شيت الـ Cell cycle - محمد الشبلي - 2003 PDF

# تنوع مروري ## Cell Cycle ### **Multiplication of Cells** - Multiplication of cells occurs by division of pre-existing cells. - Multiplication is an essential feature of embryonic development and is necessary after birth for growth and replacement of dead cells. ### **The Chromosomes** - The ch...

# تنوع مروري ## Cell Cycle ### **Multiplication of Cells** - Multiplication of cells occurs by division of pre-existing cells. - Multiplication is an essential feature of embryonic development and is necessary after birth for growth and replacement of dead cells. ### **The Chromosomes** - The chromosomes within nuclei of the cells carry genetic information. ### **Mitosis** - Mitosis: It is a type of division in which daughter cells must have chromosomes identical in number and genetic content to those in the mother cell. ### **Meiosis** - A different kind of cell division called Meiosis occurs during gamete formation. - Meiosis consists of two successive divisions called first and second meiotic divisions. - **The gametes differ from other cells in the body in that:** - The number of chromosomes is reduced to half the normal number. - The genetic information in various gametes produced is not identical. ### **The Cell Cycle** - The cell cycle is a series of events that prepare the cell for dividing into two daughter cells. ### **The Cell Cycle Is Divided Into Two Events:** 1. **Interphase:** It is a long period of time during which the cell increases its size, content, and replicates its genetic material DNA. 2. **Mitosis:** A shorter period of time during which the cell divides its nucleus and cytoplasm, giving rise to two daughter cells. ### **Notes** - Cells may cease mitosis permanently, e.g., neurons, muscle cells. - Cells that leave the cell cycle are said to be in a resting stage, the G0 (outside) phase or the stable phase. ## **Interphase** - Interphase is subdivided into three phases: 1. **G1 (Gap) Phase:** When the synthesis of macromolecules essential for DNA duplication begins. - The cells synthesize RNA, regulatory proteins, and enzymes essential to DNA replication. - The cell volume is restored to normal. - The nucleoli are re-established. - The centrioles begin to duplicate themselves and are completed in G2 phase. 2. **S (Synthetic) Phase:** When DNA is duplicated. - All of the nucleoproteins (including histones) are manufactured and incorporated into DNA, forming chromatin. - Autosomal cells contain the diploid (2n) amount of DNA before the S phase and doubled (4n) in preparation for cell division. 3. **G2 (Gap) Phase:** When the cells undergo preparation for mitosis. - It is the period between the end of DNA synthesis and the beginning of mitosis. - The RNA and proteins essential to cell division are synthesized. - The energy for mitosis is stored. - Tubulin is synthesized for assembly into microtubules required for mitosis. ## **Mitosis** - Mitosis is the process of cell division that results in the formation of two identical daughter cells. - Mitosis (M) occurs at the conclusion of the G2 phase and thus completes the cell cycle. - First, the nuclear material is divided in a process called karyokinesis. Followed by division of the cytoplasm, called cytokinesis. - Mitosis is divided into four stages: Prophase, Metaphase, Anaphase, and Telophase. ### **Prophase:** - At the beginning, the chromosomes are condensing and thus become visible microscopically. - Each chromosome consists of two parallel sister chromatids joined together at one point (the centromere). - The nucleolus disappears. - The nuclear envelope disappears. ### **Metaphase:** - The chromosomes become maximally-condensed and lined up at the equator of the mitotic spindle (Metaphase plate configuration). - Sister chromatids must be maintained in closer positions. ### **Anaphase:** - Anaphase begins when sister chromatids pull apart and begin their migration toward the opposite poles. - In the late stage of this phase, a cleavage furrow begins to form at the plasma lemma. ### **Telophase:** - It is the terminal phase of mitosis. - Cytokinesis (division of cytoplasm) occurs. - Reconstitution of the nucleus and nuclear envelope happens. - Disappearance of spindle fibers occurs. - Unwinding of the chromosomes into chromatin occurs. - Note: Each daughter cell possesses a diploid (2n) number of chromosomes. ## **Meiosis (Gametogenesis)** - Meiosis is a special type of cell division resulting in the formation of gametes (spermatozoa or ova) whose chromosome number has been reduced from diploid (2n) to haploid (1n) number. - Meiosis begins at the conclusion of interphase, and it produces germ cells, the ova and sperms. ### **This Process Has Two Crucial Results:** 1. **Reduction in the number of chromosomes from diploid (2n) to haploid (1n)**, ensuring each gamete carries the haploid amount of DNA and haploid number of chromosomes. 2. **Recombination of genes**, ensuring genetic variability and diversity. ## **Meiosis I (Reductional Division)** - It separates the homologous pairs of chromosomes, thus reducing the number from diploid to haploid. ### **Meiosis I is Divided into Four Stages:** 1. **Prophase I:** - Lasts a long time and homologous pairs of chromosomes approximate each other, lining up and making synapses, forming tetrads. - Chiasmata (crossing over sites) are formed, and random exchanges of genetic material occurs between homologous chromosomes. 2. **Metaphase I:** - Characterized by homologous pairs of chromosomes being lined up in the equatorial plate. - Note: Each chromosome is composed of two chromatids. 3. **Anaphase I:** - Homologous pairs of chromosomes migrate away from each other going to opposing poles. - Note : Each chromosome still consists of two chromatids. 4. **Telophase I:** - Similar to Telophase of mitosis. - Nuclei are reformed, and cytokinesis occurs, giving rise to two daughter cells. - Each cell possesses 23 chromosomes (haploid number). - Each chromosome is composed of two chromatids. - Each newly formed cell enters meiosis 2. ## **Meiosis II (Equatorial Division)** - Meiosis II is not preceded by the S phase (without DNA synthesis). - It is similar to mitosis and is subdivided into Prophase II, Metaphase II, Anaphase II, Telophase II, and Cytokinesis. - The chromosomes line up in the equator, and the kinetochores attach to spindle fibers, followed by chromatids migrating to opposing poles, and cytokinesis divides each cell into two cells. - This results in a total of four cells from the original diploid germ cell. - Each cell contains a haploid amount of DNA and haploid chromosome number. - Each gamete contains its own unique genetic content.