Normal Cell Division PDF

NORMAL CELL DIVISION On a day-to-day basis, cells become damaged, diseased, or worn out and then die. New cells must be produced as replacements and for growth. In addition, sperm and egg cells must be produced by cell division. Cell division is the process by which cells reproduce themselves. It...

NORMAL CELL DIVISION On a day-to-day basis, cells become damaged, diseased, or worn out and then die. New cells must be produced as replacements and for growth. In addition, sperm and egg cells must be produced by cell division. Cell division is the process by which cells reproduce themselves. It consists of a nuclear cell division and a cytoplasmic division (cytokinesis). There are two kinds of cell division:  SOMATIC CELL DIVISION – A single parent cell duplicates itself by a nuclear division called mitosis and cytokinesis. Each new daughter cell has the same number and kind of chromosomes as the original parent cell. This results in an increase in the number of body cells.  REPRODUCTIVE CELL DIVISION – This is the mechanism whereby sperm and egg cells are produced, preliminary to the formation of a new organism. The process consists of a nuclear division called meiosis plus cytokinesis.  SOMATIC CELL DIVISION When a cell reproduces, it must replicate its chromosomes so its hereditary traits may be passed on to succeeding generations of cells. A chromosome is a highly coiled DNA molecule that is partly covered by protein. The protein causes changes in length and thickness of the chromosome. The hereditary information is contained in the DNA portion of the chromosome in units called genes. When a cell is between divisions it is said to be in interphase (metabolic phase). During this stage, the replication of chromosomes occurs and RNA and protein needed to produce structures required for doubling all cellular components are manufactured. When DNA replicates, its helical structure partially unfolds. The separation occurs at the points where the nitrogen bases are connected. Each exposed nitrogen base then picks up a complementary nitrogen base with associated sugar and phosphate group from the cytoplasm of the cell. This uncoiling and complementary base pairing continues until each of the two original DNA strands is matched and joined with two newly formed DNA strands. The original DNA molecule has become two DNA molecules. Once a cell completes its replication of DNA and its production of RNA and proteins during interphase, mitosis begins. A microscopic view of a cell during interphase shows a clearly defined nuclear membrane, nucleoli, karyolymph, chromatin and a pair of centrioles. 1  MITOSIS Mitosis is the distribution of two sets of chromosomes into two separate and equal nuclei following the replication of the chromosomes of the parent nucleus. The process is divided into 4 stages:  Prophase  Metaphase  Anaphase  Telophase Actually, mitosis is a continuous process, one stage merging into another imperceptibly.  PROPHASE During this phase, the chromatin shortens and coils into chromosomes. The nucleoli become less distinct and the nuclear membrane disappears. Each prophase ‘chromosome’ is actually composed of a pair of structures called chromatids. Each chromatid is a complete chromosome consisting of a double- stranded DNA molecule and each is attached to its chromatid pair by a small spherical body called a centromere. During prophase, the chromatid pairs assemble near the equatorial plane region of the cell. The paired centrioles separate and each pair moves to an opposite pole of the cell. Between the centrioles, a series of microtubules is organised into two groups of fibres that extend from one pole of the cell to another. As they grow toward each other, the second group of microtubules develops. These are called chromosomal microtubules and extend from a centromere to a pole of the cell. Together, the continuous and chromosomal microtubules constitute the mitotic spindle and, with the centrioles, are referred to as the mitotic apparatus.  METAPHASE This is the second stage of mitosis. Here, the centromeres of the chromatid pairs line up on the equatorial plane of the cell. The centromeres of each chromatid pair form a chromosomal microtubule that attaches the centromere to a pole of the cell.  ANAPHASE Anaphase is characterised by the division of the centromeres and the movement of complete identical sets of chromatids, now called chromosomes, to opposite poles of the cell. During this movement, the centromeres attached to the chromosomal microtubules seem to drag the trailing parts of the chromosomes towards opposite poles. 2   TELOPHASE This is the final stage of mitosis and it consists of a series of events nearly the reverse of prophase. As telophase progresses, new nuclear membranes begin to enclose the chromosomes. The chromosomes start to assume their chromatin form, nucleoli reappear and the mitotic spindles disappear. The centrioles also replicate so that each cell has two centriole pairs. The formation of two nuclei identical to those of cells in interphase terminates telophase and a mitotic cycle has been completed.  CYTOKINESIS The division of the cytoplasm often begins during late anaphase and terminates at the same time as telophase. Cytokinesis begins with the formation of a cleavage furrow that extends around the cell’s equator. The furrow progresses inward, resembling a constricting ring, and cuts completely through the cell to form two separate portions of cytoplasm.  REPRODUCTIVE CELL DIVISION In sexual reproduction, each new organism is produced by the union and fusion of two different sex cells, one from each parent. The sex cells are called gametes. The ovum is produced in the female gonads (the ovaries) and the sperm is produced in the male gonads (the testes). The union and fusion of gametes is called fertilisation and the cell thus produced is known as a zygote. The zygote contains a mixture of chromosomes (DNA) from the two parents. Gametes differ from all other somatic cells with respect to the number of chromosomes in their nuclei.  Somatic cells contain 46 chromosomes in their nuclei of which 23 are a complete set that contain all the genes necessary for carrying out activities of the cell. The other 23 chromosomes are a duplicate set. The symbol n is used to designate the number of different chromosomes within the nucleus. Since somatic cells contain two sets of chromosomes, they are referred to as diploid cells, symbolised by 2n. In a diploid cell, two chromosomes that belong to the same pair are called homologous chromosomes.  If gametes had the same number of chromosomes as somatic cells, the zygote formed from their fusion would have a double number. The somatic cells of the resulting individual would have twice the number of chromosomes (4n) as the somatic cells of the parents. With every succeeding generation the number of chromosomes would double. The chromosome number, in fact, does not double with each generation because of a special nuclear division called meiosis. This occurs only in the reproduction of gametes and it causes a developing sperm or ovum to relinquish its duplicate set of chromosomes so that the mature gamete has only 23. Thus gametes are haploid cells symbolised as n. 3  MEIOSIS Meiosis occurs in two successive nuclear divisions referred to as reduction division (meiosis I) and equatorial division (meiosis II). During the interphase that precedes reduction division of meiosis, the chromosomes replicate themselves. This replication is similar to that in interphase preceding the mitosis of somatic cell division. Once chromosomal replication is complete, reduction division begins. This consists of 4 phases referred to as prophase I, metaphase I, anaphase I and telophase I.  Prophase I is an extended phase in which chromosomes shorten and thicken, the nuclear membrane and nucleoli disappear, the centrioles replicate and the spindle appears. However, in meiosis, the chromosomes line up in the nuclear region in homologous pairs. The pairing is called synapsis. The four chromatids of each homologous pair are referred to as a tetrad. A process called crossing-over occurs within the tetrad where portions of one chromatid may be exchanged with portions of another. Thus process permits an exchange of genes so that subsequent cells produced are unlike each other genetically and unlike the cell that produced them. This phenomenon accounts for the great genetic variation among humans.  In metaphase I, the paired chromosomes line up along the equatorial plane of the cell, with one member of each pair on either side.  Anaphase I is characterised by the separation of the members of each pair of chromosomes, with one member of each pair moving to the opposite pole of the cell. Unlike mitotic anaphase, the centrioles do not split and the paired chromatids, held by a centromere remain together.  Telophase I and cytokinesis are similar to telophase and cytokinesis of mitosis. The net effect of reduction division is that each resulting daughter cell contains the haploid number of chromosomes. Each cell contains only one member of each pair of the original homologous chromosomes in the starting parent cell. The interphase between reduction division and equatorial division is either brief or lacking altogether. When present, there is no replication of DNA. The equatorial division of meiosis consists of four phases referred to as prophase II, metaphase II, anaphase Ii and telophase II. These phases are essentially similar to those that occur during mitosis since the centromeres divide and chromatids separate and move toward opposite poles of the cell. The net result of meiosis is four haploid cells. 4