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BIOC 212 Lecture 26.docx

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McGill University
BIOC 212
Thomas Duchaine

Lecture 26: Cell Cycle (March 14 , 2012)th Cell Cycle (Lecture 24-26, Slide 1) - Very fundamental for all cells: bacteria, mammalian, … - They all need to duplicate: o Cell content  Cytoplasm  Proteins  Organelles o Genome  DNA - Two identical cells = the result of cell cycle - 2 separate processes involved with the cell cycle o Cell growth  Cell duplicates contents  Driven by protein synthesis o Cell division  DNA replication  Chromosome segregation  Cytokinesis - It has been studied for a long time but it’s only a few years that they could study it at a molecular level What is seen under the microscope? Figure below. (Lecture 24-26, Slide 2) - You see twoFigure 17–2 The events of eucaryotic cell division as seen under a microscope. o M phaseeasily visible processes of nuclear division (mitosis) and cell division (ytCells look completely roundled M phase, typically occupy only a small fraction of the cell cycle. The other, much longer, part of the cycle is known as interphase. The five stages of mitosis are shown: an abrupt change in the biochemical state of the cell occurs at the transition from metaphase to anaphase. A cell can pause in metaphase before this transition point, but once the point has been passed, the cell carries on to the end of mitosis and through cytokinesis into interphase. Note that DNA replication occurs in interphase. The part of interphase where DNA is replicated is called S phase (not shown).he different chromosomes  DNA has to condense into chromosomes  Chromosomes have to line up in the middle  Chromosomes are then pulled apart  Cytokinesis o Cell is divided in 2  Nuclear membrane is recreated  Spindle poles are degraded o Interphase  Cells look flat  Cells are attached to the cell culture dish (where cells are grown)  Unless you’re examining blood cells o They are liquid o They float o Difficult to tell which phase they are in o They are not attached to the cell culture dish - People still didn’t know how this worked o They would stain the DNA to see it all happen o They were curious to see how it all worked & how it was regulated Further conclusions were drawn about the cell cycle (Lecture 24-26, Slide 3) - M phase o Nuclear division o Cytoplasmic division o Maybe 1 hour - Gap phases o Not in all cells o Only present in adult cells  Used as checkpoints  To assure readiness of commitment to M or S phase  Don't want to go into S phase if DNA is damaged by carcinogens or radioactivity which lead to mutations or cancer o Repair phases o Used to assure genome integrity o Not in embryonic cells  Due to rapid production of the embryo before it gets degraded by someone else o G1 & G2 o In cancer, these checkpoints are gone  Cell although mutated, or not attached to a spindle, it still enters M phase  In normal cells there are sensors, in cancer cells these sensors disappear  Accumulation of damages occur - S phase o DNA division o Maybe 12 hours  Because one 1m long DNA - You can study this cycle in mammalian cells o Grow them in culture o Study the cycle – this was more difficult because the genomes were not sequenced back in the dayure 17–3 The phases of the cell cycle. o Mammalian cells are diploid therefore every gene isThe cell grows continuously in interphase, gene, it’s double the work which consists of three phases: DNA o Still to this day it takes a long time to remove threplication is confined to S phase; G1 isf haploid cells the gap between M phase and S phase, o Want to use less complex organisms to study – yeast is a model organism for this reasonand M phase. In M phase, the nucleus and then the cytoplasm divide. Lecture 24-26, Slide 4 Figure 17–4 A comparison of the cell cycles of fission yeasts and budding yeasts.(A) Fission yeasThe fission yeast has a typical eucaryotic cell cycle with G1, S, G2, and M phases. In - Lookcontrast with what happens in higher eucaryotic cells, however, the nuclear envelope of do (more or lessthe yeast cell does not break down during M phase. The microtubules of the mitotic spindle (light green) form inside the nucleus and are attached to spindle pole bodies Budding yeast (dark green) at its periphery. The cell divides by forming a partition (known as the cell - Diffplate) and splitting in two. The condensed mitotic chromosomes (red) are readily visible in fission yeast, but are less easily seen in budding yeasts. (B) The budding yeast has Yeast normal G1 and S phases but does not have a normal G2 phase. Instead, a microtubule- - 6000 genes vs. 30 000 genes (mammalian cells)ucleus early in the cycle, during S phase. In contrast with a fission yeast cell, the cell divides by budding. - All the genes were known back in the day (isolated & cloned) - Genes are easily removable o 2-3 days vs. 6-8 months in mammalian cells - Haploid o One copy of gene to remove o Easier to knockout as well - Single cell organism vs. multi-cell organism (mammals) - Cell cycle machine - Wine example o Sits on grape – in G1 phase because no food o Grape gets ripe & falls down o Grape gets crushed – food (glucose) is accessible to yeast o Yeast goes into cell cycle o Creates alcohol - Simpler organism - Food causes proliferation of cell rapidly – every 20 minutes = new yeast (24-28 hours in mammalian cells) Discovery of the cell cycle machines - Since they are both eukaryotic cells, yeast & mammalian cells, and they go through a similar cell cycle  cell cycle was probably conserved during evolution - You can mutate yeast DNA by UV rays or carcinogens & then you can but them at different temperatures o High temperature  Restrictive temperature  40 degrees  Stress = inactive protein, misfolding o Low temperature  Permissive temperature  20 degrees  No effects at low temperature  No changes in cell cycle o Point mutations in proteins are temperature sensitive so if one of those point mutations was important for the cell cycle you can control it with different temperatures o Used to find genes that rest at different stages in the cell cycle o Easy do see due to size & shape o Look for phenotypes at resting phases of cell cycle  isolate & study them - Yeast usually grows at 30 degrees Lecture 24-26, Slide 5 Figure 17–5 The behavior of a temperature- sensitive cdc mutant. (A) At the permissive (low) temperature, the cells divide normally and are found in all phases of the cycle (the phase of the cell is indicated by its color). (B) On warming to the restrictive (high) temperature, at which the mutant gene product functions abnormally, the mutant cells continue to progress through the cycle until they comeology of budding
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