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Cell Division and cycle notes, Study notes of Biology

Biology

Brief and summarized notes on the cycle cycle with high reference to mitosis and meiosis

Typology: Study notes

2022/2023

Uploaded on 02/08/2023

Calvganwa
Calvganwa 🇺🇬

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CELL DIVISION
In this topic, learners are supposed to know the following
1. Describe mitosis and meiosis.
2. Compare mitosis and meiosis.
3. Explain the significance of changes in the nucleus during cell
division.
4. State the significance of mitosis and meiosis.
CELL DIVISION
Cell division is the process by which a cell divides to produce two new daughter
cells.
Cell division mostly involves the chromosomes since they are hereditary units that
determine the characteristics of a cell hence they should be correctly distributed in
the daughter cells during division.
A cell has a fixed number of chromosomes and cells which contain two sets of
chromosomes are known as Diploid cells. These exist as the majority of animal
cells. Cells which contain only one set of chromosomes/ half the fixed number of
chromosomes are known as Haploid cells. These exist as gamete cells (sperm and
ovum).
Cell division exists in two forms according to the behavior of chromosomes.
In the first form, the daughter cells contain the exact same number of
chromosomes as the parent cell and this is known as Mitotic cell division(Mitosis)
This type of cell division takes place during growth of an organism.
The other form involves the daughter cells containing half the total number of
chromosomes in the parent cell. This type of cell division is known as Meiotic cell
division (Meiosis). This type of cell division takes place in the formation of
gametes.
Mitosis and meiosis are the processes which lead to division of the nucleus but the
division of the whole cell which follows shortly afterwards is known as
cytokinesis. This process leads to division of the cytoplasm that in turn divides the
cell.
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CELL DIVISION

In this topic, learners are supposed to know the following

1. Describe mitosis and meiosis.

2. Compare mitosis and meiosis.

3. Explain the significance of changes in the nucleus during cell

division.

4. State the significance of mitosis and meiosis.

CELL DIVISION Cell division is the process by which a cell divides to produce two new daughter cells. Cell division mostly involves the chromosomes since they are hereditary units that determine the characteristics of a cell hence they should be correctly distributed in the daughter cells during division. A cell has a fixed number of chromosomes and cells which contain two sets of chromosomes are known as Diploid cells. These exist as the majority of animal cells. Cells which contain only one set of chromosomes/ half the fixed number of chromosomes are known as Haploid cells. These exist as gamete cells (sperm and ovum). Cell division exists in two forms according to the behavior of chromosomes. In the first form, the daughter cells contain the exact same number of chromosomes as the parent cell and this is known as Mitotic cell division(Mitosis) This type of cell division takes place during growth of an organism. The other form involves the daughter cells containing half the total number of chromosomes in the parent cell. This type of cell division is known as Meiotic cell division (Meiosis). This type of cell division takes place in the formation of gametes. Mitosis and meiosis are the processes which lead to division of the nucleus but the division of the whole cell which follows shortly afterwards is known as cytokinesis. This process leads to division of the cytoplasm that in turn divides the cell.

MITOSIS

Mitosis is the process by which a cell nucleus divides to produce two daughter nuclei containing identical sets of chromosomes to the parent cell. It is usually followed by division of the whole cell which forms two daughter cells. Mitosis occurs in somatic cells of animals and in the apical meristems, cambium tissues, axillary and terminal buds in plants. Mitosis occurs in four stages; Prophase, Metaphase, Anaphase and Telophase. At each stage, crucial events occur particularly in regards to the chromosomes. This process involves a preparation stage known as Interphase where a cell prepares itself for division. Interphase is also known as the resting phase of the parent cell. In this stage,  The DNA of the parent cell replicates such that there is sufficient DNA for both the two daughter cells.  The chromosomes are long, thin and invisible existing as chromatin threads.  The cell builds up a sufficient amount of energy in energy reserves to carry out the process of cell division.  The cell organelles such as the mitochondria, chloroplasts and ribosomes replicate.  The centrioles which play an important role also replicate such that they now exist as pairs. b) Prophase During this stage,  The chromatin threads condense by shortening and thickening to form distinct chromosomes. Each chromosome now consists of two chromatids joined together by a centromere.  The centrioles migrate to the opposite poles and begin formation of spindle fibres. These are seen as short microtubules called asters.  The nucleolus shrinks and disappears.  At later prophase, the nuclear membrane completely disintegrates and disappears and spindle fibres are completely formed.

Cytokinesis is the process by which the cytoplasm divides which leads to division of the whole cell. This normally occurs after Telophase is completed. In preparation for this process, the cell organelles become evenly distributed at the two poles during Telophase along with the chromosomes. In animal cells, the cell surface membrane begins to constrict across the equator of the spindle forming a furrow around the outside surface of the cell. The cell surface membranes in the furrow join up and completely separate the two cells. In plant cells, the spindle fibres begin to disappear during Telophase everywhere except in the region of the equatorial plane. Here they move outwards in diameter and increase in number to form a barrel shaped region known as the phragmoplast. Cell organelles are attracted to this region where the Golgi apparatus produces a number of small fluid filled vesicles. These are guided by microtubules in order to fuse to form a cell plate that grows across the equatorial plane. The contents of the vesicles contribute to the new middle lamella and cell walls of the daughter cells and their membranes form the new cell surface membranes. The cell plate eventually fuses with the parent cell wall and separates the daughter cells. In certain areas, the vesicles of the cell plate fail to fuse and the cytoplasm of neighboring daughter cells remains intact. These cytoplasmic channels are lined by cell surface membrane to form structures known as plasmodesmata. Significance/ Importance of Mitosis  Mitosis enables genetic stability by producing two daughter nuclei identical to the parent nucleus therefore there is no variation in genetic information.  Mitosis enables growth of an organism by increasing the number of cells in an organism which is the basis of growth.  Mitosis enables an organism to replace worn out and dead cells by producing new ones.  Mitosis enables some organisms to reproduce themselves asexually by producing new individuals of a species by one parent organism such as binary fission in amoeba.  Mitosis enables regeneration of parts of some animals through production of new cells such as legs in crustacean and arms in starfish.

Differences between Mitosis in plants and in animals Plants Animals No centrioles are present Centrioles are present No asters are formed Asters are formed Cell division involves formation of a cell plate Cell division involves furrowing and cleavage of cytoplasm Occurs mainly in the meristems Occurs in all somatic cells of an organism MEIOSIS Meiosis is the type of nuclear division where the nucleus divides to form two daughter nuclei with half the total number of chromosome in the parent cell.

 The two bivalents shorten and thicken partly by coiling.  Each chromosome is now seen to consist of two chromatids joined together by a centromere.  The two chromosomes are seen to be joined at several points along their length known as chiasmata. Each chiasma is a site for exchange between chromatids. The chiasmata enable the process of Crossing over. This is the process of exchange of genes between homologous chromosomes leading to new gene combinations in the resulting chromatids. iv) Diplotene  Chromatids in the bivalent begin to repel each other and the bivalent assumes a particular shape depending upon the number of chiasmata (Bivalents having a single chiasma appear as open crosses, two chiasmata produce a ring shape and three or more chiasmata produce loops lying at right angles to each other)  The homologous chromosomes begin to separate into two pairs of chromatids. v) Diakinesis  Homologous chromosomes separate completely.  The nuclear membrane starts to disintegrate. By the end of prophase;  The nucleolus and nuclear membrane have completely disappeared.  The centrioles (if present) have migrated to opposite poles  The spindle fibres are completely formed. b) Metaphase I During this stage,  Chromosome bivalents arrange themselves at the equator of the spindle  Spindle fibres attach to the bivalents at the centromere c) Anaphase I

During this stage,  Spindle fibres shorten causing the centromere to divide and the homologous chromosomes split.  The homologous chromosomes are pulled to the opposite poles of the spindle separating them into two haploid sets, one at each pole of the spindle. d) Telophase I During this stage,  The arrival of homologous chromosomes at opposite poles marks the end of meiosis I.  Spindle fibres disappear  The centrioles replicate  The cell membrane and cytoplasm constrict from the middle of the cell and later divides producing two daughter cells. Halving the number of chromosomes has occurred but the chromosomes are still made up of two chromatids. If crossing over has occurred, these chromatids are not genetically identical and must be separated in a second meiotic division. In animal and some plant cells, the chromatids usually uncoil and a nuclear envelope re-appears at each pole and the nucleus enters interphase. In many plants, there is no Telophase, cell wall formation or interphase. The cell passes straight from anaphase I into prophase of the second meiotic division. SECOND MEIOTIC DIVISION (MEIOSIS II) a) Interphase II This stage only occurs in animal cells. During this stage,  No further replication of DNA occurs

 The creation of new variety leads to the process of evolution due to variation in characteristics of the new generation from those in the existing generation  Meiosis leads to production and fusion of haploid gametes which mixes the genotypes of parents resulting in improved off springs. Comparison between Mitosis and Meiosis i) Similarities  Both are forms of nuclear division  Both begin with a diploid number of chromosomes in a cell  Both involve a single duplication of chromosomes  Both follow similar stages of division namely interphase, prophase, metaphase, anaphase and telophase.  Both involve centrioles and spindle formation ii) Differences Mitosis Meiosis Occurs in somatic body cells Occurs in gonads which are germinal cells that produce gametes Results in formation of two daughter cells Results in formation of four daughter cells Daughter cells have diploid number of chromosomes like the parent cell Daughter cells have haploid number of chromosomes Involves a single division of the nucleus Involves double division of the nucleus Crossing over of chromosomes does not occur Crossing over of chromosomes occurs Does not result in variations among off springs Results in variations among off springs Chiasmata are not formed Chiasmata are formed Chromatids move to the opposite poles Chromosomes move to the opposite poles Daughter cells are genetically identical to the parent cell Daughter cells are genetically different from the parent cell. Chromosomes form a single row at the equator of the spindle during metaphase Chromosomes form a double row at the of the spindle during metaphase 1 It is a brief process that takes 24 hours to complete the cell cycle. It is a longer process that takes more time to complete.