lect 20: cell cycle Flashcards
what are the learning objectives of this lecture?
-outline the major events of mitosis and cytokinesis
-explain the control of the cell cycle through cyclin/Cdk
-describe the events occurring in prophase, prometaphase, metaphase, anaphase and telophase
-define meiosis and its two different stages (meiosis I and II)
what is the definition of cell division? what are the two types in eukaryotic cells?
cell division: process by which new cells arise from other living cells
-continues in certain tissues throughout life
-takes place very differently in prokaryotes and eukaryotes
two types in eukaryotic cells
1. mitosis
-generates daughter cells genetically identical to their parent
-basis for producing new cells
2. meiosis
-produces cells with half the genetic content of the parent
-basis for producing new sexually reproducing organisms
what are the phases of the cell cycle?
interphase (G1, S, G2)
-majority of the cell cycle
-lasts longer than M phase (da weeks, longer)
M phase
-mitosis=chromosome segregation
-cytokinesis= cell division
what is the G0 phase?
resting phase
-cells metabolically active but not actively preparing to divide
-temporary in some cells (external signal triggers onset of G1)
-other cells never/rarely divide (e.g. myocyte, neuron
what are the phases in interphase?
G1 (gap) phase
-end of mitosis to DNA replication
S phase (synthesis)
-DNA replication
G2 (gap) phase
-end of S phase to beginning of mitosis
what are the mitotic stages?
- prophase
- prometaphase
- metaphase
- anaphase
- telophase
-each phase characterized by a particular series of events
-each of these stages represents a segment of a continuous process
what are the four major transitions or checkpoints of the cell cycle control system?
- G1 checkpoint
- S checkpoint
- G2 checkpoint
- M checkpoint
-interphase contains 3 out of the 4
what is the G1 checkpoint?
-in yeast its called the start point
-called the restriction point in mammalian cells, to enter S phase
-check for DNA damage/favorable conditions (yeast=nutrients, humans=growth factors)
-conditions not favorable=G0
what is the S checkpoint?
-checks for DNA damage before/during replication
-prevents DNA reduplication
what is the G2 checkpoint?
-entry into mitosis
-check for DNA damage
-ensures DNA duplicated
what is the M checkpoint?
-entry to anaphase
-ensures chromosomes aligned and attached to spindle
what are two key components in the cell cycle control system (controls system as a whole)?
- cyclin-dependent protein kinases (Cdks)
-family of protein kinases
-level constant throughout cycle - cyclin
-Cdk regulatory protein: activate kinase activity (switches on and off)
-levels change cyclically
-kinase activity rises and falls as cell progresses through cycle
-pairing between individual cyclins and Cdks is specific
what is the graph that shows the concentrations of M-cyclin and M-Cdk?
what are cyclin-dependent protein kinases in yeast?
fission yeast (aka brewer’s yeast)
-single Cdk protein (only 1 type, cdc2)
-binds all classes of cyclins
-changing cyclin partner at different stages of the cycle triggers different cell-cycle events
two check points
1. START
2. G2-M transition
what are the Cdks in eukaryotes?
different cylin-Cdk complexes trigger different steps of cell cycle
-unlike yeast, there are multiple Cdks in vertebrates (bind specific cyclins)
three main cyclins found in all eukaryotic cells
-G1/S-cyclin
-S-cyclin
-M-cyclin
control 3 checkpoints
what are the mitotic cyclin classes?
what is the table of major cyclins and cdks of vertebrates and budding yeast?
what is the maturation promoting factor (MAP)?
phosphorylates many targets
what is the activation of Cdks?
-cyclin binds Cdk-> movement of flexible loop of polypeptide chain away from active site opening=partial activation
-full activation requires phosphorylation of aa near Cdk active site by Cdk-activating kinase (CAK)-> conformation change in CdK-> phosphorylation of target proteins
what are the other mechanisms influencing Cdk activity?
other than cyclin levels (primary determinant) and partial/full activation
-Cdk phosphorylation/dephosphorylation
-Cdk inhibitor proteins
-regulated proteolysis
what is Cdk phosphorylation/dephosphorylation?
-phosphorylation by protein kinase Wee1-inhibites activity
-dephosphorylation by phosphatases Cdc25-increases activity
-important in control of m-Cdk activity at beginning of mitosis
what are Cdk inhibitor proteins?
binding inactivates cyclin-Cdk complexes
-Cdk active site conformational change
important control mechanism in early cell cycle-G1, S
-e.g. budding yeast: Sic1 inhibits Cdk in G1
-e.g. human: P27 inhibits cyclin A-Cdk2 complex, S phase
what is regulated proteolysis in Cdk activity?
-cyclins undergo proteolysis at end of each phase of cell cycle
-ubiquitin-proteasome pathway
ubiquitin ligases: multi-subunit complexes
-anaphase-promoting complex (APC):
-targets S- and M- cyclins
-metaphase-to anaphase transition (M checkpoint)
what is the graph of the recap of the cell cycle?
what is the overview of prophase and prometaphase?
what is the overview of metaphase, anaphase and telophase?
what is the overview graph of mitosis and cytokinesis?
what is prophase? what happens in early prophase?
formation of mitotic chromosome
-duplicated chromosomes prepared for segregation
-mitotic machinery assembled
chromosome compaction/condensation occurs in early prophase
-condensin: protein responsible for compaction
-activated by phosphorylation of its subunits by cyclin-Cdk complex
-pulls DNA loops-> positive DNA coiling-> condenses DNA
what needs to happen prior to replication of DNA in prophase? what is cohesin?
appear as rod-like structures in two chromatids
prior to replication the DNA of each chromosome is associated with cohesin (large protein complex)
-forms a ring to encircle the two sister DNA molecules
-holds sister chromatids together through G2 and into mitosis
-mist removed during chromosome duplication in prophase
what are the similarities between cohesin and condensin?
cohesin and condensin
-similar structure organization
-both contain SMC proteins (structural maintenance of chromosome)
SMC protein structure
-fold back on themselves-> antiparallel coiled coil
-globular domains at N- and C- termini
what are centromeres and kinetochores used in prophase?
centromere
-constricted region of mitotic chromosome
-highly repeated DNA sequences
-binding site for specific proteins
kinetochore
-protein complex, assembles on centromere
-attaches chromosome to spindle microtubules
what is the graph of kinetochore?
what is the importance of centrosomes in prophase?
formation of pole of mitotic spindle
centrosome
-principal MT-organizing center (animal cells)
-duplication necessary to form two poles of mitotic spindle
-spindle begins concurrently with DNA replication
-centrosomes separate at beginning of mitosis
-nucleate array of MTs called an aster
-asters move to opposite sides of nucleus-> two poles of mitotic spindle (separation triggered by enzyme separase)
what is the formation of the mitotic spindle in prophase?
-as centromeres move apart they organize bundles of MTs that form mitotic spindle
rapid diassembly of interphase cytoskeleton
-inactivation of stabilizing proteins (e.g. MAPs)
-activation of destabilizing proteins
in higher plants and animal oocytes there are no centrosomes
-spindle pole formed by movement of motor proteins
what is the transition of prophase/prometaphase?
dissolution of nuclear envelope, partitioning of cytoplasmic organelles
what is prometaphase?
-nuclear envelope completely broken down
-mitotic spindle assembly completed
-compacted chromosomes scattered in early prometaphase
-need to be moved into position in center of cell
what are the three groups used in metaphase?
chromosomes aligned at spindle equator (metaphase plate)
three microtubule groups
1. astral MTs: position spindle apparatus in cell, determine plane of cytokinesis
2. chromosomal Mts: move chromosomes to poles
3. polar MTs: maintain spindle integrity
what does metaphase involve?
-microtubule flux in the metaphase spindle involves tubulin treadmilling toward the poles
-subunits are incorporated at the kinetochores of the chromosomal microtubules and the equatorial ends of the polar microtubules
-they are lost from the minus ends of the microtubules in the region of the poles
what is the role of proteolysis in progression through mitosis in anaphase?
-breakage of remaining cohesin linkages
-linkage is destroyed by protease called separase
-prior to beginning of anaphase, separase maintained in inactive state by securin
-securin targeted for destruction by APC (anaphase-promoting complex)
what determines APC substrate in anaphase?
what is the important of protein degradation in anaphase?
-the importance of protein degradation in regulating the events of mitosis and the reentry of cells into G1 is best revealed with the use of inhibitors
-no destruction of cyclin B (M cyclin) (by inhibiting the proteasome)-> cells remain in a late stage of mitosis
what are the events of anaphase?
chromosomes split in synchrony
anaphase A
-movement of chromosomes toward the poles
-rate 1um/min (very slow)
-driving form is loss of tubulin subunits from kinetochore MTs (both ends)
anaphase B
-spindle poles move in opposite directions
-motor proteins are driving force (kinesin and dynein)
what is the SAC in anaphase?
the spindle assembly checkpoint (SAC)
-operates at metaphase/anaphase transition to check for misaligned chromosomes
unattached kinetochores contain a protein complex that sends a “wait” signal to prevent entry into anaphase
-complex senses loss or lack of tension
-if a cell cannot postpone chromosome segregation the daughter cells receive an abnormal number of chromosomes (aneuploidy)
what is telophase?
-disassembly of mitotic spindle
-nuclear envelopes reassembles
-chromosomes become dispersed
what is cytokinesis?
thin band of actin and myosin filaments generate force to cleave cell
-creates cleavage furrow
-position determined by anaphase mitotic spindle location
-plane of cytokinesis (site of filament assembly) determined by signal coming from spindle poles
what is the contractile ring theory?
-bipolar myosin filaments essential for the ring contraction (composed of myosin II)
-force generating mechanism similar to actin-myosin mechanism during contraction of muscle
-sliding of the filaments of the contractile ring pulls cortex and attached plasma membrane toward the center of the cell
what is meiosis?
