cell biology -cell cycle and cell division

Mitosis takes a diploid cell and creates a nearly exact copy. Mitosis has two main functions: (1) it leads to the creation of all of the somatic (body) cells in humans and other living organisms; (2) in organisms that undergo asexual reproduction, diploid parent cells undergo mitosis to create identical daughter copies of themselves. Mitosis creates a daughter cell with chromosomes that are identical to the chromosomes in its parent cell. Mitosis includes four phases namely G1,S,G2 and meiosis (M phase) which occur in succession these called cell cycle .
The first three phases form in mitotic cycle as fallow :
1- Interphase
Interphase is a period between two successive nuclear divisions, during which the chromosomes are diffuse and the nuclear envelope is intact , during this period the cell is most active in transcribing and translating genetic information that is a time of active gene expression . If the cell is going to divide, it replicates its DNA during interphase , interphase consists of the G1, S, and G2 phases .

A -Growth phase one (G1) gap one
It is a period of rapid RNA protein synthesis , the daughter cells resulting from mitosis usually engaged in metabolism and growth . The genes being coding messages for the production of new protoplasm that lead to increase in cell mass .
This stage is devoted to cell growth and chemical preparation for DNA synthesis during G1 phase chromosomes are completely dispersed .


B - Synthesis phase (S)
In the S phase of interphase both DNA and histones synthesis, the nucleus replicates its chromatin , thus the amount of DNA doubles. From this point until the centromer divides , each chromosome consist of two chromatids.






C -Growth phase two (G2) gap two
After the completion of DNA duplication, the cell enters a second growth phase called G2 .It is a period between the end of DNA synthesis and beginning of prophase , In this period all the genes are function fully again and the rate of protein synthesis is high . The relative lengths of these phases differ in all organisms for example a human cell grown in tissue culture the mitotic cycle is about 18 hr.( G1 :8hr. , S:6hr. , G2 :4hr.) and mitosis (M- phase) consume about one hour the whole cycle is 19hr.

Mitotic division
It occurs during the mitotic phase or M- phase , it is divided into four phases:
1- Prophase
Prophase is the first phase of mitosis , it begins when chromosomes thread like structure .The nucleoli and nuclear envelope begin to break up and the two centriole pairs move apart. By the end of this phase ,the centriole pairs are at opposite poles of the cell. The centrioles radiate an array of microtubules called asters (little star) between the centrioles ,microtubules form a spindle of fibers that extens from pole to pole .The asters , spindle ,centrioles and microtubules are collectively called the mitotic spindle or mitotic apparatus.

spindle : an array of microtubules stretches from pole to pole of a dividing nucleus and moves the chromosomes during mitosis and in both division of meiosis


3- Metaphase
At metaphase of a nuclear division, the centromeres of a highly condensed chromosomes are all lying on a plane (equatorial plane) perpendicular to a line connecting the spindle poles. Metaphase ends abruptly as the centromeres divide and anaphase begins.
















4- Anaphase
During this phase which the shortening of the microtubules in the mitotic spindle pulls each daughter chromosome apart from its copy and toward its respective pole. Anaphase ends when all the daughter chromosomes have moved at the poles of the cell , each pole now has a complete , identical set of chromosomes .


5- Telophase
In telophase of mitosis, the chromosomes become diffuse, the spindle breaks down then nuclear envelopes form and nucleoli appear in the daughter nuclei. Telophaes is the final stage of a nuclear division.

Cytokinesis
Division of the cytoplasm is called cytokinesis. In animal cells, cytokinesis often consist of the pinching apart of the two daughter cells by the contraction of a ring of microfilaments.


















Unlike the single-cell division of mitosis, meiosis involves two cellular divisions: meiosis I and meiosis II. Each stage of meiosis runs through the same five stages as discussed in mitosis. During the first round of division, two intermediate daughter cells are produced. By the end of the second round of meiotic division (meiosis II), the original diploid (2n) cell has become four haploid (n) daughter cells. But humans and most other complex plants and animals each have a unique set of chromosomes. This diversity of chromosomes is the result of sexual reproduction, which involves the contribution of the genetic material from not one, but two parents. During sexual reproduction the father’s haploid sperm cell and the mother’s haploid ovum (egg) cell fuse to form a single-celled diploid zygote that then divides billions of times to form a whole individual.
In order for sexual reproduction to take place, however, the parents first need to have haploid sperm or ova, also called sex cells, germ cells, or gametes. Meiosis is the name for the special type of cell division that produces gametes.

























Spermatogenesis and Oogenesis
Meiosis, the process by which gametes are formed, can also be called gametogenesis, literally “creation of gametes.” The specific type of meiosis that forms sperm is called spermatogenesis, while the formation of egg cells, or ova, is called oogenesis. The most important thing you need to remember about both processes is that they occur through meiosis, but there are a few specific distinctions between them.

Spermatogenesis
The male testes have tiny tubules containing diploid cells called spermatogonium that mature to become sperm. The basic function of spermatogenesis is to turn each one of the diploid spermatogonium into four haploid sperm cells. This quadrupling is accomplished through the meiotic cell division detailed in the last section. During interphase before meiosis I, the spermatogonium’s 46 single chromosomes are replicated to form 46 pairs of sister chromatids, which then exchange genetic material through synapsis before the first meiotic division. In meiosis II, the two daughter cells go through a second division to yield four cells containing a unique set of 23 single chromosomes that ultimately mature into four sperm cells. Starting at puberty, a male will produce literally millions of sperm every single day for the rest of his life.




















Fig. spermatogonium process











Oogenesis

Just like spermatogenesis, oogenesis involves the formation of haploid cells from an original diploid cell, called a primary oocyte, through meiosis. The female ovaries contain the primary oocytes. There are two major differences between the male and female production of gametes. First of all, oogenesis only leads to the production of one final ovum, or egg cell, from each primary oocyte (in contrast to the four sperm that are generated from every spermatogonium). Of the four daughter cells that are produced when the primary oocyte divides meiotically, three come out much smaller than the fourth. These smaller cells, called polar bodies, eventually disintegrate, leaving only the larger ovum as the final product of oogenesis. The production of one egg cell via oogenesis normally occurs only once a month, from puberty to menopause.



















Fig. Oogenesis process