Mitosis
The Cell Cycle
Dividing cells undergo a regular pattern of events known as the cell cycle. For convenience of description it can be divided into stages, but, it should be kept in mind that this is a continuous process and the beginning and end of each of these stages is not always clear. There are four stages plus a 'resting' stage in each division.
Dividing cells undergo a regular pattern of events known as the cell cycle. For convenience of description it can be divided into stages, but, it should be kept in mind that this is a continuous process and the beginning and end of each of these stages is not always clear. There are four stages plus a 'resting' stage in each division.
Interphase
Although described as a 'resting' stage this is the way the cell is found for most of it's existence.
During early interphase (S1) the cell must recover from the previous division. It must increase in size and reproduce any organelles that have been lost during the division. It must produce any organelles that are lacking in number too, e.g. mitochondria.
It now has a period when it carries out its cellular function. This is different for each type of cell. The length of time is also different. Brain cells, for example, will remain in this stage near enough your whole life. Skin cells however, will spend less than a month in this stage. At this stage chromosomes are not visible. This is because the chromosome material is dispersed through the nucleus as chromatin.
When the cell is preparing to divide (nobody quite knows how it knows) it must make certain preparations. The DNA must replicate so that the cell has double the quantity of DNA that it normally has. This happens when DNA replicates (as we have learned). The two identical strands remain attached by a bundle of protein known as the centromere. There is considerable metabolic activity at this stage (S) The cell must make lots of energy as it is required for these processes.
Next the cell must prepare itself to divide. Organelle are replicated and the cell literally increases in size in preparation for the following events. (G2)
Now the cell is ready to go into Mitosis
During early interphase (S1) the cell must recover from the previous division. It must increase in size and reproduce any organelles that have been lost during the division. It must produce any organelles that are lacking in number too, e.g. mitochondria.
It now has a period when it carries out its cellular function. This is different for each type of cell. The length of time is also different. Brain cells, for example, will remain in this stage near enough your whole life. Skin cells however, will spend less than a month in this stage. At this stage chromosomes are not visible. This is because the chromosome material is dispersed through the nucleus as chromatin.
When the cell is preparing to divide (nobody quite knows how it knows) it must make certain preparations. The DNA must replicate so that the cell has double the quantity of DNA that it normally has. This happens when DNA replicates (as we have learned). The two identical strands remain attached by a bundle of protein known as the centromere. There is considerable metabolic activity at this stage (S) The cell must make lots of energy as it is required for these processes.
Next the cell must prepare itself to divide. Organelle are replicated and the cell literally increases in size in preparation for the following events. (G2)
Now the cell is ready to go into Mitosis
Prophase
The first stage of Mitosis. At this stage:
1. Chromosome condense to become shorter and thicker. They are now visible (with a microscope) as pairs of chromatids - this is how you are likely to be familiar with seeing chromosomes. This is the defining feature of this stage.
1. Chromosome condense to become shorter and thicker. They are now visible (with a microscope) as pairs of chromatids - this is how you are likely to be familiar with seeing chromosomes. This is the defining feature of this stage.
2. Centrioles (in animal and lower plant cells) have divided at the end of interphase. They can now be seen migrating to the poles of the cell (opposite ends).
3. Protein microtubules form from each centriole and and a spindle develops. The spindle extends from pole to pole.
4. Towards the end of Prophase the nuclear membrane disintegrates and the chromatids can be seen lying freely in the cytoplasm.
3. Protein microtubules form from each centriole and and a spindle develops. The spindle extends from pole to pole.
4. Towards the end of Prophase the nuclear membrane disintegrates and the chromatids can be seen lying freely in the cytoplasm.
Metaphase
During metaphase the chromatids line up at the equator of the cell. The spindle fibres attach to the centromeres of the chromatids. The spindle fibres now contract (shorten) pulling the chromatids apart slightly.
Anaphase
This is a very rapid stage. As the spindle fibres pull in opposite directions the centromere is put under increasing strain. Eventually it splits and the chromatids are pulled to opposite ends of the cell.
Telophase
The final stage of mitosis. The chromatids have reached the poles. They uncoil and lengthen again - they now become known as chromosomes once more. The spindle breaks down, centrioles replicate.
A nuclear membrane forms at each end of the cell. Temporarily giving a cell with two nuclei. This is short lived though. Finally the cell divides. This is slightly different in plants and animals due to the plant requiring a cell wall.
A nuclear membrane forms at each end of the cell. Temporarily giving a cell with two nuclei. This is short lived though. Finally the cell divides. This is slightly different in plants and animals due to the plant requiring a cell wall.