4. cellular reproduction: mitosis and cytokinesis Flashcards
Cytoskeletal dynamics during mitosis
- essentially the process of partitioning newly replicated chromosomes into separate parts of the cell
- mitosis occurs as the last step of the cell cycle
- lasts approx 1 hours
- during that time the cell must build, them disassemble mitotic spindle
mitotic spindle has 3 sets of micro tubules:
astral microtubules
kinetochore microtubules
polar microtubules
astral microtubules
radiate from centrosome, form aster (centrosome is essentially a MTOC [microtubule organizing centre])
-help position mitotic apparatus, determine cleavage plane
kinetochore microtubules
attach to chromosomes at kinetochore; pull chromosomes to different poles
Polar microtubules
interdigitate (interlocking) with opposing pair; support framework; help push centrosomes apart
microtubules have…
a polarity
alpha and beta tubulin
minus (-) ends of microtubules
alpha
all point toward the centrosome
point away from kinetochore (=point of attachment to centrosome)
Plus (+) ends microtubules
all point away from centrosome
beta end
rapidly growing end
plus and minus ends of microtubules differs in their rates of assembly
high at the +end and low at the -end
two key events in spindle assembly:
1) Formation of poles
- spindle microtubules must “attach” or anchor to poles
2) capture of chromosome some
- spindle microtubules must attach to chromosomes
which are the heads that are walked on
dyneins and kinesins
in animal cells, duplication of centrosomes (MTOCs) occurs during _______ phase; each centrosome has ________ centrioles arranged perpendicularly to each other .
S, two
what happens during prophase?
centrosomes begin to move apart and migrate to poles for orientation of mitotic apparatus
-motor proteins are involved
motor proteins
- kinesins
- dyneins
kinesins
most walk toward +end of MT
-orange
push/ enlarge
dyneins
-end directed motor proteins
-round, blue
pull, shorten
late prophase
- motor proteins help align microtubules
- centrosome alignment
- bottom end of dyneins is anchored
- moving end of protein dynein is round top
- dynein -end directed MT motor protein - centrosome separation
- kinesin +end- directed MT motor protein
- lengthening of polar tubules can push centrosomes apart
- shortening of astral MT’s can help pull centrosomes apart
- -end dynein
attachment of microtubules to chromosomes
- occurs at kinetochore
- +ends of MTs attach to outer layer
- centromeric chromatin has binding factors mediating MT attachment
- motor proteins help hold the MT and corona fibers together
Chromosome capture: rapid fluctuations in spindle MTs length occurs ___________.
- Occurs during pro-metaphase once nuclear envelope is broken down
- Dynamic MT acts a poker
what generates chromosomal movement?
A combination of micro-tubular dynamics and motors:
- rapid polymerization and depolymerization occurs at the + end of microtubules
- microtubule motors assist in two things
microtubules motors assist in:
- maintaining flow of tubulin subunits
2. tethering the kinetochore
treadmilling
in this scenario, there is no net elongation/shrinking of the MT
page slide 14
red subunits added to +end and being pushed up to -end. -end looses subunits and quickly and subunits are added
depolymerization
In this scenario, there is a net loss at the + end, and the chromosome moves to the right
smaller
polymerization
in this scenario, there is a net gain at the +end and the chromosome moves away (to the left)
larger
MT motors
there is enough room fro tubulin subunits to be added or removed from kinetochore; the motor proteins hold the MT just far enough away from the kinetochore
-Dynenin and - directed MT motor walks on its head from the kinetochore to the centromere
Spindle checkpoint
-cell monitors the status of events in metaphase before going on to anaphase
(it is important that the chromosomes are all evenly lined up as the cell goes on to anaphase, so that the separation process goes smoothly)
-if one of the chromosomes is not at the metaphase plate it waits, to give it a chance to get there
5 mechanisms of the spindle checkpoint
- sister chromatids are held together by protein called securin
- the anaphase promoting complex (APC) causes ubiquitination of securin, thereby allowing chromatids to split apart
- Cdc20 is an activator of the Anaphase promoting complex (APC)
- MAD2 protein is bound to the kinetochore of un-aligned or improperly attached metaphase chromosomes
- Mad2 inhibits cdc20
Cytokinesis is different for plants and animals cells
animal cells: cleavage furrow
Plant cells: formation of cell plate
Actin and Myosin ll form what?
the contractile ring
what does head rotation of myosin cause?
sliding of actin
what is the evidence for myosin ll in Cytokinesis?
Treatments:
-myosin II knock-outs (gene for myosin II is deleted)
-antisense inhibition of myosin II mRNA
-anti-myosin II antibodies
treatments results:
cells that can replicate their chromosomes, but are unable to divide, resulting in large, multi-nucleated cells
Why use antibodies as tool for inhibiting protein functions?
- an antibody will bind with high specificity to just one target molecule (called its antigens) (many sorts of both)
- binding an antibody to a protein MAY disrupt the normal function of that protein (due to steric hindrance)
- antibody binding to myosin prevent it from working properly, so the cells failed to divide
What is a benefit of being multinucleated?
rapid gene expression-lots of RNA and protein
mitosis without cell division in early insect embryos
during early fly development, nuclear division occur without cellular division
the early embryo is called a syncytial blastoderm
the nuclei can be studied by GFP-labeling of chromosomes
(remember not all mutlinucelated cells arise in this manner)
multinucleated striated muscle cells arise by_______
cell fusion
Meiosis is a cell division without DNA replication
- 2 sequential cell divisions without an intervening phase of DNA replication
- results in 4 haploid cells (n=1 - each cell has only one copy of every centrosome, not 2, as in a haploid organism)
- in animal cells, meiosis produces the gametes (sperm and eggs)
securin needs to be degraded in order for ________________
sister chromatids to detach
mitosis steps (5)
[prophase]- nuclear envelope disappears; chromosomes condense
[prometaphase]-mitotic spindle forms; chromosomes attach to spindle
[metaphase]-chromosomes aligned along equator
[anaphase]-chromatids separate; chromosomes migrate to poles
[telophase]-chromosomes decondense; nuclear envelope forms
