CHAPTER 5

Mitosis

Mitosis is the process of nuclear division within the cell which results in the production of two daughter cells from a single parent cell. The daughter cells are identical to one another and to the original parent cell. Mitosis occurs in all body cells (or somatic cells) except in the reproductive cells (gametes).

A somatic cell contains two complete sets of chromosomes. One set is derived from the female parent and the other from the male parent. Each set has 23 chromosomes, giving a total of 46 chromosomes in a single somatic cell.

A somatic cell with two sets of chromosomes is called a diploid cell (2n). Haploid cells ( n ) are cells with single sets of unpaired chromosomes. All reproductive cells (gametes) are haploid. If the parent cell is haploid ( n ), the mitosis producing the daughter cells will also be haploid. If the parent cell is diploid (2n), the mitosis producing the daughter cells will also be diploid.

The Significance of Mitosis

Mitosis is important in growth, cell replacement, asexual reproduction, reproduction in plants and ensuring genetical identity between parents and daughters.

Growth

Mitosis increases the number of cells in organisms and this is the basis of growth and development in multi-cellular organisms.

Cell replacement

Mitosis is important to replace dead or damaged cells from injuries with new ones. When damaged tissues are repaired, the new cells must be exact copies of the cells being replaced so as to retain the normal function of these cells.

Asexual reproduction

New organisms are formed when the division of cells by mitosis takes place in unicellular organisms. Each new cell grows and becomes an entire organism. The binary fission of an amoeba and the budding of yeast cells are examples of mitosis in unicellular organisms.

Vegetative reproduction

This is the process of asexual reproduction in plants which also involves mitosis. Tulips and onions are reproduced by bulbs, which are short stems under the ground. New bulbs sprout from the old ones and each new bulb gives rise to a new leafy plant. Potatoes are reproduced by tubers, which are enlarged parts of the short stems located underground. The "eyes" become tiny buds. Each bud becomes a shoot, which penetrates the soil and grows upwards. The buds also form roots.

Genetical identity

Mitosis ensures that all new cells that are formed in an organism carry the same genetic information, therefore sharing the same characteristics as the parent cell.

The Cell Cycle

A cell undergoes a sequence of events from the time it is formed until it divides completely into two. This sequence of activities exhibited by cells is called the cell cycle. Two main phases in the cell cycle are the interphase and the mitotic phase (M phase). The mitotic phase is the phase where the division of cell takes place through the processes of mitosis and cytokinesis.

The interphase (G1, S and G2 subphases)

The interphase is not part of mitosis but it is part of the cell cycle and accounts for 90% of the whole cycle. Before mitosis begins, three sub-phases take place in the interphase. These are the G1 (synthesis of new organelles), S (replication of chromosomes), and G2 phases (synthesis of proteins necessary for mitosis). All these sub-phases are necessary preparatory stages for a successful mitotic division.

In the interphase, the cell is engaged in metabolic activity and the chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible. The cell may contain a pair of centrioles (or microtubule organising centres in plants) both of which are the organisational sites for microtubules. During this phase, the cell enlarges and cellular organelles double in number, the DNA replicates, and protein synthesis occurs. The chromosomes are not visible and the DNA appears as uncoiled chromatin.

Just before mitosis begins, two chromomoses, each consisting of two chromatids, appear as thread-like structures. The nucleus is also large and prominent.

Mitosis (M phase)

Mitosis is divided into four phases, namely, the prophase, metaphase, anaphase and telophase. Although separation of the phases makes mitosis easier to understand, it is important to note that mitosis is a continuous process, without pause between phases.

Cytokinesis

The process of cytokinesis is different in animal and plant cells. We shall look at each of them separately.

Cytokinesis in animal cells

Cytokinesis is the process where the cytoplasm of a cell is physically divided to form two daughter cells. This occurs after the mitotic division of the nucleus. Cytokinesis in the animal cell begins shortly after the separation of the sister chromatids in the anaphase of mitosis. The process begins when a ring of actin and myosin filaments constricts the plasma membrane at the equator. Eventually, the cell breaks at the constricted region and the parent cell is divided into two cells.

Cytokinesis in plant cells

Cytokinesis in plant cells is different from that in animal cells. Unlike the animal cells, plant cells construct a cell plate in the middle of the cell. The cell plate begins to enlarge and finally comes into contact with the existing plasma membrane. At the end of the process, a new cell wall is formed on each side of the cell plate and two daughter cells are produced.

Meiosis

Meiosis is the cell division that takes place in the reproductive organs. In human beings and animals, meiosis occurs in the testes in males (to produce sperms) and the ovaries in females (to produce ovules). Meiosis in plants occurs in the anthers of flowers (to produce male gametes in the pollen) and in the ovaries of flowers (to produce egg cells in the ovules).

This cell division can be divided into two stages, namely, Meiosis I and Meiosis II. In meiosis I (first meitotic division), the homologous chromosomes in a diploid cell divide, producing four (haploid) daughter cells. It is this step in meiosis that generates genetic diversity. Meiosis I produces four daughter cells called gametes. Each daughter cell will have half of the chromosome numbers from the parent cell. The haploid cells form one set of chromosomes from each pair of homologous chromosomes. Gametes that are produced are haploid ( n ) whereas the parent cell is diploid (2n). Gametes have different genetic material from the parent cell.

The Significance of Meiosis

Allowing trait inheritance in offspring for the continuity of life

The transmission of traits from one generation to the next generation occurs during meiosis. Genes, also known as units of inheritance, are the inherited characteristics that are passed from parents to their offsprings through the genes in the sperms and ova. Genes present themselves in pairs. One is inherited from the father and the other is from the mother. During fertilisation, when the nucleus of a sperm fuses with the nucleus of an ovum, genes from both parents will be present in the nucleus of the zygote. The genes will determine the traits that will be inherited by the offsprings.

Maintaining the diploid chromosomal number from generation to generation

The maintenance of the diploid number of chromosomes in each generation is important. Meiosis produces gametes that are haploid and when fertilisation happens, a haploid sperm fuses with a haploid ovum to form a diploid zygote (one set of haploid chromosomes each from the paternal and maternal sides). The diploid chromosomal number is maintained in each generation. Thus, meiosis ensures genetic continuity from generation to generation.

Production of haploid gametes in sexual reproduction

In meiosis, the number of chromosomes is reduced by one half to produce gametes (sperm and ova). This ensures that the haploid generation receives a mixed set of genes. During sexual reproduction, the haploid gametes fuse together to form diploid offsprings and therefore restores the diploid conditions in each generation.

Producing genetic variation among offspring

Most importantly, recombination and the independent assortment of homologous chromosomes allow for a greater diversity of genotypes in the population. Meiosis results in unique combinations of maternal and paternal chromosomes during metaphase I. Crossing over during meiosis results in genetic exchange between the members of each pair of homologous chromosomes. This produces genetic variation in gametes that result in genetic and phenotypic variation in a population of offsprings.

The Process of Meiosis

Meiosis is a two-part nuclear division in which the number of chromosomes is halved. Meiosis I reduces the number of chromosomes and Meiosis II divides the double stranded chromosomes to form single stranded chromosomes. Meiosis creates daughter cells each of which receives half the number of chromosomes of the parent cell.

A human cell contains 46 chromosomes; therefore after the process of meiosis, the four daughter cells have 23 chromosomes each. Sex cells or gametes are produced in animals during meiosis. During sexual reproduction, the male and female gametes unite and create a new being called a zygote. The zygote receives two sets of 23 chromosomes from the gametes. These add to become the necessary 46 chromosomes and a diploid or 2n cell.