When cells multiply, enzymes enable the complete copying of all the DNA (not just the gene sequences) resulting in two complete sets of DNA, one for the original (parent) cell and one for the newly formed (daughter) cell. Both MITOSIS and MEIOSIS take place in the nucleus of the cell and are concerned with the division of the nucleus.
So cells reproduce exact duplicates of themselves. The exception to this being the sex cells – sperm and ova, which have only half the normal DNA complement. You’ll understand this further below when we look at meiosis.
DNA is one of the basic processes involved in making new cells. It is the process of making a copy of replica of the DNA in the nucleus. It always happens just before cell division during interphase.
The steps of DNA replication can be simplified to:
- Open up the DNA helix using an enzyme called helicase.
- Add new DNA nucleotides to make a new polymer of DNA (done by enzyme DNA polymerase)
- Open up more up the helix and restart the lagging strands
- Join together the lagging strands and twist together the double helixes
In the case of MITOSIS, the nucleus and its cell divide into two halves which then grow and form two new cells. This process is one of simple growth. Plants grow from small seedlings into mature plants by mitosis dividing the cells, usually at the root tips and the ends of the shoots (called the apical meristem). Animals also grow from fertilised cells through the stages of foetus to young animal to mature animal by mitosis. The process of the division of a cell nucleus by mitosis is illustrated.
Mitosis is an example of asexual reproduction which is where a cell divides to make two identical nuclei (in daughter cells). In mitosis the original and daughter cells are all diploid (2n).
Mitosis cell division occurs in 4 phases which is preceeded by what’s called interphase. Interphase involves cell growth and a synthesis stage during which DNA is copied or replicated to produce two identical copies of DNA.
Interphase is the phase in the cell cycle which lasts for approximately 90% of the time of the entire cell cycle. It lasts for around 23 hours normally and is made up of three sub-phases. The stages within interphase the cell prepares itself for DNA replication, before replicating into two identical DNA molecules called chromatids, then expanding in size.
The 4 stages of mitosis are as follows:
Inside the nucleus, DNA (undividing chromosomes) condenses to form visible chromosomes (the short, thick chromosomes which we see visible on microphotographs) consisting of two sister chromatids. As this continues the nucleus breaks down.
Outside the nucleus the centrioles formed move to opposite sides (poles) of the cells. As the centrioles move apart the mitotic spindle fibres are formed.
Once the chromatids and spindle fibres have formed, the chromatids connect to the spindle fibres. The point of attachment is the small protein structure called a centromere.
(Note terminology: centrosomes in plant cells and centrioles in animal cells).
The counteracting forces of the spindle cause the chromosomes to line up along an invisible centre line or plate in the centre of the cell.
The sister chromatids now separate from each other to become an individual chromosome. From this point, the spindle fibres which are attached to chromosomes shorten and ‘pull’ the chromosomes toward opposite poles in the cell.
Simultaneously, spindle fibres which were not attached to any chromatids continue to lengthen thus pushing the poles further apart.
By the end of this stage, each pole has an identical set of genes.
Spindle fibres disintegrate and a nuclear envelope reforms around each set of chromosomes, a nucleolus reappears. The mitotic spindle fibres break down.
This is the final process relating to cell division. It occurs where the cytoplasm of the cell divides by the formation of new plasma membrane and ultimately the cell divides into two after which chromosomes reappear as chromatin.
Division of the cytoplasm of a cell into two halves creates two equal cells normally referred to as daughter cells.
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