CELL DIVISION Summarized

This unit will take you approximately 3 hours to study. You should not worry if it takes you longer than expected, as we all do not learn at the same pace. The important thing is that you achieve the set objectives.

Learning objectives

At the end of this unit, you should be able to:
● define mitosis as nuclear division giving rise to genetically identical cells (details of stages are not required)
● describe mitosis simply, in terms of the exact duplication of chromosomes resulting in identical daughter nuclei (details of stages are not required)
● state the role of mitosiS in growth, repair of damaged tissues, replacement of cells and asexual reproduction
● define meiosis as reduction division in which the chromosome number is halved from diploid to haploid resulting in genetically different cells (details of stages are not required)
● state that meiosis is involved in production of gametes
● explain how meiosis produce variation by forming new combinations of material and paternal chromosomes (details are not required)
● distinguish between mitosis and meiosis.

Growth and reproduction in organisms is usually a result of cell division. Cell division is the process by which cells produce new cells. All cells are derived from pre-existing cells. Cell division differs in prokaryotes (bacteria) and eukaryotes (protists, fungi, plants, and animals).

 Cell Division

cell division is important for cell growth, reproduction of the species, and repair and replacement of damaged cell parts. Some tissues must be repaired often such as the lining of the gut, white blood cells and skin cells with a short lifespan. Other cells do not divide at all after birth, such as muscle and nerve cells. One of the most important events that happens during cell division is the exact duplication of the information within the nucleus of the cell and its distribution into the two identical daughter cells produced. In the nucleus of every living cell there are several thread-like structures, called chromosomes, made of DNA and protein. The DNA is a set of instructions for the cell These instructions tell the cell what proteins to make. A molecule of DNA is very long and may contain instructions for many different proteins. A section of a DNA molecule that gives instructions for making any one kind of protein is called a gene, which is the part of a chromosome that carries the hereditary material. Each chromosome contains many genes. Figure below shows human chromosomes magnified many thousands of times by an electron microscope. The nucleus of each cell (except egg cells and sperm cell) in the human body has 46 chromosomes.

The human karyotype can be sorted into 23 pairs. The nucleus of each gamete cell Contains one of each of the 23 kinds of human chromosomes. When they join together at fertilisation, the zygote that is produced ends up with 46 chromosomes in the nucleus. The gamete cells have haploid nuclei, which means that the nuclei contain one set of chromosomes. A haploid nucleus is represented by the symbol n.

All other cells, including the zygote, have diploid nuclei, which means tidte nucel Contain two sets of chromosomes. A diploid nucleus is represented by the symbol 2n. Each set of chromosomes will contain the same number, distribution and sequernce Or genes as the other. However, the exact form of the gene may differ between each set of chromosomes. The alternative form of each gene is called an allele, defined as an alternative form of a single gene. There are two forms of cell division. They are mitosis and meiosis.

 Mitosis

When a plant reproduces asexually, some of its cells Will divide to make the hundreds of thousands of cells in the new organism. Each of these cell divisions is done in Such a way tldt te new cells have exactly the same number and kind of chromosomes as their parent cell. Inis kind of cell division in mitosis. The identical daughter nuclei or cells have identical instructions and will grow very similar plants, provided the conditions under which they are grown are similar Mitosis also occurs in animals where identical daughter cells develop from a single cell.

During mitosis, the cell produces two genetically identical diploid daughter cells, as shown in Figure below. When a cell divides into two, the chromosomes in the nucleus make exact copies of themselves, so that each of the two new daughter cells receives a full set of chromosomes. The process of mitosis can be described as the exact duplication of chromosomes that happens just before the equal distribution of chromosomes during nuclear division.
● Before the cell divides, the DNA in each chromosome produces an exact Copy of itself. Each chromosome now has two molecules of DNA.
● The strands separate and move to opposite sides of the cell.
● A new nuclear membrane forms around each group of chromosomes. The cell appears to have two nuclei.
● The cytoplasm of the cell then divides and wraps around each nucleus. The cell eventually splits into two.

Two identical daughter cells are produced, each With identical chromosomes carrying identical genes. In humans, a diploid cell containing 46 chromosomes will therefore produce two new diploid cells. Mitosis is the type of cell division that takes place in the body of an organism when it produces cells that are needed to make an adult organism from a tertilised egg (growth) cells that are needed by your body to repair damaged tissues, such as to heal wounds cuts and broken bones, cells that your body needs to replace dead red blood cells; and other cells such as skin cells that are worn out and peel off the skin. Mitosis also takes place in all the organisms that reproduce asexually, such as potato plants, fungi, bacteria and the like. Asexual organisms reproduce oftspring that are genetically identical to themselves, since only one parent is involved in reproduction. Mitosis is important because it produces two identical daughter cells that are genetically identical to their parent. Any favourable characteristic of the parent cell, such as resistance to disease, is passed to the daughter cells.

Meiosis

Meiosis is a reduction division in which the chromosome number is halved from aiplold
to haploid resulting in genetically different cells. Meiosis OCcurs in the gamete-producing cells of plants and animals. Instead of producing daughter cells identical to each other and to the parent cell, meiosis produces four daughter cells with half the chromosome number and with a different Combination of genes. The daughter cells contain the haploid number of chromosomes, n.

This is essential to ensure that when two gamete cells fuse during fertilisation, the original diploid number is restored. This is shown diagrammatically in Figure below

During the stages of meiosis, genes from each of the original parent chromosomes can be exchanged and thiS produces chromosomes in the gamete cells with new Combinations of genes. Figure below

Shows how the genes on one pair of chromosomes in the gamete producing cell can be recombined in the gamete cells. This process produces gametes With chromosomes that are different from those of the parent. This allows variation to occur.

The process of meiosis can be described as the halving of the chromosome number and the rearrangement of genes, which produces variation. In humans, a diploid cell containing 46 chromosomes will then produce haploid cells containing 23 chromosomes

 Mitosis and meiosis compared

Mitosis

● occurs during divIsIon of somatic cells
● two daughter cells are formed
● diploid number of chromosomes
● chromosomes and genes in each cell are identical

Meiosis

● final stage of cell division leading to production of gametes
● four daughter cells are formed
● haploid number of chromosomes
● the homologous chromosomes and their genes are randomly assorted between gametes

 SUMMARY

● Most living organisms grow from a fertilised egg or zygote, which grows by dividing during the process of mitosis.
● Mitosis duplicates the chromosomes exactly, and identical daughter nuclei are formed. It is important because the daughter cells are identical to the mother cel. If the mother cell has adapted to its environment, the offspring will do so, too.
● Gametes are sex cells and are made by a different kind of cell division called meiosis. This only happens in the sex organs.
● Meiosis halves the number of chromosomes. Each gamete has halt as many chromosomes as the other cells of the organism. The full number of chromosomes is made up at fertilisation.
● Meiosis results in cells with half the chromosome number and the cells also differ from each other and from the parent cell.

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