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Biol 121 225 Freeman 227-240 (Ch. 11) How many chromosomes do human (and potato plants) have? What is the general structure of eukaryotic chromosomes? Chromatids -46 chromosomes in each cell

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University of British Columbia
BIOL 121

Biol 121 225 Freeman 227-240 (Ch. 11) Jan. 25, 10 How many chromosomes do -46 chromosomes in each cell human (and potato plants) have? What is the general structure of -eukaryotic chromosomes normally exist as extremely long, threadlike strands eukaryotic chromosomes? consisting of DNA associated with globular proteins called histones -in eukaryotes, the DNA-protein material is called chromatin Chromatids -each of the DNA copies in a replicated chromosome is called a chromatid -chromatids from the same chromosome are called sister chromatids – they represent exact copies of the same genetic material -each chromatid contains one long DNA double helix At start of M phase, each -at the start of M phase, each chromosome consists of two sister chromatids that are chromosome... attached to each other at the centromere -as mitosis begins, chromatin condenses to form a much more compact structure -during mitosis, the two sister chromatids separate to form independent chromosomes, and one copy of each chromosome goes to each of the two daughter cells Prophase -first step of mitosis -chromosomes and centrosomes have already replicated during interphase -during prophase, the chromosomes condense into compact structures -chromosomes first become visible in light microscope during prophase -in cytoplasm, prophase is marked by formation of the mitotic spindle – a structure that produces mechanical forces that pull chromosomes into the daughter cells during mitosis (via depolymerisation) -the mitotic spindle consists of an array of microtubules – components of the cytoskeleton -groups of microtubules attach to the chromosome and are called spindle fibres -in all eukaryotes, spindle fibres originate from a microtubule organizing center – the nature of which varies among species -in animal cells, this microtubule organizing center is a centrosome – a structure that contains a pair of centrioles -during prophase in all eukaryotes, the mitotic spindle either begin moving to opposite sides of the cell or form on opposite sides Prometaphase -once chromosomes have condensed, nucleolus disappears and the nuclear envelope fragments or breaks down -after the nuclear envelope has disintegrated, spindle fibres from each mitotic spindle attach to one of the two sister chromatids of each chromosome at the kinetochore -the kinetochore -the attachment between the spindle fibres and each chromatid is made at a structure called the kinetochore -kinetochores are located at the centromere region of the chromosome, where sister chromatids are attached to each other -each chromosome has two kinetochores where spindle fibres attach – one on each side -during prometaphase in animals, the centrosomes continue their movement to opposite poles of the cell -in all groups, the microtubules attached to the kinetochores begin moving the Biol 121 225 Freeman 227-240 (Ch. 11) Jan. 25, 10 chromosomes to the middle of the cell Metaphase -animal centrosomes complete their migration to the opposite poles of the cell -in all eukaryotes, the kinetochore microtubules finish moving the chromosomes to the middle of the cell -when metaphase finishes, the chromosomes are lined up along an imaginary plane called the metaphase plate -at this point, the formation of the mitotic spindle is complete -each chromatid is attached to spindle fibres that run from its kinetochore to one of the poles of the cell -each chromosome is held by kinetochore spindle fibres reaching to opposite poles and exerting the same amount of tension or pull Anaphase -at the start of anaphase, the centromeres that are holding sister chromatids together split -because they are under tension, sister chromatids are pulled apart equally – with the same amount of force – to create independent chromosomes -the kinetochore spindle fibres then begin to shorten, and motor proteins pull the chromosomes to opposite poles of the cell -the two poles of the cell are also pushed away from each other by motor proteins associated with microtubules that are not attached to chromosomes -during anaphase, replicated chromosomes split into two identical sets of unreplicated chromosomes – separation of sister chromatids Telophase -nuclear envelope begins to form around each set of chromosomes -the mitotic spindle disintegrates and the chromosomes begin to de-condense -once two independent nuclei have formed, mitosis is complete Cytokinesis -usually occurs immediately following mitosis -during cytokinesis, cytoplasm divides to form two daughter cells, each with its own nucleus and complete set of organelles -in animals, fungi, and slime molds, cytokinesis begins with the formation of a cleavage furrow -the furrow appears because a ring of actin filaments forms just inside the plasma membrane, in a plane that bisects the cell -a motor protein called myosin binds to these actin filaments -when myosin binds to ATP or ADP, part of the protein moves in a way that causes actin filaments to slide -as myosin moves the ring of actin filament on the inside of the plasma membrane, the ring shrinks in size and tightens, pulling the membrane with it -the actin and myosin filaments continue to slide past each other, tightening the ring further, until the original membrane is pinched in two and the cell division is complete Definitions of structures involved in mitosis: 1) Chromosome -chromosome – a structure composed of a DNA molecule and associated proteins 2) Chromatin -chromatin – material making up eukaryotic chromosomes, consists of DNA molecule complexed with histone proteins 3) Chromatid -chromatid – one strand of a replicated chromosome, with its assoc. pns 4) Sister chromatids -sister chromatids – two strands of a replicated chromosome (identical genetically) and when sister chromatids separate during mitosis, become independent chromosomes 5) Centromere -centromere – structure that joins sister chromatids 6) Kinetochore -kinetochore – structure on sister chromatids where spindle fibres attach Biol 121 225 Freeman 227-240 (Ch. 11) Jan. 25, 10 7) Microtubule organizing -any structure that organizes microtubules center 8) Centrosome -the microtubule organizing center in animals 9) Centriole -cylindrical structures that comprise microtubules, located inside animal centrosomes Length of cell cycle - can vary enormously among different cell types, even in the same individual -in humans, intestinal cells divide more than twice a day to renew
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