Cell division is necessary to the life of any organism. When the organism consists of a single cell, cell division permits reproduction. Even in a multicellular organism, cell division is involved in the production of cells that allow reproduction. At all times, cell division is necessary for growth and repair. The human embryo or, any embryo, begins as a single cell that repeatedly divides to produce the millions of cells needed for the development of organs. Thereafter, human cells differ in their capacity to divide.
Some cells, such as skeletal muscle cells and nerve cells, do not usually divide at all. Others, like liver cells will divide if tissue has been lost due to injury, surgery or disease. Others, for example those that produce skin cells or blood cells, divide constantly. In the adult human millions of cells must divide every second to maintain the life of the body.
During cell division, the genetic material (DNA) is distributed to the newly formed cells. Because of this, each cell contains genes that control the metabolism and the characteristics of the cell. In eukaryotic cells, these genes are enclosed in a nucleus.
Cell division
Nucleus
The eukaryotic cell contains two distinct regions: the cytoplasm and the nucleus, which is often centrally located. The nucleus is enclosed by two membranes collectively called the nuclear envelope. The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum. Numerous pores in the nuclear envelope permit substances to pass between the nucleus and the surrounding cytoplasm.
The nucleus contains a background substance known as nucleoplasm and within the nucleoplasm, one or two more nucleoli are usually found. Ribosomal RNA (rRNA) is synthesized and stored in the nucleolus. This RNA is necessary for the formation of ribosomes. Most important, the nucleoplasm also contains the threadlike chromatin where the DNA of the cell is located. In nondividing cells, the chromatin is dispersed, but during cell division, it condenses, forming the highly coiled chromosomes.
Chromosomes
Each species has a characteristic number of chromosomes. For example, maize (corn) cells have 20 chromosomes, mice cells have 40 chromosomes, and human cells have 46 chromosomes. To view clearly the chromosomes so that they can be counted, a cell can be treated and photographed just prior to dividing. The chromosomes can then be cut out of the photograph and arranged in pairs. Each pair with chromosomes of the same size and appearance is known as homologous. The resulting display of paired chromosomes is called a karyotype. A human karyotype consists of 46 chromosomes, made up of twenty-two homologous pairs and two sex chromosomes.
A karyotype
Although both male and female humans have 23 pairs of chromosomes, the chromosomes of one pair are of unequal length in the male. The larger chromosome in this pair is called the X chromosome and the smaller one is called the Y chromosome. Females have two X chromosomes in the karyotype. These Y and X chromosomes are called sex chromosomes because they contain genes that determine sex. All the other chromosomes are called autosomes.
Just prior to division, every chromosome is composed of two identical parts called chromatids. The two chromatids are genetically identical; that is, they contain the genes that control the same traits. The chromatids are constricted at the centromere.
Structure of a chromosome
Mitosis
When a cell divides, it gives rise to two cells, each cell having the same number of chromosomes as the original parent cell. This kind of cell division is called mitosis. Protozoa and many other single-celled organisms divide by mitosis.
Stages in mitosis
Growth involves cell division by mitosis. Mitosis involves a number of phases as described below:
1. Prophase: The nuclear envelope of the cell and nucleolus disappear. The nuclear material has condensed and the chromosomes are visible with a light microscope. Centrioles move to the opposite sides of the nucleus and form the spindle. Around each centriole is an aster.
2. Metaphase: Chromosomes attach to the spindle fibres and arrange themselves along the equator of the spindle so that they are at right angles to the spindle fibres. The chromosomes are now ready to divide.
3. Anaphase: The centromere divides, sister chromatids separate and move to the opposite poles of the spindle. Once the separation is complete, the chromatids are now called chromosomes. The chromosomes begin to uncoil and the nucleoli reappear. A nuclear membrane begins to form around the chromosomes.
4. Telophase: A nuclear membrane forms around chromosomes at each pole; the cytoplasm of the cell begins to cleave and divide, cytogenesis occurs resulting into two daughter cells. Mitosis is now complete.
5. Interphase: The cell nucleus is now well defined. The nucleoli are visible and the chromosomes have already duplicated but they cannot be seen with the light microscope because the chromatids are uncondensed or diffuse.
Meiosis
Meiosis is a kind of cell division used to produce gametes. It has two important functions:
(1) To form haploid cells with half the normal chromosome number
(2) To rearrange the chromosome with new combination of genes (genetic recombination)
Meiosis comprises two divisions; the second is division almost similar to mitosis but the first division is different in many respects.
Stages in meiosis
Meiosis I
Interphase: (not shown) Chromosomes have duplicated but they are not yet visible except as a mass of chromatin.
Prophase: chromatin coils into long thin chromosomes, each composed of two chromatids; chromosomes pair up, forming a structure called a tetrad. This process is called synapsis. Crossing over may occur at this time in which chromosomes exchange segments (see below).
Metaphase: chromosome tetrads align on the equator. Each chromosome is condensed so they appear short and thick. Homologous chromosomes of each tetrad are poised to move towards the opposite poles of the cell.
Anaphase: chromosomes migrate towards the two poles of the cell. Pairs of homologous chromosomes split up.
Telophase: chromosomes arrive at the poles of the cell. Each pole of the cell has haploid chromosome set. Note that each chromosome still consists of two sister chromatids.
Meiosis 2
Meiosis II is essentially similar to mitosis. The major difference is that meiosis II starts with a haploid cell.
Prophase: a spindle forms and moves the chromosomes towards the middle of the cell
Metaphase: chromosomes align on the equator.
Anaphase: the centromeres of sister chromatids separate and the sister chromatids of each pair now individual daughter chromosomes move towards opposite poles of the cell.
Telophase: nuclei form at the opposite poles of the cell and cytokinesis occurs at the same time. There are now four daughter cells (two in our case from one daughter cell), each with haploid number of (single) chromosomes.
Crossing Over (chiasma formation)
During prophase of the first meiotic division, the chromatids of homologous chromosomes come to lie along side each other in a process called synapsis, so that the locus of each gene is exactly opposite the same locus in the chromatid of the homologous chromosome. Sometimes the chromatids break and exchange equivalent portions, i.e. ‘cross over’ from one chromosome to its homologue and vice versa.
This exchange of genetic material occurs between non-sister homologous chromatids. Exchanges of genetic material can also occur between sister chromatids but are not of genetic importance since sister chromatids are identical.
Crossing over during meiosis 1
Crossing over is a means for exchanging genes between homologous chromosomes and it is important in increasing the amount of genetic variation. The gametes that receive these recombined chromosomes are termed recombinant genes.
Spermatogenesis and oogenesis
The formation of sperm by the testes is called spermatogenesis and the formation of eggs by the female ovaries is oogenesis. Both processes involve meiotic divisions. During oogenesis, meiosis produces one large cell and a small nonviable cell called the polar body. The polar body disintegrates and is lost.
The second meiotic division does not take place unless fertilisation occurs. However, complete oogenesis in females results into one single viable cell and at least two nonviable polar bodies.
In the male, the first meiotic division results into two haploid cells, each of which then goes on to give rise to two haploid cells during the second meiotic division. The result is that four haploid cells are produced from one original cell.
The outcome of spermatogenesis and oogenesis is the production of sex cells that have one-half the number of chromosomes of an ordinary cell. In humans, each female egg and each male sperm cell has 23 chromosomes.
Mitosis and meiosis compared
Mitosis | Meiosis |
Produces genetically identical cells The only variation can come through genetic mutation | Produces four non-identical daughter cells |
Chromosome number same as parent i.e. diploid | Chromosome number is halved i.e. haploid |
No chromosome combination and therefore no variation except through mutation | Chromosomes are rearranged to form combinations that are different from parent’s, which is source of genetic variability |
Organisms that produce asexually use mitosis. The offspring are identical genetically to each other and to their parent; they are clones | Organisms that produce sexually use meiosis. Sexual reproduction involves two parents using gametes and fertilisation to produce a zygote |
Asexual reproduction is faster; conserves genetic status quo. Species are not properly adapted to the environment | Sexual reproduction is slower and more complex; it introduces genetic variation due to recombination in meiosis and random fertilisation which allows species to adapt better to environment |
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