The Cell Cycle. Dr. SARRAY Sameh, Ph.D

Transcription

1 The Cell Cycle Dr. SARRAY Sameh, Ph.D

2 Overview When an organism requires additional cells (either for growth or replacement of lost cells), new cells are produced by cell division (mitosis) Somatic cells are produced by division of existing cells in a sequential manner: they duplicate their content first and then divide to produce TWO identical daughter cells: this process is called cell cycle.

3 Phases of cell cycle The cell cycle may be broadly divided into three distinct stages: interphase, mitosis and cytokinesis

4 Interphase - Is the period between successive round of division. - It is composed of 3 phases: G1, S & G2 A. G1 - Is the phase between mitosis and S phase (DNA replication) - Time for RNA and protein synthesis and for organelles and intracellular structures to duplicate

5 - The length of cell cycle is quite variable between cell types: rapidly dividing cells (embryonic) spend short time in G1, while mature cells which are no longer cycling remain permanently in G1 - Cells remained permanent in G1 and not committed to DNA synthesis are considered in resting state called: G0 phase. (Some inactive cells in G0 may re-enter active phases upon proper stimulation) - Restriction point is found within G1 phase, is critical for regulation of the cell cycle, if passed, cell will commit in DNA synthesis in S phase.

6 S phase Replication of DNA occurs during S phase. Time for completion of S phase is constant among cell types: in active cycling, cells spend approx. 6 hours in phase S.

7 G2 phase G2 is the gap between S phase and mitosis, during which the cell prepare for division It is a safety period to allow the cells to check that DNA duplication is complete before cell division G2 contain, also, a checkpoint for regulation of cell cycle and nuclear integrity This phase lasts for approx. 4 hours

8 Mitosis Is a continuous process of nuclear division (Karyokinesis) and can be divided into 5 phases: - Prophase - Prometaphase - Metaphase - Anaphase - Telophase ( 1hr) Dividing cells spend about ONE hour in mitosis

9 1. Prophase The chromatin condenses into chromosomal structure called chromatids linked together by the centromere. Each chromatid is associated with kinetochores (proteins), where chromatid will attach to the mitotic spindle Microtubule of the cytoplasm disassemble centrosomes (pair of centrioles) migrate to opposite pole of the cell and from each, microtubules grow in mitotic spindle

10 2. Prometaphase Marked with the disassembly of the nuclear envelope Microtubules from the centrosomes invade the nucelus Sister chromatids attach to microtubules from opposite centrosomes.

11 3- Metaphase Chromosomes align at the equator of the spindle, called metaphase plate (half way between the two poles) It is the phase needed in karyotyping analysis

12 4. Anaphase Centromere divide (split in 2) Sister chromatids move to opposite pole.

13 Last phase mitotic spindle disappear 5.Telophase Nuclear membrane reform around each of the 2 nuclei containing chromatids. The chromatids de-condense into chromatin and nucleoli reform in daughter nuclei

14 Cytokinesis It s a cytoplasmic division following the nuclear division creating 2 distinct separate daughter cells with identical nuclei Formation of cleavage furrow that deepens till opposing edges meet. Plasma membranes fuse on each side of deepened furrow until the two daughter cells separate.

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16 Some have rapid turnover as part of their job, thus need continuous replacement (e.g. intestinal cell & bone marrow) Some leave the active phases of the cycle (G1, S,G2) immediately after differentiations and never return (e.g. neurons). Cells that reversibly stop division, are in state of quiescence (G0 phase) (e.g. hepatocytes: retain the ability to divide throughout their life span but reenter the active cell cycle accounts for liver regrowth following injury or disease). Cells that permanently stop division (due to age or accumulated DNA damage) are senescent...

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