Here is a photomicrograph showing the cells of a plant at different stages of mitosis : Back

Sorry, you’ve got the wrong answer!!! Remember that chromosomes are made of DNA, and the chromosomes in cell 3 are duplicated. Try again...

Sorry, you’ve got the wrong answer!!! Remember that chromosomes are made of DNA, and the chromosomes in cell 3 are duplicated. Try again...

Congratulation!!! You’ve got the correct answer!!! Go to next question...

Sorry, you’ve got the wrong answer!!! Remember that chromosomes are made of DNA, and the chromosomes in cell 3 are duplicated. Try again...

Self-Learning Exercise Q: Click this button to see the information for answering this question : Which cells contain the same amount of DNA? A. 1 and 2 only B. 1 and 3 only C. 2 and 3 only D. 1, 2 and 3

Information : The photomicrograph below shows some plant cells at different stages of mitosis : Back to the question

Remember that the different stages of cell division shown is mitosis! Sorry, you’re wrong!!! Remember that the different stages of cell division shown is mitosis! Try again!!!

Remember that the different stages of cell division shown is mitosis! Sorry, you’re wrong!!! Remember that the different stages of cell division shown is mitosis! Try again!!!

Remember that the different stages of cell division shown is mitosis! Sorry, you’re wrong!!! Remember that the different stages of cell division shown is mitosis! Try again!!!

Congratulations!!! You’ve got the correct answer! Click here to end

Significance of mitosis To ensure that each daughter cell maintains exactly the same number and kind of chromosomes as the parent cell Mitosis is important in produce more cells for growth repair replacing dead and damaged cells asexual reproduction

Meiosis Producing cells with chromosome number half of the parent cell Cells having pairs of homologous chromosomes - diploid (2n) e.g. body (somatic) cells Cells having one chromosome from each homologous pair - haploid (n) e.g. gametes Two nuclear divisions Meiosis I and meiosis II Four haploid cells are produced

Prophase I Chromosomes become visible Nuclear membrane disappears

Prophase I Homologous chromosomes pair up Crossing-over may occur between homologous chromosomes

Metaphase I Homologous chromosomes lie up at the middle of the cell randomly

Anaphase I The 2 members of each homologous pair of chromosomes separate from each other and move to opposite poles of the cell

Telophase I Nuclear membrane reforms Followed by cytoplasmic cleavage Each cell has half the chromosome number as the parent cell

Second meiotic division Separation of chromatids of each chromosome 4 daughter cells with half of the chromosome number of the parent cells are formed

Occurrence of meiosis Plants: anthers and ovules Mammals: testes and ovaries

What would happen if gametes are formed by mitosis? Male (2n) Female (2n) Mitosis Sperm (2n) Egg (2n) Fertilization Zygote (4n) Mitosis Sperm/Egg (4n) Fertilization Zygote (8n)

Significance of meiosis Leads to halving of chromosome number, so to ensure that the diploid number of chromosomes can be restored after fertilization

Significance of meiosis Produce genetic variation at crossing-over between homologous chromosomes during prophase I independent assortment of chromosomes during metaphase I

Crossing-over during meiosis

Independent assortment

Sources of genetic variation Crossing-over between homologous chromosomes during meiosis Independent assortment of chromosomes during meiosis Random fusion of gametes during fertilization Mutation

Comparison between mitosis and meiosis Number of division One Two No. of daughter cell produced by one parent cell Two Four Type of cells produced Somatic cells Gametes Chromosome number of daughter cells Same as parent cell Half of parent cells Genetic make-up of daughter cells Identical to parent cell May be different from the parent cell

Comparison between mitosis and meiosis Pairing of homologous chromosomes No Yes Crossing-over No Yes Occurrence Growing tissues Reproductive tissues Role Growth, repair, replacement of old tissues, asexual reproduction Gamete formation for sexual reproduction