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=s4PaOz7e
WS8&feature=related
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
Photomicrograph showing cells from an onion root tip Note that many of the cells are in interphase
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
Cancer
When the cell cycle goes wrong
Scanning electron micrograph of dividing cancer cells
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
Introduction to Meiosis
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
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
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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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