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(1)

Cell Division

Mitosis

(2)

Mitosis

• Mitosis is just one part of the cell cycle

• The Mitotic (M) phase is the shortest part of the cell cycle

(Cytokinesis may be included in this phase)

• Mitosis is a continuum, but can be

separated into 4 phases

(3)

Mitosis

• Prophase

• Metaphase

• Anaphase

• Telophase

(4)

Prophase

• Chromatin condenses and becomes visible in the light microscope as chromosomes.

• The nucleolus disappears.

• Centrioles begin moving to opposite ends of the cell

• Mitotic spindle begins to

form

(5)

Metaphase

• Nuclear membrane dissolves

• Spindle fibres align

the chromosomes

along the middle of

the cell nucleus.

(6)

Anaphase

• The paired

chromatids separate at the kinetochores and move to opposite sides of the cell.

• Speed of 1μm/min

(7)

Telophase

• Chromatids arrive at opposite poles of cell

• New membranes form around the daughter nuclei.

• The chromosomes disperse and are no

longer visible under the light microscope.

• The spindle fibres disperse

• Cytokinesis may also

begin during this stage.

(8)

Cytokinesis

• In animal cells,

cytokinesis results when a fibre ring composed of a protein called actin

around the centre of the cell contracts pinching the cell into two daughter

cells, each with one nucleus.

• In plant cells, the rigid wall requires that a cell plate be synthesized

between the two daughter

cells.

(9)
(10)

• http://uk.youtube.com/watch?v=s4PaOz7e

WS8&feature=related

(11)

Observing Mitosis in Plant Tissues

Plant roots grow by mitotic division of the cells at the root tip

Onion root tips are an ideal source of material for observing the stages of mitosis

A scalpel is used to cut about 4 mm from the tip of the growing onion root

Acetic acid is added to the tip on a watch glass and warmed gently for about 5 minutes; the acid helps to macerate the cells

The root tip is transferred to a slide where two or three drops of aceto-orcein stain are added; this stain is taken up by the chromosomes and makes them more visible as they stain red

The tip is gently broken up with a mounted needle and the cells are spread across the slide

A coverslip is placed over the root preparation and gently squashed

The slide is examined for stages of mitosis using an optical microscope

(12)

Photomicrograph showing cells from an onion root tip Note that many of the cells are in interphase

(13)

The Timing of The Cell Cycle

The different phases of the cell cycle last for different periods of time

An onion root slide was examined for the different stages of the cell cycle and the number of cells in each phase was recorded

Stage of Cell

Cycle Number of Cells

Cell % (approx.)

Interphase 138 66

Prophase 56 27

Metaphase 6 3

Anaphase 2 1

Telophase 7 3

Total 209 100

The percentage of cells in each stage was then calculated The % of cells in each stage is used as a measure of the % of time

that the cells spend in each phase of the cell cycle

Interphase is the longest stage of the

cell cycle with anaphase

being the shortest

(14)

Cancer

When the cell cycle goes wrong

(15)

Scanning electron micrograph of dividing cancer cells

(16)

Cancer: The Cell Cycle Out of Control

When a normal body cell mutates it may divide to produce a clone of cells that form a tumour

normal

body cell mutated

body cell

tumour mitosis

mutation

Many such tumours are found to be BENIGN and do not spread from

their site of origin – they may nevertheless compress and damage adjacent tissues Malignant, cancerous tumours may spread from their site of origin

These tumours develop their own blood and lymph supply which can transport malignant cells from the tumour to other sites in the body

malignant tumour

malignant cancer cells carried to other body sites

secondary tumour

This is called metastasis these cells invade other body regions

to form secondary cancers

(17)

Introduction to Meiosis

(18)

An Introduction to Meiosis

Meiosis is another form of cell division that is associated with reproduction in many organisms

In humans, meiosis is responsible for the formation of the reproductive cells or gametes

SPERM CELL

EGG CELL

In humans, these are the egg and sperm cells

Whereas most body cells have a complement of 23 pairs of chromosomes, human gametes possess only 23 single chromosomes. A gamete’s complement of 23 single chromosomes is constituted by one chromosome

taken from each of the 23 pairs of chromosomes

Within the human ovaries and testes, gametes are produced by meiosis and this process halves the

chromosome number

Human body cells are DIPLOID as they possess two sets of chromosomes (23 pairs)

Human gametes are described as being HAPLOID as they possess only one set of chromosomes (23 chromosomes)

If the gametes were diploid then the number of chromosomes would double at every generation after

fertilisation

(19)

MEIOSIS MITOSIS

Diploid body cell

Two diploid daughter cells

The nucleus divides twice

Four haploid, genetically different gametes are produced

Meiosis is important as it ensures that, when the gametes fuse at

fertilisation, the normal diploid number of chromosomes is maintained; meiosis

is also an important source of genetic variation

(20)

20

Significance of Meiosis

• Introduces some genetic variation

• Maintains genetic stability

• Each haploid (n) gamate carries only 1 gene for a particular characteristic

• Crossing over leads to new combinations of genes

• Independent assortment of chromosomes

mixes up combinations of chromosomes

(21)

21

References

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