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Cell Cycle Regulation.docx

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University of British Columbia
BIOL 121
Carol Pollock

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 theBiol 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
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