TYPES OF REPRODUCTION

Medical biology - Lec. 4

TYPES OF REPRODUCTION
1- Mitosis (asexual ) : is a type of cell division which occurs in somatic cell by which the chromosome number is multiplied , any cell undergoes mitosis gives rise to 2 identical daughter cells , each daughter cell resembles the mother cell both having the same chromosome number.















Fig. mitosis and meiosis processes

2- Meiosis (sexual): It is a type of cell division occurs in germ cell of sexually reproducing organisms. Two successive nuclear divisions designated meiosis -1 and
meiosis – 2 take place , result in four daughter cells each with half number of chromosomes of parent cells . The specific type of meiosis that forms sperm is called spermatogenesis, while the formation of egg cells, or ova is called oogenesis , fertilization of an egg by sperm occurs in reproductive organs each contain half of the parent cell s chromosomes (fig. )












Living cell has ability divided to reproduce organisms, to grow, to replace damaged cells, to transfer genes from cell to cell. During development, the cells in the body alternately divide (mitosis) and appear to be resting (interphase) the sequence of activities exhibited by cell is called cell cycle. When a cell divides, chromatin fibers are very highly folded and become visible in the light microscope as chromosomes but during interphase (between divisions) chromatin is more extended, a form used for expression genetic information. Cell cycle includes four phases namely G1, S, G2 and mitosis (M- phase) which occur in succession as fellow:
Interphase
Interphase is a period between two successive nuclear divisions, during which the chromosomes are diffuse, nuclear envelope is intact, the cell is most active in transcribing and translating genetic information. If the cell is going to divide, it replicates its DNA during interphase; interphase consists of the G1, S, and G2 phases:

1- First growth phase
It is a period of rapid RNA and protein synthesis, the daughter cells resulting from mitosis usually engaged in growth and metabolism. 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 (fig.) .















Fig. Cell cycle


2- 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 consists of two chromatids (fig.).

3- Second growth phase
After the completion of DNA duplication, 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(fig.)
The relative lengths of these phases differ in all organisms for example a human cell grown in tissue culture the cell cycle is about 18 hr.( G1 :8hr. , S:6hr. , G2 :4hr.) and mitosis (M- phase) consume about one hour the whole cycle is 19hr.

Cell division
It occurs during the mitotic phase (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 fibers that extend from pole to pole .The spindle , centrioles and microtubules are collectively called the mitotic spindle or mitotic apparatus (fig.).

Spindle : an array of microtubules stretches from pole to pole of a dividing nucleus and pull the chromosomes during mitosis and meiosis .












Fig. Mitotic spindle


The centriol: It is usually lies adjacent to outer side of the nuclear membrane towards the larger area of cytoplasm. The cytoplasm immediately surrounding it is called the centrosome. The centriol appears as a small hollow cylinder, it plays an important role in cell division during mitotic prophase, the centriole divided into two and followed by the division of the centrosome and each migrates towards the opposite pole with the radiating astral rays.

2- Metaphase
At metaphase of a nuclear division, the centromeres of 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 (fig.).

3- Anaphase
During this phase which the shortening of microtubules in 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 (fig.).
4- - Telophase
In telophase of mitosis, chromosomes become diffuse, spindle breaks down then nuclear envelopes form and nucleoli appear in the daughter nuclei. Telophaes is the final stage of a nuclear division (fig.).

Cytokinesis
Division of cytoplasm is called cytokinesis, in animal cells cytokinesis often consist of a pinching apart of the two daughter cells by the contraction of a ring of microfilaments and in plant, cell plate forming to separate into two cells (fig.).























Fig. mitotic division phases














Fig. cytokinesis