Cell Division

Purposes of Cell Division Increase the number of cells for growth and repair of worn out tissues What examples in the human body can you think of? Transmit genetic information to later generations Why do you think this is important?

Whether for growth or reproduction, all cell division types rely on the ‘brain’ of the cell... the NUCLEUS The nucleus contains the most important parts of the cell with regards to genetics… the CHROMOSOMES

What's inside the nucleus? chromosome nucleus chromatins

What is a chromosome? Genetic material found inside the nucleus of a cell. Made of protein and DNA, bound together and wound up. DNA contains the genetic information (blueprint) of an organism. Each organism in a species has unique DNA, made up of genes.

A chromosome carries a lot of genes A gene is a short length of DNA on a 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. These are called… 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. Each form of a gene on each chromosome is called an allele. allele for brown skin colour – gene for skin colour allele for white skin colour – gene for skin colour

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

One homologous chromosome comes from your mother and one comes from your father. So your father might have given you the allele for blue eyes while your mother gave you the allele for brown eyes. However, the combination of the alleles allows for variation in the offspring.

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

Chromosome Number Each species has a unique number of chromosomes. In humans the full compliment chromosome number is 46. Each somatic cell contains 46 chromosomes and are called diploid (2n). Each gamete contains 23 chromosomes and are called haploid (n)

Steps of cell division Although there are more than 2 steps, essentially cell division is grouped divided into: Nuclear division Cytoplasmic cleavage

Types of nuclear division Mitosis – somatic (body) cells Meiosis – gametes (sperm and egg)

Mitosis Produces two identical daughter cells Each daughter cell has the same kind and number of chromosomes as the original parent cell. Used in somatic cells. Whole process is about 24 hours, with 20 of those being spent in interphase.

Interphase (Resting stage) Chromosomes cannot be seen because un-wound. Chromosomes duplicate and double in number. Each chromosome now has an identical copy. These identical copies are called SISTER CHROMATIDS. Sister chromatids join together at the center point which is called a centromere. Total number of chromosomes doubles from 46 to 92. Same in mitosis and meiosis. Not a dividing stage. Rather, a pre-stage to double up chromosome numbers before division.

Sister Chromatids vs Homologous Chromosomes Do not confuse these terms. Homologous chromosomes are chromosomes which contain the same genes but different alleles. Sister chromatids are identical copies of each individual chromosome.

Prophase Chromosomes contract and become darker and more visible Each chromosome now consists of two identical sister chromatids CENTRIOLES move towards the opposite ends (poles) of the cell MICROTUBULES extend 92 chromosomes total, 46 pairs

Metaphase Chromosomes line up at the equator (center of the cell) Spindles are formed to attach to the centromere of each chromosome 92 chromosomes total, 46 pairs

Anaphase Sister chromatids separate as individual chromosomes They move apart towards the opposite poles 92 chromosomes total, 46 pairs. 23 pairs moving to each side.

Telophase Chromosomes gradually disappear Nuclear membrane is forming around each set of chromosomes 23 pairs in each half of the cell in telophase, 46 in each cell. Cytoplasm begins to divide – Cytoplasmic division

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

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

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 Homologous chromosomes pair up Crossing-over may occur between homologous chromosomes 92 chromosomes total

Crossing-over during meiosis

Metaphase I Homologous chromosomes line up at the middle of the cell – independent assortment 92 chromosomes total

Independent assortment

Anaphase I The 2 members of each homologous pair of chromosomes separate from each other and move to opposite poles of the cell 92 chromosomes total

Telophase I Nuclear membrane reforms Followed by cytoplasmic cleavage Each cell has half the chromosome number as the parent cell – 46 in each cell

Second meiotic division Prophase 2: Same as prophase 1, 2 cells, 46 chromosomes in each cell. No crossing over! Metaphase 2: Same as metaphase 1, 2 cells, 46 chromosomes in each cell. No independent assortment! Anaphase 2: Same as anaphase 1, 2 cells, 46 chromosomes in each cell. 23 on each side of cell Telophase 2: Same as telophase 1, 4 cells, 23 chromosomes in each cell. Cells are haploid.

Prophase 2

Metaphase 2

Anaphase 2

Telophase 2

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

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