Mitosis
The process by which a nucleus divides into two daughter nuclei, each of which contains the same number of chromosomes as the one found in the parent nucleus.
Meiosis
The process by which a nucleus divides into four daughter nuclei, each of which contains half the number of chromosomes as the one found in the parent nucleus.
Meiosis is also known as
Reduction division since it halves the number of chromosomes in the parent nucleus.
Interphase
- G1 (First growth phase)
- S phase (Synthesis phase)
- G2 (Second growth phase)
Mitosis sequence
- Prophase
- Metaphase
- Anaphase
- Telophase
Prophase
- The chromatins shorten, condense & thicken. The shortening occurs as DNA molecule coils up several times to produce supercoil.
- Individual chromosomes visible as two sister chromatids attached at the centromeres.
- The centrioles move to opposite poles of the cell. Short microtubules are seen radiating from the centrioles.
- At the end of prophase, the nuclear membrane disintegrates & the nucleolus disintegrates. The chromosome now free in the cytoplasm
Metaphase
- The centrioles have reached opposite poles and microtubules extend forming spindle fibres.
- The chromosomes are lined up across the equator of the spindle fibres.
- Each chromosome becomes attached to the spindle fibres by its centromere
Anaphase
- Each chromosome splits at its centromere
- The spindle fibres shorten and as a result, pull the sister chromatids apart.
- The sister chromatids now move towards the opposite poles, centromere first
Telophase
- The chromatids have now reached the opposite poles
- The DNA uncoils and unwinds. The chromosome appear long, thin and thread like : chromatins
- The nuclear membrane reforms around each group of chromatin
- The nucleolus reappears
- The spindle fibres disintegrates
- Two daughter nuclei are formed
Functions of centromere during mitosis
- To hold the two sister chromatids together
2. To attach the chromosomes to the spindle fibres
Importants of mitosis
- Allows the growth of unicellular zygotes into multicellular organisms. Growth may occur over the whole body in animals or be confined to certain areas called meristems of plants
- Mitosis occurs during the repair of damaged and worn-out tissues. Cells are constantly dying and being replaced by identical cells. In the human body, for example, skin cells and the cells lining the gut are constant dying and replaced by identical cells.
- Mitosis is the basis of asexual reproduction
- Immune response (reproduction of B/T lymphocytes)
Stem cells
Cells that can divide an unlimited number of times by mitosis. They are undifferentiated cells and so they are not adapted to perform a specific function
What is a stem cell potency
Refers to a stem cell’s ability to differentiate into different cell types. The greater the number of possible cell types, the higher the potency
Stem cell potency
- Totipotent
- Pluripotent
- Multipotent
Totipotent
Can develop to any type of cell (e.g. zygote)
Pluripotent
Lose ability to produce placenta cells but can produce all cells to develop into embryo
Multipotent
Can only produce few types of cells; in bone marrow (in adults)
Use of stem cells
Treat and cure diseases that are currently uncurable.
They could be use to replace damaged nerve cells in the spinal cord, so that a person who has become paralysed could walk again.
Problems with the use of stem cells
- Adults stem cells are only able to differentiate into very limited types of cell
- There are ethical tissues about using stem cells from embryos as this involve ‘killing’ the embryos. Even, if successful, the cells will be genetically different from the adult into whom they are to be transplanted, producing problems of tissue rejection
Cancer cells
Cells which divide excessively as a result of uncontrolled cell division, to produce a mass of abnormally arranged cells known as tumours
Benign tumour
Consist of a slowly growing mass of cells which remain at the original site. It may not cause serious problems but it can physically compress surrounding tissues and restrict blood flow to them. The tumour can be completely removed by surgery.
Malignant tumour
Consists of fast growing cells which break away from their primary location and enter either the lymphatic or blood circulatory system to invade other parts of the body where they give rise to more tumours (secondary tumours). The cells may even block blood vessels and passageways. This spread of cancer cells beyond their original sites is called metastasis. It is very difficult to remove or treat the cancer once it has spread this way
A factor which cause mutation is called
mutagen
Agent that causes cancer is called
Carcinogen (cancer causing substance) and is described as carcinogenic
