Cell Cycle Biology II (PDF)

Cell Cycle CELL CYCLE The cell cycle is a series of events that occur in the cell to prepare it for division into two daughter cells. It is divided into mitosis, the period in which the cell divides, and interphase, the longest stage in which the cell increases its size, its content and replicate...

Cell Cycle CELL CYCLE The cell cycle is a series of events that occur in the cell to prepare it for division into two daughter cells. It is divided into mitosis, the period in which the cell divides, and interphase, the longest stage in which the cell increases its size, its content and replicates its genetic material. INTERPHASE It is the time between the stages of mitosis. It is divided into three phases: - Phase G1: first stage of the cell cycle. It is a period of cellular growth, where the cell regains its size that had decreased when the cell divided into two after mitosis. Nucleoli are restored, synthesis of RNA, enzymes and regulatory proteins necessary for all cell functions. - Phase S: stage of DNA synthesis during the cell cycle. The cell duplicates its DNA content. - Phase G2: synthesis of RNA and proteins necessary for cell division. Mitosis (Gap 2) (Gap 1) (DNA synthesis) Interphase Cell division • Mitosis – Asexual reproducción – No reduction of genetic material • Meiosis – Sexual reproduction – Reduction of genetic material MITOSIS It is the process in which the cell divides giving rise to two identical daughter cells. First the nucleus is divided by a process that we call cariokinesis and then the cytoplasm is divided by a process that is known as cytokinesis. Stem cell (diploid) 2c 2c Stem cell (haploid) R c R 2c R Daughter cells (diploids) c R R c R Daughter cells (haploids) • Phases: – – – – – – Prophase Prometaphase – Cariokinesis: Nucleus division Metaphase Anaphase Telophase Cytokinesis: Cytoplasm division 2n Cytokinesis Microtubules remains Contractile ecuatorial ring During the telophase a segmentation groove begins to appear in the cytoplasm. This groove progresses until only a small bridge of cytoplasm and some microtubules hold the two daughter cells together. These microtubules are surrounded by a contractile ring of actin microfilaments. This ring contracts until the two daughter cells are separated while the microtubules and the elements of the contractile ring are dismantled. Meiosis is a special type of cell division that gives rise to gametes (ovules and sperm), that is, the number of chromosomes is reduced from 2n to n. Reducing the number of chromosomes to n, assures that each gamete carries half of chromosomes so that when merged with another gamete, we obtain the number 2n proper to the species. But also, during meiosis, gene recombination occurs and that enables genetic variability and diversity. In this process two consecutive divisions take place: • FIRST DIVISION • PROPHASE I • Leptotene • Zygotene • Pachytene • Diplotene • Diakynesis • PROMETAPHASE I • METAPHASE I • ANAPHASE I • TELOPHASE I • INTERKINESIS • SECOND DIVISION • PROPHASE II • PROMETAPHASE II • METAPHASE II • ANAPHASE II • TELOPHASE II 1. 2. 3. 4. 5. 6. 7. In the S phase, the DNA content is doubled as in any division. Prophase I: chromosomes condense and become visible in the nucleus. The homologous chromosomes are paired and fragments of genetic material exchanged, whereby the chromatids no longer have the identical genetic information than in the mother cell. Prometaphase I: the nuclear envelope disappears and the spindle begins to form. The homologs remain together, so the microtubules of the spindle will be joined this time to pairs of chromosomes instead of individual chromosomes. Metaphase I: The pairs of homologous chromosomes are aligned at the equator of the spindle. Anaphase I: the spindle begins to be disorganized and the microtubules drag a complete chromosome of each pair with it. Telophase I: the two child nuclei are formed with one chromosome of each pair. That is, they are already haploid nuclei (n), but even if there are no pairs of homologous chromosomes, we still have complete chromosomes, with their two copies of DNA. Meiosis II begins and will follow the same steps as normal mitosis. At the end of meiosis I, two daughter cells were obtained and, in meiosis II, each of them gives place to two new daughter cells. So at the end of meiosis we get 4 haploid cells (n). Mitosis vs. Meiosis https://youtu.be/jjEcHra3484 Mitosis vs. Meiosis Cell Death The cell cycle can be interrupted by various factors that cause the death of the cell. These factors can be injuries or accidents, attack of pathogens, absence of vascular supply or even by genetic programming. There are two mechanisms by which cells may die: • Apoptosis: it is a programmed cell death; this means that it is a physiological process. This happens for example in cells that are programmed from the embryonic stage for their destruction so that the organism can develop properly. Or also in cells that must die to establish a balance with cell proliferation. • Necrosis: is a process that occurs due to attacks or traumatic injuries. It is no longer a regulated process, and it may damage the organism. Apoptosis Normal Macrophages Modification of the cell shape and decrease in cell size → loss of contact with other cells Condensation of the cytoplasm (↓volume). No changes in the organelles Apoptotic Bodies Condensation and fragmentation of nuclear chromatin (pyknosis). Fragmentation of the cell: these fragments surrounded by PM are called apoptotic bodies The cell membrane is not destroyed. The apoptotic bodies are rapidly phagocytized. There is no inflammatory reaction. Pyknosis Embrionic development There are cells that during the embryonic development are genetically programmed to undergo apoptosis, as for example those that would form membranes between the fingers and toes or those that would give rise to the formation of the tail in the human being. Maintaining body homeostasis Participates in the continuous remodeling and maturation of all organs and tissues. It helps maintain the balance between proliferation and cell death. Basal membrane Basal cells Activated Caspase 3. Mammary duct Apoptosis Proliferation Protection against damaged or tumoral cells When intracellular damage occurs and the intervention of the immune system or an inflammatory reaction is not appropriate, apoptosis is activated in an intrinsic manner. Physic agents damage (radiation) Chemic agents damage (toxics) Tumoral cells Necrosis Increase of cell size (the cell swells). Reversible process at the beginning. Mitochondrial damage. Breakage of the cell membrane. Release of the content to the extracellular medium. Minimum modifications in the core. Dissolution of the organelles. Neighboring cells submitted to this process. There is an inflammatory reaction. Tissue damage. Normal Apoptosis Normal Macrophages Apoptotic Bodies Pyknosis Necrosis Normal