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Chapter

BIOL 121 Chapter Notes -Microtubule Organizing Center, Spindle Apparatus, Sister Chromatids


Department
Biology
Course Code
BIOL 121
Professor
Carol Pollock

Page:
of 6
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
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
Biol 121 225
Freeman 227-240 (Ch. 11) Jan. 25, 10
7) Microtubule organizing
center
8) Centrosome
9) Centriole
-any structure that organizes microtubules
-the microtubule organizing center in animals
-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 tissue lost during
digestion; mature human nerve and muscle cells do not divide at all
-most of these differences are due to variation in the length of the G1 phase
-in rapidly dividing cells, G1 is essentially eliminated
-most non-dividing cells in contrast are permanently stuck in G1
G0
-as said before, most non-dividing cells in contrast are permanently stuck in G1
-this arrested stage is called the G0 state (“G zero”)
-cells in G0 have effectively exited the cell cycle and sometimes referred to as post-
mitotic
-nerve cells, muscle cells, and many other cell types enter G0 once matured
Changes in conditions and how
they affect division rate
-changes in conditions can affect rate
-human liver cells divide once a year
-but if part of liver is damaged/lost, remaining cells divide every one or two days until
repair is accomplished
-cells of unicellular organisms like yeasts, bacteria and archaea divide rapidly only if
the environment is rich in nutrients; otherwise they enter a quiescent/inactive state
MPF
Composition/subunits
Role of subunits
-mitosis-promoting factor
-a molecule that induces mitosis in all eukaryotes
-made of two distinct polypeptide units
-one is a protein kinase (enzyme that catalyzes transfer of phosphate group from ATP
to a target protein)
-because addition of a phosphate group changes target protein’s shape and activity,
protein kinases frequently act as regulatory elements in the cell
-the other subunit belongs to the family of proteins called cyclins
-concentration of cyclins fluctuates throughout cell cycle
-builds up in [ ] in interphase and peaks during M phase
-this increase is important b/c the kinase subunit in MPF can be active only when
bound to the cyclin subunit
-therefore, the protein kinase subunit is called a cyclin-dependent kinase, or Cdk
-MPF is a dimer consisting of a cyclin subunit and a cyclin-dependent kinase subunit
the cyclin subunit functions as a regulatory protein; the kinase subunit is the part that
catalyzes the phosphorylation of other proteins to start mitosis
Fluctuation of MPF Cyclin
-affects fluctuation of complete MPF dimmers
-MPF Cdk remains constant throughout cell cycle
-MPF Cyclin increases regularly from G1 to M phase, and then decreases to the end of
M phase, and then increases from the start of G1 to M phase and so on
Why doesn’t the increasing
[MPF] trigger the onset of M
phase? (in interphase)
-MPF’s Cdk subunit becomes phosphorylated at two sites after it binds to cyclin
-when Cdk is phosphorylated, its conformation changes in a way that renders the
protein inactive
-late in G2, enzymes cause one of the phosphate groups on the Cdk subunit to drop
off this dephosphorylation reaction changes MPF’s shape in a way that activates it
Mechanism MPF uses to trigger
-once MPF activated, it binds to specific proteins and catalyzes their phosphorylation