Cell Division

Mitosis

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

Mitosis

• Prophase• Metaphase• Anaphase• Telophase

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

Metaphase

• Nuclear membrane dissolves

• Spindle fibres align the chromosomes along the middle of the cell nucleus.

Anaphase

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

• Speed of 1μm/min

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.

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.

• http://uk.youtube.com/watch?v=s4PaOz7eWS8&feature=related

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 stagesof 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

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

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 andthe 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 timethat the cells spend in each phase of the cell cycle

Interphase isthe longest stage of the

cell cyclewith anaphase

being theshortest

Cancer

When the cell cycle goes wrong

Scanning electron micrograph of dividing cancer cells

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

normalbody cell

mutatedbody cell

tumour

mitosismutation

Many such tumours are found to be BENIGN and do not spread fromtheir 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 transportmalignant cells from the tumour to other sites in the body

malignant tumour

malignant cancercells carried toother body sites

secondarytumour

This is calledmetastasis

these cells invadeother body regionsto form secondary

cancers

Introduction to Meiosis

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

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

SPERMCELL

EGGCELL

In humans, these are the egg and sperm cellsWhereas most body cells have a complement of

23 pairs of chromosomes, human gametes possessonly 23 single chromosomes. A gamete’s complement of

23 single chromosomes is constituted by one chromosometaken from each of the 23 pairs of chromosomes

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

chromosome number

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

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

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

fertilisation

MEIOSISMITOSIS

Diploid bodycell

Two diploid daughter cells

The nucleusdivides twice

Four haploid, genetically different gametes are produced

Meiosis is important as it ensuresthat, when the gametes fuse at

fertilisation, the normal diploid numberof chromosomes is maintained; meiosis

is also an important source of genetic variation

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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

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