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

nucleus chromosome

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.

chromosome

gene controlling skin colour

A gene is a short length of DNA on a chromosome which controls an inherited character of the organism

gene controlling tongue rolling

gene controlling eye colour

gene controlling blood group

A chromosome carries a lot of genes

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

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

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.

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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)

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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.

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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

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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.

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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

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Cytoplasmic division

Division of cytoplasm Separate from nuclear division

Animal cells : by formation of cleavage furrow Plant cells : by formation of cell

plate

http://www.google.ca/url?sa=i&rct=j&q=cell+plate&source=images&cd=&cad=rja&docid=w5nUGhREzgDMxM&tbnid=Hhr9iEydRdnr1M:&ved=0CAUQjRw&url=http%3A%2F%2Fbiologytb.net23.net%2Ftext%2Fchapter9%2Fconcept9.3.html&ei=sfHKUeWECoasigLRs4CwCQ&bvm=bv.48340889,d.cGE&psig=AFQjCNHbmLa80Fxp3nS16cEZMnRln1tp4Q&ust=1372341032580253http://www.google.ca/url?sa=i&rct=j&q=cleavage%2Bfurrow&source=images&cd=&cad=rja&docid=a4iG-_6Ayu3YHM&tbnid=wHGOstVrRp7cNM:&ved=0CAUQjRw&url=http%3A%2F%2Fdj003.k12.sd.us%2FSCHOOL%2520NOTES%2Fchapter_02.htm&ei=5_HKUfrsGeOmigLarYC4CQ&bvm=bv.48340889,d.cGE&psig=AFQjCNEVhU2aT7XM7Xsimjo2ZfV4UgxgMQ&ust=1372341063832882

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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?

metaphase

anaphase

prophase

interphase

telophase

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

Sorry, youve got the wrong answer!!!

This is not a correct sequence of mitosis.

Try again...

Sorry, youve got the wrong answer!!!

This is not a correct sequence of mitosis.

Try again...

Congratulation!!!

Youve got the correct answer!!!

Click here

Sorry, youve got the wrong answer!!!

This is not a correct sequence of mitosis.

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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, youve got the wrong answer!!!

Remember that chromosomes are made of DNA, and the

chromosomes in cell 3 are duplicated.

Try again...

Sorry, youve got the wrong answer!!!

Remember that chromosomes are made of DNA, and the

chromosomes in cell 3 are duplicated.

Try again...

Youve got the correct answer!!!

Go to next question...

Congratulation!!!

Sorry, youve got the wrong answer!!!

Remember that chromosomes are made of DNA, and the

chromosomes in cell 3 are duplicated.

Try again...

Self-Learning Exercise

A. 1 and 2 only

B. 1 and 3 only

C. 2 and 3 only

D. 1, 2 and 3

Q: Click this button to see the information for answering this question : Which cells contain the same amount of DNA?

Information : The photomicrograph below shows some plant cells at different stages of mitosis :

Back to the question

Sorry, youre wrong!!!

Remember that the different stages of cell division shown is

mitosis!

Try again!!!

Sorry, youre wrong!!!

Remember that the different stages of cell division shown is

mitosis!

Try again!!!

Sorry, youre wrong!!!

Remember that the different stages of cell division shown is

mitosis!

Try again!!!

Congratulations!!!

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)

Sperm (2n) Egg (2n)

Zygote (4n)

Sperm/Egg (4n)

Zygote (8n)

Mitosis

Fertilization

Mitosis

Fertilization

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

Mitosis Meiosis

Number of division One Two

No. of daughter cell produced by one parent cell

Type of cells produced

Chromosome number of daughter cells

Genetic make-up of daughter cells

Two

Somatic cells

Same as parent cell

Identical to parent cell

Four

Gametes

Half of parent cells

May be different from the parent cell

Comparison between mitosis and meiosis

Mitosis Meiosis

Pairing of homologous chromosomes

No Yes

Occurrence

Role

Growing tissues

Growth, repair, replacement of old tissues, asexual reproduction

Reproductive tissues

Gamete formation for sexual reproduction

Crossing-over No Yes

Slide Number 1Purposes of Cell DivisionSlide Number 3Slide Number 4What is a chromosome?Slide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10Slide Number 11Chromosome NumberSlide Number 13Steps of cell divisionTypes of nuclear divisionMitosisInterphase (Resting stage)Slide Number 18Sister Chromatids vs Homologous ChromosomesProphaseSlide Number 21MetaphaseSlide Number 23AnaphaseSlide Number 25TelophaseSlide Number 27Cytoplasmic divisionSlide Number 29Slide Number 30InterphaseProphaseMetaphaseAnaphaseTelophaseCan you identify the different stages of mitosis from the diagram below?Slide Number 37Here is a photomicrograph showing the cells of a plant at different stages of mitosis :Slide Number 39Slide Number 40Slide Number 41Slide Number 42Slide Number 43Here is a photomicrograph showing the cells of a plant at different stages of mitosis :Slide Number 45Slide Number 46Slide Number 47Slide Number 48Slide Number 49Slide Number 50Slide Number 51Slide Number 52Slide Number 53Slide Number 54Significance of mitosisMeiosisProphase ISlide Number 58Crossing-over during meiosisMetaphase IIndependent assortmentSlide Number 62Anaphase ISlide Number 64Telophase ISlide Number 66Second meiotic divisionProphase 2Metaphase 2 Anaphase 2Telophase 2Occurrence of meiosisWhat would happen if gametes are formed by mitosis?Significance of meiosisSignificance of meiosisSources of genetic variationComparison between mitosis and meiosisComparison between mitosis and meiosis