Biol 121- 2010.01.25- Genetics- Mitosis, Cytokinesis, Chromosome Structure, Cell Cycle Regulation.docx

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Biol 121 225
Freeman 227-240 (Ch. 11) Jan. 25, 10
How many chromosomes do
human (and potato plants)
have?
-46 chromosomes in each cell
What is the general structure of
eukaryotic chromosomes?
-eukaryotic chromosomes normally exist as extremely long, threadlike strands
consisting of DNA associated with globular proteins called histones
-in eukaryotes, the DNA-protein material is called chromatin
Chromatids
At start of M phase, each
chromosome...
-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 the start of M phase, each chromosome consists of two sister chromatids that are
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
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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
2) Chromatin
3) Chromatid
4) Sister chromatids
5) Centromere
6) Kinetochore
-chromosome a structure composed of a DNA molecule and associated proteins
-chromatin material making up eukaryotic chromosomes, consists of DNA molecule
complexed with histone proteins
-chromatid one strand of a replicated chromosome, with its assoc. pns
-sister chromatids two strands of a replicated chromosome (identical genetically)
and when sister chromatids separate during mitosis, become independent
chromosomes
-centromere structure that joins sister chromatids
-kinetochore structure on sister chromatids where spindle fibres attach
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