Biology Unit 2 Revision notes

The Cell Cycle And Reproduction

1. Normal cells (i.e. not gamete producing cells) replicate by mitosis. The two daughter cells will 

contain the full number of chromosomes (diploid), and will be identical to the parent cell.

2. The cell cycle has five main stages:

a. Interphase – The cell grows to full size, and carries out its specialised functions. Towards the end of the interphase, DNA replication occurs.

b. Prophase – The chromosomes coil together and shorten, forming two chromatids joined at 

the centromere. The two chromatids are identical. The nuclear membrane begins to disappear, and the centrioles go to opposite ends of the cell, beginning to produce spindle fibres.

c. Metaphase  – The spindle fibres reach across the whole cell, and the nuclear  membrane has disappeared. The chromatid pairs line up on the equator of the spindle.

d. Anaphase – The chromatids are pulled apart by the spindle fibres to the poles of the cell.

e. Telophase  – A nuclear membrane is formed around each set of chromosomes, and they form chromatin threads. The centrioles divide. Cytokinesis occurs, so that the cytoplasm 

divides in half to form two separate cells. The interphase will now begin in each cell.

3. Vegetative propagation is the natural asexual reproduction of plants:

a. Runners, stolons, rhizomes and tubers are all used to produce new plants asexually.

b. These are meristematic regions, whereby rapid growth takes place.

c. Plants produced in this way will be clones of the original plant – so any good properties of the parent will be passed on, but so will any vulnerability to diseases etc.

d. This can be carried out artificially by using cuttings containing a meristematic region.

4. Tissue culture is used to commercially grow plants:

a. The principle of totipotency states that any part of a plant can be made from any other 

part (in theory).

b. The meristem is isolated, and cultured to form a callous (mass of undifferentiated cells).

c. This is then sub-divided, and plantlets are grown and cultured.

5. Micropropagation is also used commercially to grow plants, but leaf axil cuttings are taken as the meristem, grown into a shoot, and then further divided. This is repeated to give as many shoots as 

needed, and then the shoots are placed…