BS 1007 Lecture 11 Cell cycle 2
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Created by:
zelkova on April 11, 2012
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BS1007 Molecular & Cell Biology
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Terms | Definitions |
|---|---|
 Cell Division | - Cells divide into identical copies by packaging a complete set of genetic info (chromosomes) into each new cell - Cell cycle: cycle of duplication & division - Cancer: Uncontrolled cell growth |
| Apoptosis | - Programmed cell death - ↓ no. of somatic cells - Opposite of mitosis |
| M Phase | - Mitosis + Cytokinesis - Separate and segregate of chromosomes must be coordinated 6 Stages 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase 6. Cytokinesis - Period between 2 M phase called interphase |
| Forms of DNA during replication | Chromatin: - DNA + associated proteins - condense into chromosome Chromatids: - duplicated & attached chromosome after replication - held tgt by cohesin protein |
| Chromosome condensation | - human chromosome condense 10,000 fold - 45nm → 10-20µm forms Nucleosome - DNA wrapped around histones - reduce length of DNA 7 fold - further compaction → solenoidal fiber - Condensin complex use ATP to drive coiling of chromosome (interphase) |
| Mitochrondria division | Dynamic division - mtDNA replication not limited to S phase - dynamin-like protein for fission |
| Nuclear Envelope, Golgi & ER during replication | Extensive fragmentation: - allows partitioning into daughter cells - Catalyzed by Mitosis-specific reactions |
| Interphase (G1,S,G2) | - Cell increases in size Replication of - Organelles - Centrosome - DNA G2/M Checkpoint |
| Prophase | Inside Nucleus: - Replicated chromosomes condense Outside the Nucleus: - Mitotic spindle assembles btw 2 centrosomes - Centrosomes move apart towards the pole |
| Prometaphase | 1. Centrosomes at spindle poles 2. Chromosomes attach to spindle MT via kinetochores 3. Breakdown of nuclear envelope - due to Phosphorylation of nuclear lamina (intermediate filaments) by M-Cdk |
| Metaphase | 1. Chromosomes align at equator 2. Kinetochore microtubules attach sister chromatids to opposite poles of the spindle Metaphase checkpoint |
| Anaphase | 1. Sister chromatids synchronously separate - form two daughter chromosomes - each is pulled towards the spindle pole it faces 2. Kinetochore microtubules get shorter 3. Spindle poles move apart. |
| Telophase | 1. Chromosomes arrive at the spindle poles & decondense 2. New nuclear envelope reassembles around each set of chromosomes → 2 nuclei End of mitosis |
| Cytokinesis | 1. Contractile ring made of actin & myosin filaments assembled betw 2 daughter nuclei 2. Cytoplasm divided by contracile ring, which pinches the cell into 2 daughter cells |
| Mitosis Vs Cytokinesis Cytoskeletal machines | Mitosis - bipolar mitotic spindle & motor proteins Cytokinesis - contractile ring made up of actin & myosin |
| Mitosis → Cytokinesis tut: Cytokinesis does not precede mitosis | 2 mechanisms 1. During mitosis: activate proteins for mitosis & inactivate proteins required for cytokinesis - Cytokinesis starts when M-cdk inactivated at end of mitosis 2. Presence of residual central region of the spindle is required to maintain a functional contractile ring for cytokinesis |
| Centriole Replication | Centrosome: - Pair of Centriole & amorphous material - Principle MTOC in animal cells Centrosome cycle: - G1: pair move apart few µm - S: daughter centriole grows - G2: growth completed - M: 2 pairs split and move apart |
| Mitotic spindle basic concepts | - microtubule + associated proteins - assembly & function depends on motor proteins (KRP and dynein) Function: - Pull the chromosomes toward poles and move the poles apart 3 Classes of MT Astral: separate centrosome poles, orientate & position spindle Kinetochore: attach chromosome to spindle Overlap: bipolar shape of spindle |
| Microtubule instability increases at M phase | Dynamic instability: - rapid assembly/disassembly of microtubules - ↑ faster at M phase - due to ↑ in catastrophes - due to phosphorylation by M-Cdk (Motor proteins & MAPs) |
| Mitotic spindle (Astral) assembly and function | - Random microtubules interact in overlap zone - Kinesin related motor protein cross link the MT & push the centrosome - (-)end motors form the foci - (+)end motors form overlap zone & slide MT past each other - Both act together to form the spindle |
| Length of Spindle: Balance betw + and - end motor protein | - ↑ (-)end directed motor protein = Short spindles - ↑ (+)end directed motor protein = Long spindles |
| Kinetochore and Kinetochore microtubule | Prometaphase - Nuclear Envelope breakdown - Microtubules capture condensed chromosome at kinetochore Kinetochore: Complex protein assembled at centromere |
| Tut: Chromosomes captured by MT even though the nuclear envelope does not break down in yeast | - Spindle pole body in yeast embedded on the nuclear membrane - Thus the mitotic spindle is formed inside the nucleus - Allows for the capture of chromosomes by MT |
| Kinetochores function: Search & Capture | Search and Capture: - MT from centrosome grow toward chromosome - MT that attach to a chromosome become stabilised - MT eventually attached to kinetochore → kinetochore microtubule - Free kinetochore captures MT from opp spindle pole, convert unipolar → bipolar attachment |
| Opposing forces drive the chromosome to Metaphase plate | - Astral/interpolar MT push chromosome - Kinetochore MT pull chromosome |
| Chromosomes at metaphase plate | - Chromosomes attached on both sides (Bipolar attachment) - Chromosomes tugged back & forth Metaphase plate - Chromosomes assume a position equidistant betw 2 poles - Oscillate gently until signal to separate |
| Checkpoints |  Ans: Role of cell cycle checkpoints: - Ensure each phase of cell cycle is completed successfully prior to starting next phase of cell cycle - Halt the cell cycle, if they detect any abnormalities by generating/sending a signal - Give the cell more time to correct/repair any defects Checkpoint fails - G2/M: DNA not completely replicated the cells will proceed prematurely into mitosis with incompletely replicated DNA, suicidal mitosis - M/A: Kinetochore not properly attached, aneuploidy occurs _______________________________ 2 major checkpoints 1. entry into mitosis (G2/M checkpoint). 2. metaphase to anaphase transition (metaphase checkpoint) G2/M checkpoint - Ensures DNA replication is completed successfully before initiation of mitosis occurs Metaphase checkpoint - monitors attachment by both poles to kinetochores & tension by mitotic spindle attachment - before proceeding to anaphase - unattached kinetochore→ halts anaphase - > time for attachment - ensure ↑ fidelity of chromosome separation - prevents aneuploidy |
| APC (anaphase-promoting complex) |  Ans: - lead to the production of cells with imbalances in # of chromosomes (Aneuploidy) Action: Securin inhibits separase (a protease which degrades cohesin molecules holding the sister chromatids) 1. APC mediated ubiquitination → destruction of securin 2. thereby releasing separase, to carry out its function - APC activated by mitotic Cdk complex (MPF) - Degradates proteins: 1. Anaphase inhibitor 2. Cyclin in mitotic Cdk complex (M-cyclin) - Separate & Segregate sister chromatid |
| Chromosome separation at Anaphase | 1. APC activated by mitotic Cdk complex (MPF) 2. APC targets securin & M-cyclin for degradation (securin usually keep separase inactive) 3. Separase activated, cleaves cohesin (betw sister chromatid) 4. Release cohesin linkage - Delay anaphase entry: by delaying securin ubiquitination |
| Anaphase A | Pull daughter chromosomes to nearest pole via 1. Motor proteins operating at the kinetochore 2. Depolymerisation of kinetochore MT |
| Anaphase B | 2 spindle poles move apart via 1. elongation & sliding of overlap MTs past one another 2. outward force exerted by astral microtubules. |
| Plane of contractile ring | - Mitotic spindle specifies location of contractile ring - Contractile ring forms in plane of metaphase plate - Ensure division betw 2 sets of chromosome - studied by astral stimulation |
| Asymmetric cell division | - produce 2 cells that differ in size/contents or both - mother segregate "fate determinants" - spindle has to be moved Spindle rotation → Specific pattern of cleavage - (-)end motors move towards the pole - but they are immobilized, spindle pole come towards them |
| Cytokinesis in plants | - Plant cells with semi rigid cell wall - No contractile ring - forms cell plate (new cell wall) |
| Mitosis without cytokinesis | Eg. First 13 nuclear divisions in Drosophila Embryo to create syncytium - Cellularisation: coordinated cytokinesis by nuclear migration to cortex |
| Prokaryotic cell division | Binary fission - circular chromosome - Single oriC, origin of replication - divide twice in the time it takes to complete DNA replication - Replicate each daughter chromosome before completion of the first replication |
| Min Oscillations | - bacteria also need to find center to divide - Min is a protein that oscillates back & forth to find the middle of the cell |
| Segregation mechanism Bacteria Vs Yeast Vs Animals | Bacteria - Binary fission Yeast - Divide with nuclear envelope still intact - microtubule associate with nuclear envelope Animal - Nuclear envelope breaks down - Kinetochore microtubule separate from nuclear envelope |
| Life cycle of higher and some lower eukaryotes | - Diploid phase predominates - Haploid phase short - Haploid cells specialized for sexual fusion Two types of haploid cells: 1. One large, non motile (Ovum) 2. Small, motile (Spermatozoon) |
| Meiosis Basic Concepts | - Chromosomes: Autosomes + Sex chromosomes Diploid organism: - Two similar versions of each chromosome - similar but not identical DNA sequence - homologs - Replicated chromosomes tightly linked and called sister chromatids |
| First cell division of meiosis | - Replicated chromosomes pair with replicated homolog form bivalent - contains 4 chromatids - Pairing mediated by complementary DNA base pair interactions - Haploid number of chromosomes - Diploid amount of DNA |
| Second cell division of meiosis | Meiosis I: - Does not produce haploid cells - Each daughter inherit two copies of one homolog - Two copies identical except where genetic recombination has taken place Meiosis II: - Produce haploid gametes - Each daughter inherit a copy of one homolog - Chromosomes fail to separate normally into four haploid cells → nondisjunction - Some haploid lack a chromosome while others have >1 (down's syndrome) |
| Mitosis vs Meiosis | Meiosis: DNA replication →2 cell divisions (4n→2n→n) Mitosis: DNA replication → 1 cell division (4n→2n) |
| Genome reshuffling | - Independent assortment of maternal and paternal homologs during meiosis I - 2^n genetically different gametes (n=haploid #) - Crossing over during Prophase I exchanges segment of homologous chromosomes - re-assorts genes on individual chromosomes - average 2 to 3 crossovers on each pair of chromosomes |
| Crossing over | - Recombination between homologs - enzymatic process - impt role in creation of genetic diversity - Impt in repair of DNA damage |
| Tut: Cell ensure that passage through the cell cycle is unidirectional and irreversible | 1. What: Degradation of critical molecules at specific points in the cycle. 2. Mechanism: Proteins are polyubiquitinated and degraded by proteasome Eg: Proteolysis of the anaphase inhibitor at the beginning of anaphase, proteolysis of cyclin B in late anaphase, and proteolysis of the S-phase Cdk inhibitor at the start of S |
| Tut: Condensin Vs Cohesins | Condensins - drive condensing/coiling of chromosomes prior to mitosis Cohesins - hold replicated sister chromatids together at the centromere till anaphase |
| Tut: Do bacterial cells perform mitosis? | No - Mitosis is nuclear division and bacteria do not have a nucleus |
| Tut: Do the X and Y chromosomes exchange genetic material? | Yes. - There are regions within these chromosomes with DNA sequence similarity. |
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