Dr. Mahmood S. Choudhery, PhD, Postdoc (USA) Assistant Professor Tissue Engineering & Regenerative Medicine King Edward Medical University

Transcription

1 CELL DIVISION Dr. Mahmood S. Choudhery, PhD, Postdoc (USA) Assistant Professor Tissue Engineering & Regenerative Medicine King Edward Medical University

2 Cell Division The key roles of cell division Unicellular organisms: Reproduction Multicellular Organisms: Reproduction Growth Repair

3 Four Events Must Occur for Cell Division 1. Reproductive signal 2. Replication of DNA 3. Segregation 4. Cytokinesis

4 Cell Division in Prokaryotes Prokaryotes divide by binary fission External factors such as food, temperature act as reproductive signals DNA replication occurs after a cell get a reproductive signal Segregation occurs when cell grows in size Cytokinesis occurs by pinching in of plasma membrane

5 Cell Division in Prokaryotes Single, circular chromosome, attached to membrane at Ori point Prokaryotes divide by binary fission

6 Cell Division in Eukaryotes Eukaryotic cells divide by mitosis or meiosis Cell division in Eukaryotes is not related to the environment of single cell More than one chromosomes Distinct nucleus Newly replicated chromosomes remain attached as sister chromatids

7 Cell Division in Eukaryotes.. Cell Cycle Process of cell division in eukaryotes occurs in a series of events collectively 7 called as cell cycle

8 Phases of Cell Cycle Cell Cycle Interphase Mitotic Phase Mitosis Cytokinesis Division of Nucleus Division of cytoplasm During interphase cells do not divide while during Mitotic phase the cells actually divide.

9 Phases of Cell Cycle 4 Mitotic Phase Nucleus and cytoplasm divide 3 G0 DNA repair Additional growth Makes proteins and organelles Beginning of chromosome condensation G G1 11 S 8 2 Cell grows in size, Organelles are duplicated Performs normal functions Molecular machinery for DNA replication is made

10 Mitosis (karyokinesis) Division of nucleus Phases of mitosis 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase

11 Phases of mitosis cont.. 1. Prophase Chromatids condense and become visible Nucleolus disappears Centrosomes move to opposite ends of the cell Spindle fibers form Kinetochore spindle fibers Polar spindle fibers Search and capture mechanism

12 2. Prometaphase Nuclear membrane disappears Kinetochores develop in centromere region

13 3. Metaphase All the centromeres arrive at the equatorial plate Chromosomes are maximally condensed Sister chromatids line up at equatorial plate At the end of metaphase, the chromatid pairs separate

14 How do chromatid pairs separate at end of metaphase? Separase Spindle checkpoint?

15 4. Anaphase Begins with separation of Chromatids Sister chromatids move to opposite poles Mechanisms that propels chromosomes towards poles 1. Cytoplasmic dynein 2. Kinetochore Microtubules shortening from poles Poles are pushed farther apart Movement of chromosomes is very slow Pole separation

16 5. Telophase Spindle fibers disintegrate Nuclear membrane reforms Chromosomes uncoil and become less visible Nucleoli reappear

17 Cytokinesis Division of cytoplasm Cytokinesis differs in animal and plant cells Animal Cells Plant Cells

18 Cell cycle Video

19 Growth Factors can Stimulate Cells to Divide The cells in G0 must be stimulated by external chemical signals for cell division 1. Platelets release PDGF that stimulate cells to divide 2. Interleukins and erythropoietin stimulate division and specialization, respectively, of precursor cells of white blood cells and red blood cells.

20 Meiosis Occurs during formation of gametes (Eggs & Sperms) Overall Functions of Meiosis 1. To reduce chromosome number from diploid to haploid 2. To ensure that each new haploid product has a complete set of chromosomes 3. To promote genetic diversity 20

21 Meiosis: Two Part Cell Division Although nucleus divides twice during meiosis the DNA is replicated only once Meiosis 1 Meiosis II 21

22 Meiosis First cell division: Meiosis I Prophase I Metaphase I Anaphase I Telophase I Second cell division: Meiosis II Prophase II Metaphase II Anaphase II Telophase II

23 Phases of Meiosis I 1. Prophase I Mitotic prophase Chromatin material condenses and becomes visible Pairing of homologous chromosomes, Coiling and shortening chromosomes continue Crossing over occurs

24 Phases of Meiosis I 2. Prometaphase 1 Nuclear envelop breakdown Nucleolus disappear One kinetochore forms per chromosome Chromosomes attach to spindle fibers

25 Phases of Meiosis I 3. Metaphase 1 Homologous chromosomes line up on equatorial plate Mitotic Metaphase

26 Phases of Meiosis I 4. Anaphase 1 Homologous chromosomes move to opposite poles of cell Meiotic anaphase Mitotic anaphase

27 Phases of Meiosis I 5. Telophase 1 Chromosomes reach to opposite poles Chromosome stay condensed Nuclear envelope may or may not appear around chromosomes

28 Interkinesis Period between meiosis I and meiosis II Typically brief Chromatin material is partially uncoiled No S phase (no DNA replication)

29 Meiosis II Meiosis II is similar to Mitosis Prophase II Metaphase II Anaphase II Telophase II Main differences between Mitosis and Meiosis II? 1. DNA replicates before mitosis, but not before meiosis II. 2. Sister chromatids differ 3. Number of chromosomes is half

30 Video Meiosis I and II

31 Regulation of Cell Cycle Checkpoints monitor cell for its progression in cell cycle Three main checkpoints that control the cell cycle in eukaryotic cells are 1. G1 checkpoint 2. G2 checkpoint 3. Metaphase checkpoint

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33 Activation of CDKs Phosphorylation Different Combinations of CDK and Cyclin 1. Cyclin D-Cdk4 acts during G1. 2. Cyclin E-Cdk2 also acts in G1. 3. Cyclin A-Cdk2 acts during S 4. Cyclin B-Cdk1 acts at the G2 boundary

34 RB/E2F Regulation of cell Cycle

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