Cell biologyFor the first stage
Meiosis(reductive division)
Meiosis (reductive division)
Meiosis: when an egg cell and sperm cell are combined to form a zygote, their chromosomes are recombined into a single cell, because humans are diploid organisms, therefore it must be a mechanism to reduce (reduct) the number of chromosomes in gametes to the haploid state.The primary mechanism by which haploid gametes are formed from diploid precursor cells is termed meiosis.
There are two cell division steps occur during meiosis, each meiotic division has been divided into stages with the same names as those of mitosis, but the processes involved in some of the stages are quite different.
During meiosis I, often called the reduction division stage,
two haploid cells are formed a diploid cell, these diploid cells are Oogonia in females and the Spermatogonia in males.Following meiosis I, a second meiosis, the equational division, take place, and each haploid cell is replicated.
Meiosis I:
1-Interphase I :during this phase, as in mitotic interphase, important processes such as replication of chromosomal DNA take place.
The chromosome separated into two chromatids and not visible under light microscope.
2-Prophase I: is complicated and includes many of events that distinguish meiosis from mitosis ,begins as the chromatin strands coil and condense, causing them to become visible as chromosomes.
During a process called synapsis, the homologous chromosomes pair up, side by side, lying together in perfect alignment, this pairing of homologous chromosomes is an important part of the cell cycle that does not occur in mitosis.
As prophase I continues, the chromatids of the two chromosomes intertwine, each pair of intertwined homologous chromosomes is called a bivalent (two chromosomes in the unit) or tetrad (four chromatids in the unit).
A second feature of prophase I is the formation of chiasmata (plural of chiasma), each chiasma indicates a point at which the homologous chromosomes (non-sister chromatids) exchange genetic material, this process called crossing over, produces chromosomes consisting of combinations of parts of the original chromosomes which leads to increase the genetic variation in gametes.
At the end of prophase I:
• the bivalents begin to move toward the equatorial plane,
• a spindle apparatus begins to form in the cytoplasm, and
• the nuclear membrane dissipates.
3-Metaphase I: characterized by the completion of spindle formation and the alignment of the bivalents (which are still attached at the chiasmata),in the equatorial plane, and attached to the spindle fibers.
4-Anaphase I: the
• chiasmata disappear, and
• the homologous chromosomes are pulled by the spindle fibers toward opposite poles of the cell,
• while sister chromatids remain associated at their centromeres.
5-Telophase I: begins when the chromosomes reach opposite sides of the cell,
the chromosomes uncoil and
a new nuclear membrane begins to form, and the spindle disappears.
The two daughter cells each contain the haploid number of chromosomes, each having two sister chromatids.
In humans, cytokinesis also occurs during this phase..
Meiosis II
1-Interphase II: the important feature of interphase II is that, unlike interphase I and mitotic interphase(no replication of DNA occurs).2-Prophase II: the nuclear membrane disappears ,and new spindle fibers are formed.
3-Metaphase II: the spindle fibers pull the chromosomes into alignment at the equatorial plane ,attached to the kinetochores of sister chromatids.
4-Anaphase II:the link between the centromeres of sister chromatids splits, each carrying a single chromatid toward a pole of the cell ,the chromatids have now separated(produce newly separated sister chromatids that may not be identical :because of the chiasma formation and crossing over ).
5-Telophase II: begins when the chromosomes reach opposite poles of the cell, they begin to uncoil, the spindle disappear, new nuclear membranes are formed around each group of chromosomes,
and cytokinesis occurs.
In gametes formed in males, the cytoplasm is again divided equally between the two daughter cells, the end result of male meiosis is thus four functional daughter cells, each of which has an equal amount of cytoplasm.
In female gametes, unequal division of the cytoplasm again occurs, forming the egg cell and an other polar body(large egg cell and three polar bodies.
The polar body: a small nonfunctional cell that degenerates.
Gametogenesis
The formation of gametes, in mature males, the seminiferous tubules of the testes are populated by spermatogonia,which are diploid cells (46 chromosomes)After going through several mitotic divisions, the spermatogonia produce primary spermatocytes (46 chromosomes).
Each primary spermatocytes which is also diploid, undergoes meiosis I to produce a pair of secondary spermatocytes (haploid) ,each of them contains 23 double stranded chromosomes, these undergo meiosis II, and each produces a pair of spermatids that contain 23 single-stranded chromosomes(haploid).
The spermatids then lose most of their cytoplasm and develop tails for swimming as they become mature sperm cells , this process known as spermatogenesis.., which continues throughout the life of the mature male.
In spermatogenesis, each diploid spermatogonium produces four haploid sperm cells.
Oogenesis
The process in female gametes are formed, differs in several important ways from spermatogenesis, whereas the cycle of spermatogenesis is constantly recurring in males, much of female oogenesis is completed before birth.
The diploid oogonia divide mitotically to produce primary oocytes(diploid) by the third month of fetal development.., more than 2 million primary oocytes are formed during gestation,
In meiosis I, the primary oocyte produces one Secondary oocyte"haploid"(containing the cytoplasm) and one polar body ,the secondary oocyte then emerges from the follicle and proceeds down the fallopian tube with the polar body attached to it.
Meiosis II begins only if the secondary oocyte is fertilized by a sperm cell, if this occurs, one haploid mature ovum ,containing the cytoplasm and another haploid polar body are produced, the polar bodies disintegrate.
About an hour after fertilization, the nuclei of the sperm cell and ovum fuse, forming a diploid zygote, the zygote then begins its development into an embryo through a series of mitotic division.
