Cell Cycle (Interphase and Mitosis) PDF

HNU Faculty of Medicine Histology & Cell Biology Department Year 1 Principle of Human Body Structure HBS-101 Lecture 10 Cell cycle (Interphase/Mitosis) ...

HNU Faculty of Medicine Histology & Cell Biology Department Year 1 Principle of Human Body Structure HBS-101 Lecture 10 Cell cycle (Interphase/Mitosis)  Objectives Define cell cycle. List phases of the cell cycle. Describe the main events of the interphase. Describe the main events of mitosis.  Cell cycle Definition: It is a sequence of events that controls cell growth and cell division. The cell cycle in somatic cells is composed of two phases The interphase: The mitosis (M phase ): During which the cell A process that results in becomes larger and formation of two identical duplicates its genetic daughter cells. material.  The interphase The interphase is the period between two successive cell divisions. Interphase is subdivided into three phases: G1 (gap 1) phase, when the cell prepares to synthesize DNA. S (synthetic) phase, when DNA is replicated G2 (gap 2) phase, the cell prepares for the mitotic event.  G1 phase: G1 phase is the time gap between the end of mitosis and the beginning of DNA replication (S phase). It is the longest period of the cycle. Its duration is variable according to the rate of cell division. Events of G1 phase: ✓Synthesis of RNA and proteins (as proteins necessary for DNA replication). ✓Growth of the daughter cell and restoration of the original cell volume. S phase (the synthetic phase) It is the phase between G1 and G2. About 8 hours. Events during this phase: ✓Duplication (Replication) of DNA ✓Duplication of centrosome.  During duplication of DNA, the s-chromosome (single chromosome that contains one molecule of DNA) synthesizes another identical copy and changes into d-chromosome (double chromosome that is formed of two sister chromatids joined at centromere). Each sister chromatid contains one molecule of DNA. The cell at S phase contains 46 d-chromosomes. N.B. The s-chromosome = chromatid G2 phase: It is the interval between the end of DNA synthesis and the beginning of mitosis. It is about 4 hours. Events during this phase: ✓Synthesis of the RNA and proteins required for cell division. ✓Synthesis of tubulin proteins for formation of microtubules required for mitosis. ✓Storage of increased amounts of ATP for cell division. ✓ The newly synthesized DNA is checked for possible errors and, if present, are corrected. G0 phase (the resting phase or outside phase) Some differentiated cells exit the cycle after M phase (mitosis). These cells are said to be in G0 phase. The G0 phase may be permanent as in nerve cells and cardiac muscle cells (i.e. these cells don not enter the cell cycle again, so do not divide) Or temporary as in liver cells and smooth muscle cells. These cells may reenter the cell cycle after an appropriate stimulus (as in massive liver injury).  Cell division Definition: It is a biological process by which the cell can reproduce itself. Types: 1) Mitosis: Occurs in somatic cells. Gives two daughter cells having the same “diploid” number of chromosomes. 2) Meiosis: Occurs in the germ cells (in testes and ovaries). Gives four daughter cells having “haploid” number of chromosomes. The process of cell division includes division of both the nucleus (karyokinesis) and the cytoplasm (cytokinesis).  Mitosis (M phase) Definition: A type of cell division that occurs in the somatic cells producing two daughter cells, each has the same diploid number of chromosomes as the mother cell (46 S-chromosomes). Duration: lasts about 1 hour. Phases of mitosis: ✓Prophase ✓Metaphase ✓Anaphase ✓Telophase  Remember Before the beginning of the prophase stage, the cell is present in the interphase. - It contains: 4 centrioles and 46 d-chromosomes.  Prophase Events that occur during prophase: 1- The replicated chromatin condense and become visible with LM as fine threads. 2- The nucleolus disappear 3- The nuclear envelope disappear (late in prophase). 4- Each pair of the centrioles move to the opposite pole of the cell. 5- Formation of microtubules for mitotic spindles from microtubule organizing centers (MTOC). 6- Specific protein complex called a kinetochore appears on each sister chromatid opposite to the centromere.  Metaphase Events that occur during metaphase: Chromosomes (46 d-chromosomes) become maximally condensed (shorter, thicker & become visible). Chromosomes migrate to the equatorial plate of the cell (metaphase plate). Each chromosome is attached to the mitotic spindles at the kinetochores.  The mitotic spindles consist of 3 types of microtubules that arise from MTOC: Kinetochore microtubules (chromosomal microtubules): They are attached to the kinetochores of chromosomes Polar microtubules interdigitate near the equatorial plate. Astral microtubules they are short microtubules arising from MTOC like a star around centrioles. They anchor the spindle to the cell membrane.  Anaphase Each d-chromosome splits longitudinally at the centromere resulting in separation of the two sister chromatids. Each sister chromatid migrates to the opposite pole of the cell. By the end of anaphase, two groups of identical chromosomes are clustered at opposite poles of the cell.  Telophase Uncoiling of chromosomes (46 s-chromosomes). Nuclear envelope is reformed around each set of s-chromosomes. The nucleoli reappear Formation of a contractile ring of actin and myosin filaments at the equatorial plate of the parent cell. During cytokinesis, at the end of telophase, constriction of this ring produces a cleavage furrow and progresses until the cytoplasm and its organelles are divided into two daughter cells, each with one nucleus. Checkpoints and regulation of cell cycle Each phase of the cell cycle has one or more checkpoints that monitor and check the quality of specific cell activities. Progression to the next phase of the cycle does not occur until all activities of the preceding phase are completed.  Three important checkpoints including The start or restriction (G1/S) checkpoint Check that the nutrients and enzymes are sufficient for DNA synthesis. The G2/M checkpoint Ensures that DNA replication is complete. The metaphase /anaphase checkpoint Ensures that all chromosomes are attached to mitotic spindle and will be segregated. Regulation of the overall cell cycle depends on specific cytoplasmic proteins called cyclins and cyclin-dependent kinases (CDKs).  Important Tips 1- At the beginning of mitosis , there must be 46 d-chromosomes. 2- Each resulting new daughter cells after mitosis contains 46 s-chromosomes (chromatids) (one thread of DNA molecule). 3- This daughter cell, to divide again must duplicate its DNA molecules to change its single chromatids into d-chromosomes (2 sister chromatids connected by centromere). 4- This duplication occurs in S stage of the interphase stage of cell cycle.  Reference Junqueira's Basic Histology Text and Atlas, 17th edition by Anthony Mescher 28