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Chapter 2

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Robert Boulianne

Chapter 2 Notes – Mitosis & Meiosis Eukaryotic Chromosomes • Genetic material of eukaryotes is often distributed among multiple linear chromosomes. • Chromosome (definition): In eukaryotic cells, a linear structure composed of a single DNA molecule complexed with protein. Each species has a characteristic number of chromosomes in the nucleus of its cells. Haploid and Diploid • Eukaryotes that have two copies of each type of chromosome in their nuclei – Diploid or 2n • Eukaryotes that have only one set of chromosomes…usually the gametes – Haploid or “n” (actually 1n but we leave off the 1 since it is understood). • Example: Human somatic cells have 46 chromosomes, while our gametes have 23. – 2n=46, n=23 Homologous Chromosomes • In diploids, chromosomes that contain the same genes and that pair during meiosis are called homologous chromosomes. – Each member is called a homolog – One homolog is inherited from each parent. • In humans, one is the paternal homolog while the other is the maternal homolog. • Chromosomes that contain different genes and that do not pair during meiosis are called nonhomologous chromosomes. Autosomes and Sex Chromosomes • Chromosomes that differ in number or morphologically between two sexes are called sex chromosomes. – One sex may have a matched pair of sex chromosomes, while the other sex may have an unmatched pair or just a single sex chromosome. – Example: in humans, females have two X chromosomes (XX) while males have only one X and a Y chromosome (XY). Chromosomes other than sex chromosomes are called autosomes Karyotype • A complete set of all the metaphase chromosomes of a cell is called a karyotype. • Notice for this human karyotype there are 22 autosomes and one of each of X and Y. • Conventions: – Largest chromosome is #1 – Homologues are paired Additional Nomenclature for Chromosomes • When displayed in a karyotype the chromosomes are arranged from largest to smallest, and each chromosome is orientated with the largest chromosomal arm down, and the smallest on top. • The short arm is designated as p and the long arm is designated as q. – Used for the location of markers along the chromosomes – Eg. BRCA1 is at location 17q21 (on chromosome 17 on the long arm in region 21). P Q Interphase and the Cell Cycle • In both unicellular and multicellular eukaryotes cellular reproduction is necessary for growth. • This is a cyclical process that includes growth and cell division…the cell cycle. • The cell cycle is divided into two stages: – Interphase and cell division Interphase • Interphase is the interval between cell divisions and is subdivided into: – G (gap 1) Presynthesis stage where the cell prepares for DNA replication and grows. 1 – S (synthesis) Stage of DNA synthesis or replication. – G 2gap 2) Postsynthesis stage where the cell prepares for the onset of mitosis or cell division. – G 0 nondividing quiescent state some cells enter. • These cells are nondividing. • The length of G 1an vary among the different cell types – Minutes in rapidly dividing cells such as cancer cells and fetal cells, to years for some differentiated cells such as nerve cells. • In contrast S and G 2re the same for most cell types. – Once started a cascade of events occur to transition the cells through the stage. • Series of controls determine if cells continue through G to1S, or withdraw from cell cycle into G0. • G0cells are metabolically active and may re-enter cell cycle if stimulated. • Cancer cells avoid G 0 or spend only a short time in G co0pared to other cells. Cell Cycle Checkpoints • The cell cycle has 3 major checkpoints in the mitotic cell cycle. • Controlled by the activity of kinases and cyclins. Mitosis • Mitosis occurs in both haploid and diploid cells. – Outcome--two cells, each with the same number and type of chromosomes as the original parent cell. – The process is divided into four cytologically distinguishable phases: – Prophase, metaphase, anaphase and telophase. Mitotic Spindle • During prophase, the mitotic spindle assembles outside of the nucleus from microtubule organizing centres (MTOCs). – In animal cells, the MTOCs are the centrioles which divide prior to S phase. • The pair of centrioles separate during prophase aligning themselves at opposite poles of the cell. • From each centriole a sunburst of microtubules are projected across the cytoplasm . • The centrioles establish the axis for the upcoming mitotic division. • These microtubules bind to the centromere regions of the chromosomes forming
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