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BIOL 112 Lecture Notes Part 2.doc

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Biology (Sci)
BIOL 112
Joseph Dent

BIOL 112Winter 2011Lecture Notes Part IIBIOL 112 Winter 2011 notes for Part II Information and Heredity Professor Dents section From February 16 2011 to whenever the semester endsIntroductionProf does not have office hoursemail him to schedule an appointment or just email him3 sets of practice problemsdo themA nice story about a central Asian monk named Temujin around 1220 AD and unknown paternityaka Genghis KhanHad to give his land to his son but paternity issues some sort of controversySort of illustrates the concept of heredity which is what were learning nowThe mystery of heredityAll organisms come from other organisms except for the base case which ostensibly was abiogenesisAll organisms resemble their parentsSiblings are not identicalThe magic that is conceptionAn egg becomes fertilised and undergoes cleavage cell division until eventually you get an organismCell cycleAll organisms consist of cells which divide to produce new cellsHigher organisms fuse their cells sperm and ova to produce a new organismCell division results in more cells etcDividing cells devote a lot of resources to segregating chromosomesie organising them in a line pulling them apart so that each cell gets an equal number of chromosomesChromosome single string of DNACircular bacteriaLinear most other organisms including usWhen a cell is ready to divide chromosomes condense associate with proteins eg histonesCombination of DNAproteinschromatin which is dark and easy to seeNormal DNA is difficult to see under a light microscope but when its ready to divide easier to seeThe DNA double helix wraps around histones which wraps around other proteins etcall bunched togetherKaryotype organising and identifying chromosomesTake a cell that is getting ready to divideFlatten it with glass mitotic squashStain it with a dye gives them stripes take a picture and cut them up etcShows that chromosomes come in pairshomologs except for the sex chromosomes which dont look like each otherThe number of chromosomes is characteristic of a specie with some exceptionsHumans typically have 46 roundworms have 2 pigeons 80Doesnt really correspond to complexity or anythingJust before cell division each chromosome has been replicated once to produce two chromatids at the top of the chromosomeTwo chromatids for each chromosome in a pair held together by a centromereNormally a chromosome is a single piece of DNABut when we look at a karyotype we are seeing two pieces of DNAthe two chromatids bound by a centromereWe call these mitotic chromosomes to show that they are a special caseSegregating chromosomes is exacting a cell needs to give each daughter cell the right number of chromosomesWith random segregation only half the time does each daughter get one chromatid the right resultSo there needs to be some sort of mechanism that ensures that the right result is enforced every timeOrganisms need at least one of each chromosome and typically exactly onemore can cause problemsex Down syndromean extra chromosome 21 which is incidentally the smallest chromosomeSo the chromosomes must obviously be duplicated BEFORE the cell dividesNOW THE ACTUAL CELL CYCLE steps of cell divisionChromosome DNA replication S phaseMitosis M somatic cells divide into daughter cells each of which inherits one copy of each chromosomeOR Meiosis M for germ lines gametesnonidentical copies creating daughter cells that have one of each homologCytokinesis dividing the cytoplasm in two optionalStarts with Gap 1 G1cell is growing accumulating resources it needs to divideThen it duplicates its chromosomesDNA synthesis S phaseGap 2 G2another rest phase preparing to divideThen it undergoes mitosis M and the daughter cells go through the cell cycle againTo ensure that this occurs in order there is a checkpoint between each phaseFrom S to G2 cell checks that each chromatid has been duplicatedWhat evidence of this checkpoint do we haveDrug hydroxyurea blocks chrosome replicationSo cell division is put on hiatus indefinitelyAnother caffeine which disables the checkpointThis is usually not a problem as chromosome replication occurs without problemsHowever if you both block chromosome replication AND disable the checkpoint you have problemsBecause the cell will attempt to continue to the mitosis stage but will find that its not able to divide properlyCell cycle tightly regulated to ensure that in particular each cell gets the right number of chromosomesDiagram later maybeMore on the G1 to S transitionThere is a protein called Cdk4 which is always present during the cell cycle but doesnt really do anythingBut then it associates with another protein called Cyclin D which is only produced during S phaseThis complex interacts with other proteins to say that the cell is ready to go into SCyclin is degraded Cdk4 is released no longer functionalSo this activity of proteins tells the cell what cycle its in when to start transitionAlthough cyclins and cdks are involved in all parts of the cell cycle important in different transitions etcHow do cells know when to divideMost somatic cells not dividingarrested in the G1 phase of the cycleOften waiting for signals from other cells to tell them to divide
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