Module 1 - Cells

Mitosis is involved in:

• Growth, e.g. cells in an embryo divide to form tissue stem cells.
• Replacement of cells, e.g. tissue stem cells in bone marrow divide to produce red blood cells and neutrophils.
• Repair, e.g. in wound healing.
• Asexual reproduction.

The result of mitosis:

• The number of chromosomes in a nucleus stays the same.
• The genetic information passed to the daughter cells is identical.
• Two new nuclei are formed.
• no genetic variation occurs.

All cells in an organism have the same genetic information. If they did not, the organism would probably treat the cells as foreign and reject them.

There are four stages of mitosis; Prophase, Metaphase, Anaphase and Telophase.

Prophase:

• The DNA in chromosomes is packaged.
• Chromosomes coil by shortening and thickening which means that they can be seen under a microscope.
• Each chromosome now has two chromatids.
• The nuclear envelope begins to break up into small pieces and disperse throughout the cell.
• Centrioles move to opposite ends of the cell to make poles.

Metaphase

• Chromosomes come the middle of the cell.
• They line up along the equator of the cell.
• centrioles organise microtubules into spindle fibres which stretch across the cell.
• The spindle fibres attach to the centromeres of the chromosomes.

Anaphase

• Chromatids break apart at the centromere and are pulled by the spindle towards the poles.
• Once separated, the chromatids are chromosomes.

Telophase

• Nuclear envelopes re-form around each group of chromosomes at either end of the cell.
• The chromosomes uncoil.

Telophase is followed by an interphase in which DNA may replicate if the cell is going to divide again.

During telophase, the cell may divide. In Animal cells, microtubules form a 'draw string' just inside the membrane, which then fuses as it is 'pinched in'.

In plant cells, microtubules direct vesicles to the middle of the cell to form a cell plate, which forms a new cell wall. New cell surface membrane is made on either side to enclose two separate cells. This stage of the cell cycle is called cytokinesis.

Cells that have the ability to divide by mitosis and give rise to cells that change into specialised cells are called stem cells in animal cells, and meristematic cells in plants.

The process of changing from dividing cell into a specialised cell is called differentiation.