The Cell Cycle
The cell cycle starts when a cell has been produced by cell division and ends with the cell dividing to produce two identical cells.
In eukaryotic cells there are two main phases: interphase and mitotic phase.
The first growth phase: the proteins from which organelles are synthesised are produced, and organelles replicate. The cell increases in size.
Synthesis phase: DNA is replicated in the nucleus
The second growth phase: the cell continues to increase in size, energy stores are increased and the duplicated DNA is checked for errors
During the mitotic phase, two stages occur.
Mitosis - the nucleus divides
Cytokinesis - the cytoplasm divides and two cells are produced.
Mitosis - Prophase
1. Chromatin fibres coil and condense to form chromosomes. The nucleolus disappears. The chromosomes get shorter and fatter.
2. Centrioles (cylindrical bundles of protein) start moving to opposite ends of the cell.
3. Protein microtubules form spindle-shaped structures linking the poles of the cell. The fibres forming the spindle are necessary to move the chromosomes into the correct positions before division.
4. The nuclear membrane begins to break down.
Mitosis - Metaphase
1. The chromosomes line up along the middle of the cell
2. The chromosomes become attached to the spindle by their centromere
3. At the metaphase checkpoint, the cell checks that all the chromosomes are attached to the spindle befroe mitosis can continue
Mitosis - Anaphase
1. The centromeres divide, separating each pair of sister chromatids
2. The spindles contract, pulling the chromatids to opposite ends of the cell.
Mitosis - Telophase
1. The chromatids have reached the poles and are now called chromosomes.
2. They uncoil and become long and thin again
3. The two new sets of chromosomes assemble at each pole and the nuclear envelope reforms around them so there are two nuclei.
4. The nucleolus forms again.
Mitosis - Cytokinesis
1. The cytoplasm divides
There are two daughter cells that are genetically identical to the original cell and to each other.
Meiosis - Prophase 1
1. The chromosomes have replicated to form chromatids joined by a centromere.
2. The chromatids continue to shorten and become visible
3. The homologous chromosomes pair up to form bivalents - one chromosome having originated from the father, the other from the mother.
4. Crossing over occurs where lengths of chromatids are swapped - corresponding sections of the sections break and rejoin, forming a cross-shaped structure called chiasmata.
Meiosis - Metaphase 1
1. The spindle forms
2. The bivalents line up on the equator randomly.
The orientation of each homologous pair on the equator is random and independent of any other homologous pair. This is called independent assortment and can result in many different combinations of alleles facing the poles. This results in genetic variation
Meiosis - Anaphase 1
1. The spindle pulls the homologous pairs to opposite ends of the cell
2. The centromeres do not split apart
3. The homologous pairs separate to each pole - each centromere is still attached to two chromatids.
Meiosis - Telophase 1
1. The spindle fibres disappear
2. The nuclear membranes form around the chromosomes
3. Chromosomes uncoil.
4. Cytokinesis occurs and the cell divides into two cells.
Meiosis - Prophase 2
1. The nuclear membrane breaks down
2. The centrioles move to the poles and spindle formation begins.
3. The chromasomes condense and become visible again
Meiosis - Metaphase 2
1. Individual chromosomes line up on the equator
2. The chromatids are no longer identical so there is more independent assortment and genetic variation.
Meiosis - Anaphase 2
1. The centromeres divide by pulling the chromatids to opposite poles
Meiosis - Telophase 2
1. The chromatids assemble at the poles.
2. The chromosomes uncoil and form chromatin again
3. The nuclear membranes form again and the nucleolus becomes visible
4. 4 haploid games have formed.