Mitosis Flashcards
The importance of mitosis
Mitosis is the term usually used to describe the entire process of cell division in eukaryotic cells. It actually refers to nuclear division (division of the nucleus), an essential stage in cell division. Mitosis ensures that both daughter cells produced when a parent cell divides are genetically identical (except in the rare events where mutations occur). Each new cell will have an exact copy of the DNA present in the parent cell and the same number of chromosomes.
When is mitosis necessary
Mitosis is necessary when all the daughter cells have to be identical. This is the case during growth, replacement and repair of tissues in multicellular organisms such as animals, plants and fungi. Mitosis is also necessary for asexual reproduction, which is the production of genetically identical offspring from one parent in multicellular organisms including plants, fungi and some animals, and also in eukaryotic single-celled organisms such as Anoeba species. Prokaryotic organisms, including bacteria, do not have a nucleus and they reproduce asexually by a different process known as binary fission.
Chromosomes
Before mitosis can occur, all of the DNA in the nucleus is replicated during interphase. Each DNA molecule (chromosome) is converted into two identical DNA molecules, called chromatids.
The two chromatids are joined together at a region called the centromere. It is necessary to keep the chromatids together during mitosis so that they can be precisely manoeuvred and segregated equally, one each into the two new daughter cells.
The stages of mitosis
Four phases-prophase, metaphase, anaphase and telophase. We describe them separately but in fact they flow seamlessly from one to another. Each of these phases can be viewed and identified using a light microscope. Dividing cells can be easily obtained from growing root type of plants. The root tips can be treated with a chemical to allow the cells to be separated-then they can be squashed to form a single layer of cells on a microscope slide. Stains that bind DNA are used to make the chromosomes clearly visible.
Prophase
1)During prophase, chromatin fibres (complex made up of various proteins, RNA and DNA) begin to coil and condense to form chromosomes in that will take up stain to become visible under the light microscope. The nucleolus, a distinct area of the nucleus responsible for RNA synthesis, disappears. The nuclear membrane begins to break down.
2)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.
3)In animal cells and some plant cells, two centrioles migrate to opposite poles of the cell. The centrioles are cylindrical bundles of proteins that help in the formation of the spindle.
4)The spindle fibres attach to specific areas on the centromeres and start to move the chromosomes to the centre of the cell.
5)By the end of prophase the nuclear envelope has disappeared.
Metaphase
During metaphase the chromosomes are moved by the spindle fibres to form a plane in the centre of the cell, called the metaphase plate, and then held in position.
Anaphase
The centromeres holding together the pairs of chromatids in each chromosome divide during anaphase. The chromatids are separated-pulled to opposite poles of the cell by the shortening spindle fibres.
The characteristic ‘V’ shape of the chromatids moving towards the poles is a result of them being dragged by their centromeres through the liquid cytosol.
Telophase
In telophase the chromatids have reached the poles and are now called chromosomes. The two new sets of chromosomes assemble at each pole and the nuclear envelope reforms around them. The chromosomes start to uncoil and the nucleolus is formed. Cell division-or cytokinesis, begins.
Cytokinesis-animal cells
-Cytokinesis, the actual division of the cell into two separate cells, begins during telophase.
-In animal cells a cleavage furrow forms around the middle of the cell. The cell-surface membrane is pulled inwards by the cytoskeleton until it is close enough to fuse around the middle, forming two cells.
Cytokinesis-plant cells
Plant cells have cell walls so it is it possible for a cleavage furrow to be formed. Vesicles from the Golgi apparatus begin to assemble in the same place as where the metaphase plate was formed. The vesicles fuse with each other and the cell surface membrane, dividing the cell into two.
New sections of cell wall then form along the new sections of membrane (if the dividing cell wall were formed before the daughter cells separated they would immediately undergo osmotic lysis from the surrounding water).
