Cell Cycle and Cell Division Lecture 3 PDF

Lecture 3 Cell Biology Cell cycle & Cell division Cell cycle Definition: it is the alternation between interphase and mitosis. I- Interphase: a longer period: 1. The cell increases in size. 2. Performs its normal functions. 3. Replicates its DNA to prepare itself for division. II-...

Lecture 3 Cell Biology Cell cycle & Cell division Cell cycle Definition: it is the alternation between interphase and mitosis. I- Interphase: a longer period: 1. The cell increases in size. 2. Performs its normal functions. 3. Replicates its DNA to prepare itself for division. II- Mitosis: a shorter period during which the parent cell gives 2 daughter cells each containing the same number of chromosomes (identical to the parent cell - 46 chromosomes). I- Interphase  Interphase 1. First gap phase (G1 phase) It is the longest period of the cell cycle between the end of mitosis and the beginning of DNA replication: 1. The RNA and protein synthesis occurs. 2. The cell attains its full size. 3. The cell performs its function. 4. Duplication of centrosomes occurs near the transition between G, and S phase. The GO phase Definition: Differentiation of the cell to carry out specialized function and no longer divide (outside the cycle). ❖ GO may be permanent or temporary.  Interphase 2. DNA synthesis phase (S phase) Replication of DNA, thus the amount of DNA is doubled but not the total chromosomal number. Types of chromosomes s-chromosomes: made of one DNA molecule. d-chromosomes (mitotic chromosomes): are formed during the S phase. Each d- chromosome is formed of two chromatids, linked at the centromere. Each chromatid is made of a DNA molecule. S chromosome=chromatin-chromatid  Interphase 3. Second gap phase (G2 phase) It starts at the end of the DNA replication and lasts until the beginning of mitosis. 1. Proteins and energy essential to mitosis are stored. 2. Duplication of the centrosome is completed. Regulation of the cell cycle Several Checkpoints regulate the cell cycle and control the transition between the cycle stages. Checkpoints detect external or internal problems and stop the cycle until the problem is solved. 1. The restriction checkpoint It occurs in the G1 phase. It detects the cell size & its interactions with the surrounding environment. Cells that do not receive appropriate growth stimuli do not progress past this point (G1 phase) and will die by apoptosis. It is the most important checkpoint in the cycle The point of no return 2. DNA damage checkpoints It occur in G1, S, and G2 phases. It blocks cell cycle progression until repair of the damaged DNA or cell apoptosis Tumor suppressor protein p53 3. The un-replicated DNA checkpoint It occurs in the G2 phase. It prevents the cycle’s progression into mitosis before the complete synthesis of DNA. Mitosis  23 46 23 46 46 23 46 23 Mitosis Definition division of the somatic cell into two daughter cells identical to the mother cell. Importance 1. Growth and development of the organism. 2. Renewal and repair of cells. Mitosis Please Make Another Two Cells Mitosis 1. Prophase 1. Condensation of chromatin gives rise to 46 rod-shaped short d- chromosomes. 2. Each centrosome migrates to the cell’s opposite pole, forming the mitotic spindles. 3. The nucleolus disappears. 4. The nuclear envelope breaks up into small vesicles. Mitosis 2. Metaphase 1. The 46 d-chromosomes become maximally condensed. 2. The chromosomes aligned at the equatorial plate of the cell. Each pair of sister chromatid is attached to the mitotic spindles at the kinetochore. Why is karyotyping done at the metaphase? Mitosis 3. Anaphase 1. Division of the centromere results in the separation of the sister chromatids. 2. Each 46 chromatids migrate toward the opposite poles of the cell. 3. In late anaphase, a constriction (cleavage furrow) develops at the equatorial plate of the cell. Mitosis 4. Telophase 1. The chromosomes start uncoiling (46 s- chromosomes). 2. The mitotic spindle disappears. 3. The nucleolus reappears. 4. The nuclear envelope is reformed around the new sets of chromosomes. 5. Division of the cytoplasm (cytokinesis): the cleavage furrow becomes deeper due to the formation of a contractile ring of microfilaments until it divides the cytoplasm and its organelles in half resulting in two daughter cells. 1. The spindle-assembly checkpoint (the metaphase checkpoint) It occurs in mitosis. It prevents entry into anaphase until all chromosomes have attached properly to the mitotic spindle. Aneuploidy ✓ An abnormality in the number of chromosomes in a cell due to loss or duplication ,a common cause of genetic disorders, including Down syndrome (trisomy 21). 2. The chromosome-segregation checkpoint It occurs in telophase. It prevents cytokinesis until all chromosomes have been correctly separated. Cancer chemotherapy Some chemotherapeutic drugs bind to microtubules and inhibit the formation of the mitotic spindle, thus prevent the proliferation of cancer cells. Meiosis  Meiosis Definition: It is a type of cell division occurs in germ cells and results in the formation of gametes. It results in formation of 4 daughter cells (each contains 23 s- chromosomes-haploid number). It consists of two successive divisions; with an intervening short interphase without out an S phase. Please Make Another Two Cells I. First meiotic division (reductional division) It is preceded by interphase with the S phase, in which the chromosomes are replicated (46 s chromosomes 46 d chromosomes). I. First meiotic division (reductional division) 1. Prophase I: A. Pairing of the homologous chromosomes occurs forming tetrads (bivalent). They begin to make connections (synapsis) by synaptonemal complex. B. Crossing over occurs between the chromatids of the homologous chromosomes at sites called chiasmata. So, each chromosome contains some genes inherited from the mother and other genes inherited from the father. C. The nucleolus and the nuclear envelope disappear and the mitotic spindles are formed. I. First meiotic division (reductional division) 1. Prophase I: C. The nucleolus and the nuclear envelope disappear and the mitotic spindles are formed. I. First meiotic division (reductional division) 2. Metaphase 1 The paired chromosomes arrange themselves at the equatorial plate of the cell. 3. Anaphase 1 The centromeres do not divide, instead, each chromosome of homologous pairs moves separately towards the opposite poles of the cell. 3. Telophase 1 Cytokinesis occurs results in two daughter cells each containing the haploid number (23 d-chromosomes) II- Second meiotic division (equatorial division( It is similar to mitosis but, it is preceded with a short interphase but without S phase. It results in formation of 4 daughter cells, each contains 23 s-chromosomes (haploid number). Non-disjunction Definition: It is the failure of chromosomes to disjoin correctly during meiosis. This results in the production of gametes containing a greater or lesser chromosomal amount than normal ones. Consequently the individual may develop a trisomal or monosomal syndrome. Genetic variation in meiosis Occurs due to: 1. During prophase I: Crossing over between homologous chromosomes results in exchange of genetic materials. Genetic variation in meiosis Occurs due to: 2. During metaphase I: Independent assortment of homologous chromosomes (segregation of the alleles in different gametes independently of each others) due to random distribution of the maternal and paternal chromosomes at the equator. Meiosis I and Meiosis II