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Chapter 2

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Wilfrid Laurier University
Robert Boulianne

Chapter 2 – Mitosis & Meiosis th - Cytologists and geneticists in early 20 century determined that genes were carried on chromosomes - Plant cells have an outer covering called cell wall made of a polysaccharide called cellulose (over the plasma membrane) - In order to understand heredity, we need to understand the behavior of chromosomes during the cell cycle, mitosis and meiosis. - Animal cells have glycocalyx, consists of glycoproteins & polysaccharides covering over the plasma membrane too - Genetic material of eukaryotes is often distributed among multiple linear chromosomes. - Chromosome – In eukaryotic cells, a linear structure composed of a single DNA molecule complex with protein. Each species has a certain # of chromosomes in the nucleus of its cells - In nucleolus, ribosomal RNA (rRNA) is made - Areas of DNA that encode rRNA are call nucleolus organizer region (NOR) - Cytoplasm consists of mostly tubulin – derived microtubules & actin-derived microfilaments - Endosymbiotic hypothesis – Mitochondria & Chloroplast were once free living organisms that formed a symbiotic relationship with a primitive eukaryotic cell - Centrioles – Cytoplasmic bodies at a specialized region called centrosome - function is to organize spindle fibers during mit/mei - Some centrioles are from the basal body (to do with the formation of cilia & flagella) - Spindle fibers made up of arrays of microtubules and consisting of polymers of polypeptide subunits of the protein tubulin - Centromere – condensed/constricted region on the chromosome - may differ in arm length of the centromere: 1. Metacentric – Centromere in the middle 2. Submetacentric – B/n middle and end - shorter arm = p (petite) and the other is q 3. Acrocentric – Close to the end 4. Telocentric – At the end - Diploid (2n) – two copies of genetic material subdivided into chromosome - usually somatic cells - Chromosome exists in pairs; having two of each Crs. -Haploid (n) – One copy of genetic material (one set of Crs.) - Usually gametic Crs. in eukaryotes - Ex. Human somatic cells have 46 Crs. Gametes have 23. 2n = 46 n = 23 - Homologous Chromosomes – Crs. that contain the same genes & pairs during meiosis in diploids - Each member called a homolog - One homolog is inherited from each parent Ex. Humans, one is the paternal (male) and the other is maternal (female) homolog - Nonhomologous Chromosomes – Chromosomes that contain diff. genes & do not pair during meiosis - Sex Chromosomes – Crs. that differ in # or shape between two sexes - one sex may have a matched pair of sex Crs., while the other sex may have an unmatched pair/ just a single sex Crs. - Examples: Humans, Male = XX Female = XY - Autosomes – Chromosomes other then sex chromosomes - Karotype – A complete set of all the metaphase chromosomes - Ordered by: Largest Chromosomes is #1(First) Homologous are paired Sex Chromosomes are usually shown at the bottom together - Example: Human Karyotype: 22 autosomes and one of X and one of Y (sec chromosomes) - Chromosomes displayed in a karyotype are arranged largest to smallest, & each chromosomes is orientated with the largest chromosomal arm down & smallest arm up Example: BRCAI is at location 17q21 (on chromosome 17 on the long arm in region 21) - Locus – the site or place on a Crs. where a particular gene is located - Biparental inhertitance – genetics inheritance of a maternal & paternal allele of a gene Interphase and the Cell Cycle - Cellular reproduction is cyclical process – cell cycle - Cell Cycle divided into 2 stages: Interphase & cell division - Initial size of new daughter cell is half the size of a parent cell - Nucleus is the same size - Most variation is the length of time spent in G1 phase in different cells - mins in rapidly diviing cells (cancer, fetal cells) - years for some (nerve cells) - S phase & G a2e the same for most cell types - Interphase is the interval between cell divisions & subdivided into: - G1 (gap 1) – Presynthesis stage, cell prepares for mitosis - S (synthesis) – DNA replicate of each Crs. and DNA synthesis - G2 (gap 2) – Post synthesis stage, cell prepares for mitosis - G0 – A nondividing quiescent state some cells enter - Still metabolically active and may re-enter cell cycle if stimulated - Cancer cells avoid G0/ spend little time in G0 - Series of controls determine if cells continue through G1 to S1 or withdraw from cell cycle into G0 - Mitosis takes about an hour & one tenth cells in mitosis - Therefore the length of the entire cell cycle is 10 hr - The cell cycle has 3 major checkpoints in the mitotic cell cycle - controlled by the activity of kinases & cyclins and are the normal products of mutated genes - Cyclins bind to kinases, activating them at appropriate times - Activated kinases then phosphorylate other target proteins that regulate the progress of the cell cycle - The length of S phase can be determined with a pulse – chase experiment - H – CTP (precursors – radio active) – Pulse with it - chase with cold CTP - makes slides of the DNA - Mitosis occurs in both haploid & diploid cells - Outcome - 2 cells, each with the same # and type of chromosomes as the original parent cell - Mitosis Stages 1. Prophase – Migration of 2 pairs of centrioles to opposite ends - organizing cytoplasmic microtubles into spindle fibres (in plant b/c no centrioles) - Nuclear envelope breaks down - Chromatin fibers condenses & coils  chromosomes visible (sister chromotids) - Mitotic spindle assembles outside of the nucleus from microtuble organizing centres (MTOCs) Example: Human’s MTOC are the centrioles 2. Prometaphase –Chromosomes are clearly doubled structures - Centrioles reach ends & spindle fibers form 3. Metaphase – Begins when nuclear envelope has completely disappeared - Chromosomes in one plane halfway between the two spindle poles - This plane is called the metaphase plate – chromosomes align - Microtubles bind to the c
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