Cell Division Notes PDF

Cytogenetics 1- identify stages of Cell Cycle 2- list the stage of Mitotic division 3- enumerate general characteristics of meiosis 4- identify the changes occurred in prophase 1 of meiosis 5- compare between mitosis and meiosis I-The interphase: 2-Cell Division(mitosis)  1- Gl-phase(Gap or pr...

Cytogenetics 1- identify stages of Cell Cycle 2- list the stage of Mitotic division 3- enumerate general characteristics of meiosis 4- identify the changes occurred in prophase 1 of meiosis 5- compare between mitosis and meiosis I-The interphase: 2-Cell Division(mitosis)  1- Gl-phase(Gap or presynthesis: the cell restores its normal size after mitosis, forms RNA and protein, needed for its growth. 2- S-phase(DNA synthesis) during which, duplication of DNA & centrioles takes place. 3- G2-phase(Post DNA duplication): is the phase in which the cell produces the energy needed for the next cell division  according to the ability to divide Cells that cannot divide------Nerve cells. ( GO) Cells unable to divide but after degeneration are replaced by stem cells of the same family----- Cells of small intestine. Cells that divide only at time of need-- Cells of liver and endocrine gland cells.  Prophase Metaphase Anaphase Telophase occurred in testis and ovary produces the germ cells, ova and sperms (by process called oogenesis and spermatogenesis).  General Characteristics It is a reductional cell division that occurs in the germ cells of testis and ovary. It consists of two divisions, but only one DNA replication First meiotic division brings about separation of homologous chromosomes (d-chromosomes). A second meiotic division occurs without the DNA duplication. brings about separation of sister chromatids (s-chromosome). The mother cell gives rise to four daughter cells with haploid number of chromosomes (s- chromosome=23 chromosome). In testis four sperms are produced, while in ovary, it gives one ovum and three Polar bodies (degenerate).  Prophase I: It is the longest phase of meiosis, Leptotene stage: Chromosomes appear as thin threads as in early prophase of mitosis. Zygotene stage: Chromosomes are arranged in homologous pairs. Each pair forms a bivalent (one chromosome is of paternal origin from the father, while the other is its maternal homologue from the mother). Pachytene stage: Each chromosome splits lengthwise into 2 chromatids, which remain attached only at their centromeres. The bivalent therefore consists of four chromatids and is called a tetrad. Diplotene stage: Homologous chromatids move slightly a part and the tetrad formation becomes more obvious. At points along their length the homologous chromatids form chiasma (X-shaped crossing over). These chiasmata are the points where crossing over of genetic material takes place. Diakinesis: The nuclear membrane and the nucleolus disappear, and the tetrads migrate to the equator of the cell.    Metaphase I: It is similar to the metaphase of mitotic division except that pairs of chromosomes (bivalents), not single chromosomes are arranged in the equatorial plate and attach to the microtubules of the spindle.  Anaphase 1: Homologous chromosomes each consisting of 2 chromatids are separated (centromeres have not split) and move towards the corresponding poles of the cell. 23 chromosomes (d- chromosome) move towards each pole. Each chromosome contains a double amount of DNA so that the total amount of DNA within the resulting cells remains diploid (d-chromosome  Telophase 1: Chromosomes form the chromatin of the new nucleus and the cytoplasm separates into two parts.  The Second meiotic division: Prophase II: It is similar to mitosis without a period of DNA replication. Metaphase II: The 23 d-chromosomes arrange themselves in the equatorial plane. There is duplication of centromere DNA after which the centromere splits. AnaphaseI1: The two chromatids, of each chromosome, separate and move towards the corresponding poles of the cell. TelophaseI1 Chromosomes form the chromatin of the new nucleus and the cytoplasm separates into two parts, resulting in cells with haploid number of chromosomes and haploid amount of DNA. Unlike mitosis the sister chromatids are not identical due to crossing over that occurred during the first meiotic prophase.