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BI226 Semester Notes.pdf

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Chapter 2Lecture Notes 130414 522 PMChromosome and Cellular Reproduction th Cytologists and geneticists in the early 20 century determined that genes were carried on chromosomesIn order to understand heredity we need to understand the behavior of chromosomes during the cell cycle mitosis and meiosis Eukaryotic ChromosomesGenetic material of eukaryotes is often distributed among multiple linear chromosomesChromosomes a linear structure composed of a single DNA molecule complexed with protein Each species has a characteristic number of chromosomes in the nucleus of a cellHaploid and DiploidHaploidNOne copy of genetic material subdivided into chromosomesNonhomologousDiploid2NTwo copies of genetic material subdivided into chromosomesHomologous Diagram on SlideEukaryotes have 2 copies of each type of chromosome in their nucleiDiploid or 2NEukaryotes that have only one set of chromosomes Usually gametesHaploid or N Example human somatic cells have 46 chromosomes while our gametes only have 232N46 N23 Homologous ChromosomesIn diploids chromosomes that contain the same genes and that pair during meiosis are called homologous chromosomesEach member is called a homologOne homolog is inherited from each parent o In humans one is the paternal homolog while the other is maternal homologChromosomes that contain different gene and that do not pair during meiosis are called nonhomologous chromosomes Autosomes and Sex ChromosomesChromosomes that differ in the number or morphologically between two sexes are called sex chromosomesOne sex may have a matched pair of sex chromosomes while the other sex may have an unmatched pair or just a single sex chromosomeChromosomes other then sex chromosomes are called autosomesDiagram on SlideVariation in ChromosomesChromosomes may also vary in lengthIn humans there is a wide range of chromosome lengthsIn mice all of the chromosomes are of similar sizeKaryotypeA complete set of all the metaphase chromosomes of a cell are calle da karyotypeNotice for this human karyotype there are 22 autosomes and one of each X and YConventionsLargest chromosome is 1Homologues are pairedSex chromosomes often shown at the bottom togetherAdditional Nomenclature for ChromosomesWhen displayed in a karyotype the chromosomes are arranged from largest to smallest and each chromosome is oriented with the largest chromosome down and the smallest on topThe short arm is designated as p and the long arm is designated as qUsed for the location of markers along the chromosomeEg BRCA1 is at location 17q21 on chromosome 17 on the long arm in region 21Interphase and the Cell CycleIn both uni and multicellular eukaryotes cellular reproduction is necessary for growthThis is a cyclical process that includes growth and cell division the cell cycleThe cell cycle is divided into two stages interphase and cell divisionDiagram on SlideInterphaseThe interval between cell division and is subdivided intoG1 presynthesis stage where the cell prepares for DNA replication and growsSstage of DNA synthesis or replicationG2postsynthesis stage where the cell prepares for the onset of mitosis or cell divisionG0a nondividing quiescent state some cells enter o These cells are non invadingCell CycleThe length of G1 can vary among the different cell typesMinuets in rapidly dividing cells such as cancer and fetal cells and years for some differentiated cells such as nerve cellsIn contrast for S and G2 are the same for most cellsOnce started a cascade of events occur to transition the cells through the stageG0Series of controls determine if cells continue through G1 to S or withdraw from the cell cycle into G0G0 cells are metabolically active and may reenter cell cycle if stimulatedCancer cells avoid G0 or spend only a short time in G0 compared to other cellsCell Cycle Check PointsThe cell cycle has 3 major checkpoints in the mitotic cell cycleControlled by the activity of kinases and cyclinsDiagram on SlideHow do you Determine Length of the StagesThe duration of the S phase can be determined with a pulse chase experimentMitosisOccurs in both haploid and diploid cellsOutcometwo cells each with the same number and type of chromosome as the original parents cellThe process is divided into 4 cytologically distinguishable phases
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