Cells increase in number by means of Mitosis.
Mitosis makes exact duplicates of the nucleus and divides the cytoplasm (cytokinesis).
The resultant two daughter cells are identical to each other, as well as the original mother cell. The daughter cells will then mature and grow.
Mitosis is responsible for:
- growth of an organism
- replacement/ repair of damaged/ worn tissues
- in some simple organisms, it is the mode of reproduction
Chromosomes are the structures that carry our inherited information or hereditary material.
When our DNA is visible as chromosomes, it has replicated and so there is double the DNA in that cell. This happens just before cell division.
Mitosis is a cycle (it keeps going on and on and on, while an organism lives).
A cell’s life can be divided into two stages – interphase and division phase.
During interphase, a cell performs it’s normal roles and functions; it does all the activities and jobs it was made (specialised) to do.
- Growth period
- Chromosomes, in nucleus, are long and thin and, while are seperate, look like a network (Chromatin network)
- When cell division is about to happen, the chromosomes, in addition to other organelles (such as the mitochondria and vacuoles) replicate themselves. These replicates will be part of the new cell.
The division phase is split up into a series of shorter phases: Prophase, Metaphase, Anaphase, Telophase.
INTERPHASE >> PROPHASE >> METAPHASE >> ANAPHASE >> TELOPHASE
PROPHASE: (Longest phase of cell devision)
- Chromosomes become shorter and fatter.
- Nuclear membrane and nucleolus are absorbed into the cytoplasm.
- (In animal cells) – the centrioles in the cytoplasm migrate to opposite sides of the cell.
- As the centrioles migrate, a spindle (consisting of thin protein fibres) stretches between them. Some of the fibres stretch from one centriole to another; others stretch from the centriole to the chromatids.
- Chromosomes line up at the equator/ middle of the cell.
- The spindle fibres of one sister chromatid extend to one pole (centriole) and the fibres of the other sister extend to the opposite pole (centriole).
- Centromeres, joining the two chromatids of each pair, split apart.
- The chromatids of each pair separate from one another.
- The spindle fibres attached to each chromatid shorten, and pull the chromatids apart, moving them to opposite sides of the cell.
- The two sets of chromatids reach the opposite poles of the cell.
- The spindle fibres are absorbed into the cytoplasm.
- A nuclear membrane forms around each group of chromosomes.The cell membrane invaginates (goes inward) in preparation for separating the two cells.
- Cytokinesis occurs – the cytoplasm is plait and the two cells are fully separated from one another (This process differs in plant and animal cells).