Cell biology - Eukaryotic cell structure

Cell biology - Eukaryotic cell structure
In this lecture:

TYPES OF REPRODUCTION


TYPES OF REPRODUCTION
--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.

-- 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. )
Mitosis and Meiosis

Meiosis examples

Question


What are the differences between Mitosis and Meiosis processes?

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.
Shape of chromosome
chromosome
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:
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.
3- Second growth phase – G2
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.
Cell cycle
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
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.).
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. 
Spindle
Spindle : an array of microtubules stretches from pole to pole of a dividing nucleus and pull the chromosomes during mitosis and meiosis .
Prophase
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.
Mitosis phases
Metaphase
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.
Mitosis phases
Anaphase
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.
Mitosis phases
Telophase
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.
Question of lecture


Show the role of cytoskelton classes during cell division
The answer
In prophase an array of microtubules stretches from pole to pole of a dividing nucleus and pull the chromosomes during mitosis and meiosis .

in animal cells cytokinesis often consist of a pinching apart of the two daughter cells by the contraction of a ring of microfilaments.









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