18.3 The Events of the M Phase Flashcards
M phase involves a () of all cell components
major reorganization
4 primary stages of mitosis:
- prophase
- metaphase
- anaphase
- telophase
beginning of prophase is markes by the appreance of () (each of which consists of 2 sister chromatids; daughter DNA molecules produced in S)
condensed chromosomes
sister chromatids are attached to each other at the ()
centromere
the centromere is a chromosomal region to which proteins bind to form the() → site of eventual attachment of mitotic spindle microtubules
kinetochore
cytoplasmic changes leading to the development of mitotic spindle are initiated; duplicated centrosomes move towards opposite sides of the nucleus → serve as the two poles of the () (begins to form during late prophase)
mitotic spindle
in yeasts, the nuclear envelope remains intact ()
closed mitosis
in higher eukaryotes, prophase ends when the nuclear envelope breaks down ()
open mitosis
transition period between prophase and metaphase
prometaphase
what happens in prometaphase
spindle microtubules (from both poles) attach to the kinetochores of condensed chromosomes
chromosomes shuffle back and forth until they eventually align on the metaphase plate in the center of the spindle → beginning of ()
metaphase
transition from metaphase to anaphase is triggered by the ()
breakage of the link between sister chromatids
mitosis phase where nuclei reform and chromosomes decondense
telophase
() usually begins during late anaphase and is almost complete by the end of telophase
cytokinesis
() is the master regulator of M phase transition
Cdk1/cycB protein kinase (MPF)
mitotic protein kinases: (1) and (2) families are activated coordinately with Cdk1 to signal entry into M phase
- Aurora kinase (Aurora A and B)
- Polo-like kinase
Cdk1, Aurora, and Polo-like kinases are activated in a positive feedback loop that works to initiate the ff. downstream pathways needed for M phase transition:
- chromatin condensation
- nuclear envelope breakdown
- fragmentation of Golgi apparatus
- spindle formation
nuclear envelope breakdown involves changes in all of nuclear envelope components:
- fragmentation of nuclear membranes
- dissociation of nuclear pore complexes
- depolymerization of nuclear lamina
depolymerization of nuclear lamina results from the (1) by (2)
- phosphorylation of lamins
- Cdk1/cycB
Golgi breakdown is mediated by phosphorylation of proteins by ()
Cdk1 and Polo-like kinases
centrosome maturation, separation, and spindle assembly during prophase are driven by phosphorylation of Aurora and Polo-like kinases at the ()
centrosomes
during prophase, spindle formation results from the ()
dynamic instability of microtubules
() allows spindle microtubules to attach to chromosomes at the kinetochores
breakdown of nuclear envelope
proteins assembled at the kinetochore include () → involved in the interactions of chromosomes with microtubules
Aurora B kinase
chromosomes in prometaphase shuffle back and forth due to activity of () at the kinetochore and centrosomes
microtubule motors
balance of forces acting on the chromosomes leads to their alignment on the ()
metaphase plate
the mitotic spindle consists of:
- kinetochore microtubules: attached to the chromosomes
- interpolar microtubules: overlap in the center of the cell
- astral microtubules: radiate outward from centrosome towards cell periphery
chromosome condensation is driven by (1) and (2) (members of structural maintenance of chromatin or SMC proteins), which contribute to chromosome segregation during mitosis
- condensins
- cohesins
() bind to DNA in S phase and maintain linkage between sister chromatids after replication
cohesins
as the cell enters M phase, () are activated by phosphorylation of both Cdk1 and Aurora B kinase
condensins
once condensins are activated by (1), they replace (2) along most of the length of the chromosome → sister chromatids remain linked only at the centromere
- Cdk1 and Aurora B kinase
- cohesins
condensins then induce chromatin condensation by forming () → formation of metaphase chromosomes
DNA loops
the () monitors the alignment of chromosomes on the metaphase spindle
spindle assembly checkpoint
progression to anaphase is mediated by activation of () → results from phosphorylation by Cdk1/cycB
APC/C ubiquitin ligase
presence of () is sufficient to prevent APC/C activation
even 1 unaligned chromosome
unattached kinetochores lead to the assembly of the (), which inhibits APC/C
mitotic checkpoint complex (MCC)
MCC is no longer formed once () → APC/C is activated
all chromosomes are aligned on the spindle
MCC consists of 4 proteins:
- BubR1
- Bub3
- Mad2
- Cdc20
() is required for activation of APC/C, but it remains inactivated when bound to Mad and Bub proteins in the MCC
Cdc20
Cdc20 is required for activation of APC/C, but it remains inactivated when bound to () proteins in the MCC
Mad and Bub
activation of APC/C results in the ubiquitylation and degradation of 2 key target proteins that trigger metaphase to anaphase transition:
- degradation of cycB → Cdk1 inactivation → cell proceeds to exit mitosis
- degradation of securin (component of cohesins)→ dissociation of sister chromatids, allowing them to move to opposite poles of the mitotic spindle
degradation of securin activates () → degrades cohesin
separase
cytokinesis usually starts shortly after (1) starts; triggered by the (2)
- anaphase
- inactivation of Cdk1
() remain active in anaphase and play important roles in coordinating nuclear and cytoplasmic division of the cell
Aurora and Polo-like kinases
cytokinesis of yeast and animal cells is mediated by a () (forms beneath the plasma membrane)
contractile ring of actin and myosin II filaments
ring formation is activated by ()
Aurora and Polo-like kinases
in plant cells, cytokinesis proceeds by the formation of ()
new cell walls and plasma membranes
in early telophase, vesicles carrying cell wall precursors (from Golgi) accumulate at the ()
former site of the metaphase plate
vesicles fuse to form a membrane-enclosed disk; polysaccharides from the matrix of the new cell wall called a ()
cell plate
summarize how cytokinesis occurs in plant cells
- in early telophase, vesicles carrying cell wall precursors (from Golgi) accumulate at the former site of the metaphase plate
- vesicles fuse to form a membrane-enclosed disk; polysaccharides from the matrix of the new cell wall (called a cell plate)
- cell plate expands outward (perpendicular to the spindle) until it reaches the PM
- membrane surrounding cell plate then fuses with parental PM → divides the plant cell
() between daughter plant cells are formed as a result of incomplete vesicle fusion
plasmodesmata
