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Chapter

BIOL 121 Chapter Notes -Microtubule, Cytokinesis, Tubulin


Department
Biology
Course Code
BIOL 121
Professor
Robert Adamson

Page:
of 3
Chapter 11-Cell Cycle
-multicellular eukaryotes start as single-celled embryo and grow thru cell division
Chromosomes-carry hereditary material/instructions for building + operating cell
-made up of single DNA double helix wrapped around proteins
Gamete-haploid reproductive cell, fuse w/ another haploid cell to form zygote (eg.
egg and sperm cells)
Somatic cells-body cells (cell in multicellular org. that isn’t sperm/egg)
Meiosis/reduction division-hereditary material in parent cell reduced by half, occurs
in animals just to form gametes
Mitosis-daughter cells genetically identical to parent cell (hereditary material is
constant), followed by..
Cytokinesis (“cell movement”)-cytoplasm divides into the 2 daughter cells
In multicellular eukaryotes, Mitosis + Cytokinesis responsible for…
1. Growth-all identical cells that make up body are from one fertilized egg
(result of sperm+egg)
2. Wound repair-cells that repair skin generated by M + C
3. Reproduction-asexual reproduction (offspring genetically identical to parent)
eg. yeast cells
11.1 Mitosis and the Cell Cycle
Gene-segment of DNA that codes for certain protein or RNA in cell
Mitotic (M)-dividing phase (M + C) where copied chrom. given to daughter cells, no
replication
Interphase (between phase)-nondividing (cells spend most time here, chro.
replication)
Synthesis (S) phase-DNA synthesized, chromosomes replicated
Cell cycle-starts with formation of eukaryotic cell to time it undergoes cell division
M phase (6-8 hrs) -> G₁ (7-9) -> S phase (6-8) -> G₂ (4-5) -> M phase, etc.
Gap phases-no DNA synthesis, organelles replicate, more cytoplasm made (prep
parent cell for division-make sure big enough)
11.2 How Does Mitosis Take Place?
Histones-proteins associated with DNA
Chromatin-DNA + protein (i.e. histones) complex that compose eukaryotic
chromosomes
Chromatid-each of the DNA copies in a replicated chromosome, each has 1 long
DNA double helix
Sister Chromatids-chromatids from the same chromosome, joined along length and
centromere
1. Interphase
a) G₁ (parent cell with unreplicated chromosomes)
b) S Phase (chromosomes in parent cell replicate to form sister chromatids)
c) G₂ (at this point, each chromosome consists of 2 sister chromatids)
1. Mitosis
a) Chromatin condense, more compact
b) Sisters separate + form independent chromosomes
Centrioles-pair forms centrosome-the microtubule organizing center in animals
IPPMAT
1. Prophase
-chromosomes condense (now visible in microscope)
-mitotic spindles form (made of microtubules, produces mechanical forces),
move toward poles
2. Prometaphase
-nuclear envelope breaks down, spindle fibers from each mitotic spindle
attach to kinetochores
-centrosomes keep moving to poles (in animals), microtubules start moving
chrom. to middle)
3. Metaphase (middle phase)
-centrosomes at poles, chromosomes lined along metaphase plate (formation
of mitotic spindle now complete) and are pulled by microtubules
4. Anaphase (against phase)
-centromeres/sisters split, kinetochore spindle fibers shorten and pull chrom.
to poles, unattached fibers lengthen and push poles further apart
5. Telophase (end-phase)
-nuclear envelopes form, mitotic spindle disintegrates, chromosomes de-
condense
-before M phase, mitochondria, lysosomes, chloroplasts, etc. already replicated
Cytokinesis-cytoplasm splits to form 2 daughter cells, right after mitosis
Animals-cleavage furrow (myosin bind to ring of actin filaments, ring shrinks)
Plants-cell plate forms (vesicles from golgi carry cell wall parts and plasma
membrane)
-microtubules are made of α-tubulin and β-tubulin dimmers (length determined by
# of these)
-have plus and minus end, grows at plus end
-during anaphase, microtubules shorten at kinetochore b/c plus end loses
tubulin
-motor proteins (eg. dynein) on kinetochore crown “walk” down microtubules
(see pg.233)
11.3 Control of the Cell Cycle
-length of cell cycles vary
-in rapidly dividing cells, G₁ basically eliminated
-in nondividing cells, stuck in G₁-called G₀ state (cells called post-mitotic) eg.
nerve + muscle cells
-depends on conditions (eg. damaged liver, envt rich in nutrients-yeast)
-found that when cells in M and interphase were fused, nucleus of interphase cell
started M phase
-cytoplasm of M phase has regulatory molecule that induces cells to enter M
phase
Mitosis-promoting factor (MPF)-induces mitosis in all eukaryotes
MPF
-made up of 2 subunits:
Protein kinase-catalyzes transfer of phosphate group from ATP to protein
(activates/deactivates protein)
Cyclin-protein, concentration fluctuates (builds up during interphase, peaks at M),
regulatory
-kinase is Cdk (cyclin-dependent kinase), active only when bound to cyclin
-Cdk phosphorylated -> inactive -> G₂, one phosphate group drops -> MPF shape
changes, active -> binds to proteins + catalyzes their phosphorylation (chrom
condense, nuclear envelop breaks, mitotic spindle starts, cyclin degrades) –negative
feedback
Cell-cycle checkpoint-critical point of cycle where progression is regulated
-defective cells lack checkpoint and keep dividing instead of stopping growth
(tumour)
late in G₁: check if cell big enough, rich in nutrients, DNA damaged
-if damaged, p53 protein stops cycle or causes apoptosis (cell suicide)-
p53 fail=cancer
btwn G₂, + M: chromosome replication complete, DNA damage, MPF
activated-if not, stay in G₂
during M: chromosomes attached to mitotic spindle (if not, daughter cells get
wrong # of chrom)