Ch.18 - Cell Division Cycle Flashcards
18A - Overview 18B - Cell-Cycle Control System 18C - G1 Phase 18D - S Phase 18E - M Phase 18F - Mitosis 18G - Cytokinesis 18H - Control of Cell #s
The euk CC typ incl wh four phases? Briefly summarize.
M Phase - nuclear division (mitosis) and cell splitting in two (cytokinesis).
Interphase - period b/w M phases; comprised of G1, S, and G2 phases.
- S phase - DNA repl; cytoplasmic organelles (incl centrosome) also duplicated.
- G1 & G2 - gap/growth; cell monitors IC/EC environ to ensure favorable conditions; also checks that prep is complete.
T/F: During all of interphase, gene xcr typ continues.
True
During all of interphase, gene xcr typ continues → proteins synthd → cell grows.
- G1 and G2 phases are nearly absent during some embryonic CC’s; cell div (prolif) occurs v rapidly.
The euk ___________________ is a complex network of regulatory proteins that helps ensure cells repl all DNA/organelles and divide in orderly manner.
Euk cell-cycle control system (CCCS) is a complex network of regulatory proteins that helps ensure that cells repl all DNA/organelles and divide in orderly manner.
- Guarantees CC (DNA repl, mitosis, etc.) occur in set seq and that ea process completed before next one begins.
CCCS is itself regulated at certain critical points by feedback. How does slowed/stalled DNA synth, such as due to DNA damage, impact the rest of the CC?
If DNA synth is slowed/stalled → mitosis and CD must also be delayed.
- If DNA is damaged → CC must arrest in G1, S, or G2 so cell can repair damage, either before DNA repl is started/completed or before cell enters M phase.
- CCCS employs molecular brakes (“checkpoints”) to pause CC at certain transition points → prevents trigger of next step unless/until cell is properly prepared.
CCCS regulates progression thru CC at wh three main transition points?
CCCS regulates progression thru CC at three main transition points.
G1 to S phase - before committing to DNA repl, CCCS confirms that IC/EC environ is favorable for prolif.
- Partic imp in animal cells - cell prolif in animals reqs both sufficient nutrients (IC) and specific signals (EC).
- If conditions unfav → cell can transiently delay progress thru G1; may even enter G0.
G2 to M phase - before entering mitosis, CCCS confirms DNA is undamaged (i.e. intact) and fully repl.
During mitosis - before spindle pulls chromos apart/segr into daughter cells (anaphase), CCCS ensures dupl chromos are properly attached to mitotic spindle.
The core of CCCS is a series of molecular ______ that operate in a defined seq and coord main events of CC, incl _______ and ____________.
The core of CCCS is a series of molecular switches that operate in a defined seq and coord main events of CC, incl DNA repl and segr of dupl chromos.
CCCS governs CC machinery by cyclically in/activating key proteins/complexes that initiate or regulate DNA repl, mitosis, and cytokinesis. This regulation is carried out largely thru ________________ of regulatory proteins.
CCCS governs CC machinery by cyclically in/activating key proteins/complexes that initiate or regulate DNA repl, mitosis, and cytokinesis. This regulation is carried out largely thru de/phos of regulatory proteins.
T/F: Protein kinases at core of CCCS are present in proliferating cells thru/o CC.
True
Protein kinases at core of CCCS are present in proliferating cells thru/o CC; activated, h/e, only at partic times in CC, after wh they are quickly inactivated.
- Activity of ea CCCS kinase rises/falls in cyclical fashion.
- Recall: switching on/off CCCS kinases is also partly regulated by cyclins; hence “cyclin-dep protein kinases” (Cdks).
The concen of both cyclin-dep protein kinases (Cdks) and cyclins rises/falls in cyclical fashion during CC.
False
The activity of Cdks and concen of cyclins rises/falls in cyclical fashion.
- I.e. Cdks are present thru/o CC, but activated only at partic times, then quickly inactivated.
As S cyclin concen ↑ → ______ (concen/activation) of S-Cdk __ (↑/↓) → drives cell into ___ phase.
As S cyclin concen ↑ → activation of S-Cdk ↑ → drives cell into S phase.
Diff cyclin–Cdk complexes trigger diff steps of CC. Describe the activity of four cyclin-Cdk complexes, incl when active and effect on CC.
M cyclin - acts in G2 to trigger entry into M phase; forms active complex (M-Cdk) w specific Cdk.
S cyclins and G1/S cyclins also bind distinct Cdks late in G1 to form S-Cdk and G1/S-Cdk, resp → help launch S phase.
G1 cyclins - act earlier in G1, bind other Cdks to form G1-Cdks → help drive cell thru G1 toward S phase.
- Formation of these G1-Cdks in animal cells typ deps on EC signals that stim cell to divide.
Each cyclin–Cdk complex phos’s diff set of target proteins in cell; e.g. G1-Cdks phos regulatory proteins that activate xcr of genes reqd for DNA repl.
- By activating diff sets of target proteins, ea type of complex triggers diff transition step.
T/F: ea cyclin–Cdk complex phos’s diff set of target proteins in cell.
True
Each cyclin–Cdk complex phos’s diff set of target proteins in cell.
- E.g. G1-Cdks phos regulatory proteins that activate xcr of genes reqd for DNA repl.
- By activating diff sets of target proteins, ea type of complex triggers diff transition step.
Cyclin concens are regulated by ________ and ________. Over the course of CC, concen of ea cyclin rises _______ (abruptly/gradually) then falls _______ (abruptly/gradually).
Cyclin concens are regulated by xcr and proteolysis. Over the course of CC, concen of ea cyclin rises gradually then falls abruptly.
- Gradual ↑ in cyclin concens stems fr ↑ xcr of cyclin genes.
- Rapid ↓ in cyclin concens is precipitated by a full-scale targeted destruction of the protein via ubiquitylation by anaphase-promoting complex (APC).
The gradual ↑ in cyclin concens stems fr ↑ xcr of cyclin genes. By contrast, the rapid ↓ in cyclin concens is precipitated by a full-scale targeted destruction of the protein via ____.
Gradual ↑ in cyclin concens stems fr ↑ xcr of cyclin genes.
Rapid ↓ in cyclin concens is precipitated by a full-scale targeted destruction of the protein via ubiquitylation by anaphase-promoting complex (APC).
- Abrupt degrad of M and S cyclins partway thru M phase deps on large enzyme complex (APC), wh tags M and S cyclins w chain of ubiquitin (ubiquitylation) → cyclin degrad → Cdk returns to inactive state → helps drive transition fr one CC phase to next.
- e.g. M-cyclin degrad → inactivatopm of M-Cdk → cell taken out of mitosis.
Cyclin concens ↑ gradually, but activity of cyclin-Cdk complexes tends to switch on abruptly. Describe the mechanism by wh cyclin-Cdk complexes are activated.
Cyclin–Cdk complexes contain inhibitory P’s→ must be dephos by specific protein phosphatase to become active.
E.g. as soon as M-Cdk complex formed → phos’d at two adj sites by an inhibitory protein kinase (Wee1) → keeps M-Cdk inactive until Pi’s are removed by an activating protein phosphatase (Cdc25)
- Note only one of two phos’d site shown in Fig.
- Still not clear how timing of Cdc25 phosphatase is controlled.
Explain how both protein kinases and phosphatases regulate activity of specific cyclin–Cdk complexes → control CC progression.
Cyclin-dep protein kinases (Cdks) are at the core of CCCS; present thru/o CC, but activity rises/falls in cyclical fashion.
Switching Cdks on/off is partly regulated by cyclins, whose concen rises/falls in cyclical fashion.
Cyclin-Cdk complexes are phos’d—and inhibited—at two sites by Wee1 shortly after formation → inactive until Cdc25—an activating protein phosphatase—dephos’s both inhibitory P’s.
In addition to de/phos, activity of Cdks can also be modulated by binding of _________ proteins, such as ___.
In addition to de/phos, activity of Cdks can also be modulated by binding of Cdk inhibitor proteins, such as p27.
- Inhibitors can block assembly/activity of certain cyclin–Cdk complexes.
-
p27 binds an activated cyclin-Cdk complex → prevents Cdk fr phos’ing target proteins reqd for progression thru G1 into S phase.
- i.e. p27 maintain Cdk in inactive state during G1 phase → delay progression into S phase and provide more time for cell growth or more favorable EC conditions.
Describe the function of p27 in the CCCS.
p27 binds an activated cyclin-Cdk complex → prevents Cdk fr phos’ing target proteins reqd for progression thru G1 into S phase.
- i.e. p27 inactivate Cdk during G1 phase → delay progression into S phase and provide more time for cell growth or more favorable EC conditions.
CCCS uses a combo of mechanisms to transiently delay progress at transition points and ensure proper conditions and set seq of events. Describe the mechanisms that can be activated at three major transition points.
At G1-to-S transition, Cdk inhibitors (p27) can prevent cells fr entering S phase and replicating their DNA; i.e. waits for favorable IC/EC conditions or until DNA damage repaired.
At G2-to-M transition, can inhibit activating phosphatase (Cdc25) reqd to activate M-Cdk; i.e. triggers mitosis only after DNA completely repl.
At exit fr mitosis, can inhibit activation of APC → prevents ubiquitylation & degrad of M cyclin; i.e. initiates chromo sep only after dupl chromos correctly aligned on mitotic spindle.
Why is the G1 to S phase transition such an imp checkpoint?
G1 is an imp checkpoint: based on IC signals (cell size) and EC signals (growth factors), CCCS can either hold cell transiently in G1 (or G0) or allow prep for entry into S phase of another CC.
-
Beyond this critical G1-to-S transition, cell typ progresses thru rest of CC quickly—typ w/i 12–24 hours in mammals.
- In yeasts, G1-to-S transition sometimes called “Start”, bc passing it reps commitment to a full CC
Cdks are stably ________ (in/activated) in G1.
Cdks are stably inactivated in G1.
Recall: during M phase, cells are actively dividing and awash w active cyclin–Cdk complexes (S-, M-Cdks). These complexes must be inactivated by end of M phase, or cell will immediately repl DNA and initiate another round of division, w/o spending any signif time in G1 or G2 phases.
- Cdk inhibitor proteins (e.g. p27) bind/inactivate cyclin Cdk complexes.
- APC ubiquitylates cyclin → cyclin degrads → Cdk inactivated.
- Synth of new cyclins is also blocked.
Why do you suppose cells have evolved a special G0 phase, rather than just stopping in G1 and not moving on to S phase?
For multicell orgs, control of CD is v imp. Individual cells must not prolif unless it is to benefit whole org. G0 offers protection fr aberrant activation of CD, bc CCCS is largely dismantled.
Alt, if cell just paused in G1, it would still contain all CCCS and could readily be induced to divide. The cell would also have to remake the “decision” not to divide almost continuously. To re-enter CC fr G0, cell has to resynth all components that have disappeared.
Typ, mammalian cells will multiply only if stim’d by ___ (IC/EC) signals, such as _______, produced by other cells.
W/o prolif signals → CC arrests in ___ phase.
W/o prolif signals for long period → cell withdraws fr CC and enters ___.
Typ, mammalian cells will multiply only if stim’d by EC signals (mitogens) produced by other cells.
W/o EC prolif signals → CC arrests in G1.
W/o EC prolif signals for long period → cell withdraws fr CC and enters nonprolif state (e.g. G0) for variable time.
Escape fr CC arrest or certain non-prolif states reqs accumulation of ______.
Escape fr CC arrest or certain non-prolif states reqs accumulation of cyclins.
Mitogens switch on signal paths that stim synth of G1 cyclins, G1/S cyclins, and other proteins involved in DNA synth/chromo dupl → buildup of cyclins triggers wave of G1/S-Cdk activity → relieves negative controls (block progression fr G1 to S phase).
Escape fr CC arrest or certain non-prolif states reqs accumulation of cyclins. _______ switch on signal paths that stim synth of G1 cyclins, G1/S cyclins, and other proteins involved in DNA synth/chromo dupl → buildup of cyclins triggers wave of ________ activity → relieves ____ (neg/pos) controls—that is, it blocks progression fr __ to __ phase.
Escape fr CC arrest or certain non-prolif states reqs accumulation of cyclins. Mitogens switch on signal paths that stim synth of G1 cyclins, G1/S cyclins, and other proteins involved in DNA synth/chromo dupl → buildup of cyclins triggers wave of G1- and G1/S-Cdk activity → relieves neg controls—that is, it blocks progression fr G1 to S phase.
- E.g. Retinoblastoma (Rb) - rare childhood eye tumor; Rb protein is missing/defective.
Retinoblastoma (Rb) is a rare childhood eye tumor in wh Rb protein is missing/defective. In dephos state, Rb binds/inhibits partic xcr regulators that normally turn on genes reqd for cell prolif. Describe what happens when EC signals like mitogen bind a cell w normal Rb as well as one w Rb missing/defective.
In dephos state, Rb binds/inhibits partic xcr regulators that normally turn on genes reqd for cell prolif; i.e. negative control.
Recall enzyme-coupled receptors & RTKs: in active state, Ras initiates phos cascade → ser/thr protein kinases phos/activate one/an in seq → activates mitogen-activated protein kinase (MAP kinase) signal path.
In cell w normal Rb: mitogens trigger activation of G1- and G1/S-Cdks → phos/inactivate Rb → Rb changes conform and releases bound xcr regulator → xcr regulator free to activate genes reqd for cell prolif, thus relieving Rb “brake”/neg control.
In cell w missing/defective Rb: mitogens trigger activation of G1- and G1/S-Cdks → Rb not present → xcr regulator unbound and continuously activates genes reqd for cell prolif.
In repsonse to DNA damage in G1, specific protein _______ (kinases/phosphatases) _______ (in/activate) p53—a xcr regulator—and halt its ______ (synth/degrad) → p53 ________ (in/activates) xcr of gene encoding p21, a Cdk inhibitor → p21 binds ___-Cdk and ___-Cdk and prevents them fr driving cell into S phase → cell arrests in ___ → cell repairs damaged DNA before repl.
In repsonse to DNA damage in G1, specific protein kinases activate p53—a xcr regulator—and halt its degrad (↑ p53 concen) → p53 activates xcr p21, a Cdk inhibitor → p21 binds G1/S-Cdk and S-Cdk and prevents them fr driving cell into S phase → cell arrests in G1 → cell repairs damaged DNA before repl.
- If DNA damage is beyond repair, p53 can induce apoptosis (see later).
- If p53 missing/defective → unrestrained repl of damaged DNA → high rate of mutation/cancer.
- If protein kinases that normally phos/activate p53 are blocked → damaged DNA replicated.
p53—a xcr regulator—is stabilized and activated by protein kinases in response to DNA damage. What happens to the IC concen of p21 when p53 is activated? What happens if DNA damage is beyond repair, or if p53 is missing/defective?
DNA damage in G1 → specific protein kinases activate p53—a xcr regulator—and halt its degrad (thus, ↑ p53 concen) → p53 activates xcr of gene encoding p21, a Cdk inhibitor → p21 binds G1/S- and S-Cdk and prevents them fr driving cell into S phase → cell arrests in G1 → cell repairs damaged DNA before repl.
- If DNA damage is beyond repair, p53 can induce apoptosis.
- If p53 missing/defective → unrestrained repl of damaged DNA → high rate of mutation/cancer.
Cells can delay division for prolonged periods by temporarily or permanently entering specialized non-dividing states. What causes cells to enter such a state, and in what types of cells is this desirable?
Many human cells (e.g. nerve/muscle) permanently stop dividing when they differentiate → completely dismantles CCCS and irreversibly shuts down genes encoding relevant cyclins/Cdks.
In absence of approp signals, other cell types withdraw fr CC only temporarily → enter arrested/quiescence state: G0.
- Cells in G0 retain ability to reassemble CCCS quickly and divide again; e.g. most liver cells are in G0, but can be stimulated to prolif if liver is damaged.
T/F: cells in G0 retain ability to reassemble CCCS quickly and divide again.
True
Cells in G0 retain ability to reassemble CCCS quickly and divide again; e.g. most liver cells are in G0, but can be stimulated to prolif if liver is damaged.
Alt, many human cells (e.g. nerve/muscle) permanently stop dividing when they differentiate → completely dismantles CCCS and irreversibly shuts down genes encoding relevant cyclins/Cdks.
Prep for DNA repl begins early in G1, when proteins/complexes are recruited to __________ along ea chromo.
Prep for DNA repl begins early in G1, when proteins/complexes are recruited to origins of replication along ea chromo.
- Origin recog complex (ORC) remains perched atop repl origins thru/o CC.
The origin recog complex (ORC) remains perched atop repl origins thru/o CC. In the first step of repl initiation, ORC recruits _____, whose concen rises early in G1 → recruit pair of DNA _______, wh bind DNA duplex adj to ORC and cause ____ to dissoc → pre-repl complex now formed and origin is ready to fire.
The origin recog complex (ORC) remains perched atop repl origins thru/o CC. In the first step of repl initiation, ORC recruits Cdc6, whose concen rises early in G1 → recruit pair of DNA helicases, wh bind DNA duplex adj to ORC and cause Cdc6 to dissoc → pre-repl complex now formed and origin is ready to fire.
Repl initiation signal comes fr wh cyclin-Cdk complex?
Repl initiation signal comes fr S-Cdk, wh triggers S phase.
- S-Cdk is assembled and activated at end of G1.
- At start of S phase, S-Cdk activates DNA helicases in pre-repl complex (ORC-helicase-helicase) and promotes assembly of DNA pol/other proteins that form repl fork.
- I.e. S-Cdk “pulls the trigger” that initiates DNA repl.
S-Cdk also helps prevent re-repl by helping phos Cdc6→ marked for degrad.
- Eliminating Cdc6 helps ensure DNA repl cannot be reinitiated later in same CC.
Besides ‘pulling the trigger’ that initiates DNA repl, how does S-Cdk help prevent re-repl of DNA?
S-Cdk also helps prevent re-repl by helping phos Cdc6→ marked for degrad.
- Eliminating Cdc6 helps ensure DNA repl cannot be reinitiated later in same CC.
Explain how CCCS can delay entry into M phase if errors occur during DNA repl, or if repl is delayed.
For cell to progress into mitosis, Cdc25—a protein phosphatase—must dephos M-Cdks at partic sites.
- I.e. M-Cdk is inhibited by phos at partic sites → inhibitory P’s removed by Cdc25 → progress into mitosis.
- When DNA is damaged or incompletely repl, Cdc25 is inhibited → prevents removal of inhibitory Ps → M-Cdk remains inactive and M phase delayed until DNA repaired or repl complete.
Remarkably, a single protein complex, _____, brings about all the intricate rearrangements that occur in early stages of mitosis.
Remarkably, a single protein complex, M-Cdk, brings about all the intricate rearrangements that occur in early stages of mitosis.
M-Cdk helps prep duplicated chromos for segr and induces assembly of mitotic spindle (pulls chromos apart). When do M-Cdks accumulate, and how are they activated?
Inactive M-Cdk complexes accumulate thru/o G2 → at end of G2, Cdc25 (phosphatase) removes inhibitory P’s, activating M-Cdk.
- Activation is self-reinforcing (pos feedback): ea activated M-Cdk can indirectly activate additional M-Cdks by phos/activating more Cdc25.
- Activated M-Cdk also shuts down Wee1 (inhibitory kinase) → further activation of M-Cdk.
- Net conseq: M-Cdks are dephos/activated by Cdc25 → ignites explosive ↑ in M-Cdk activity: activated M-Cdks phos/activate more Cdc25 and phos/inactivate Wee1 → drives cell abruptly fr G2 into M phase.
When activated at end of G2, M-Cdk activity sharply ↑ via pos feedback and drives cell fr G2 into M phase. Explain.
M-Cdk activation is self-reinforcing (pos feedback): ea activated M-Cdk can indirectly activate additional M-Cdks by phos/activating more Cdc25 as well as phos/inactivating Wee1 (inhibitory kinase).
The hallmark of prophase is duplicated chromos condensing and becoming visible under microscope. What two protein complexes help configure duplicated chromos for segr?
Cohesins and condensins help configure duplicated chromos for segr.
Sister chromatids are held t/g by _______—protein complexes that assemble _______ (on ends/along length) of ea chromatid as DNA is repl.
________ are protein complexes phos/activated by _____ → assemble on DNA and help carry out condensation → more easily segregated w/i crowded, dividing cell.
Sister chromatids are held t/g by cohesins—protein complexes that assemble along length of ea chromatid as DNA is repl.
Condensins are protein complexes phos/activated by M-Cdks → assemble on DNA and help carry out condensation → more easily segregated w/i crowded, dividing cell.
Cohesins and condensins are structurally related; both thought to form ring structures around chromosomal DNA.
After dupl chromos have condensed, wh two complex cytoskeletal machines assemble in seq to carry out M phase?
Mitotic spindle carries out nuclear division (mitosis).
- Composed of mtubs and various proteins, incl mtub-assoc motor proteins.
- In all euks, mitotic spindle seps dupl chromos and allocates one copy to ea daughter cell.
In animal cells/many unicell euks, contractile ring carries out cytoplasmic division (cytokinesis).
- Consists mainly of actin (mfils) and myosin fils arranged in a ring around equator of cell.
- Starts to assemble just beneath pmem toward end of mitosis → ring contracts → pulls mem inward → pinches cell in two.
- V diff mechanism in plants.
Both strucs disassemble rapidly after task.
T/F: Cytokinesis begins before mitosis ends.
True
Cytokinesis begins before mitosis ends.
Describe what is meant by the “centrosome cycle”.
Centrosome cycle - centrosome dupl (fr S to G2 phase) → sep during mitosis → help form ‘poles’ of mitotic spindle → ea daughter cell receives its own centrosome.
- Centrosome dupl is triggered by same Cdks—G1/S-Cdk and S-Cdk—that initiate DNA repl.
- Initially, when centrosome duplicates, both copies remain t/g as single complex on one side of nucleus.
- As mitosis begins and centrosomes sep, ea nucleates a radial array of mtubs (an aster) → two asters move to opp sides of nucleus to form two poles of mitotic spindle
Centrosome dupl is triggered by same Cdks—G1/S-Cdk and S-Cdk—that initiate DNA repl. The duplicated centrosomes initially remain t/g as single complex. describe what happens to centrosomes as CC progress into/thru M phase.
Mitosis begins and centrosomes sep → ea nucleate a radial array of mtubs (an aster; “astral mtubs”) → two asters move to opp sides of nucleus to form two poles of mitotic spindle.
At start of mitosis (______), stability of mtubs __ (↑/↓), in part bc _____ phos/activate mtub-assoc proteins → mtubs rapidly grow/shrink, extending/exploring in all directions fr ea centrosome.
At start of mitosis (prophase), stability of mtubs ↓, in part bc M-Cdks phos/activate mtub-assoc proteins → during prophase, mtubs rapidly grow/shrink, extending/exploring in all directions fr ea centrosome.
During prophase, as mtubs rapidly grow/shrink and explore cell. When do mtubs become ‘interpolar mtubs’, and when do centrosomes become ‘spindle poles’?
During prophase, as mtubs rapidly grow/shrink and explore cell, some mtubs fr ea centrosome interact w ea/o → stabilizes mtubs → prevent them fr depoly and joins mtubs t/g to form basic framework of mitotic spindle w characteristic bipolar shape; now called ‘interpolar mtubs’.
- Two centrosomes that give rise to bipolar mitotic spindles now called ‘spindle poles’.
- Interpolar mtub assembly is driven, in part, by motor proteins → help cross-link mtubs.
After NE breakdown, spindle mtubs, wh have been lying in wait outside nucleus, now gain access to dupl chromos and capture them. Describe this mechanism.
After NE breakdown, instable/probing spindle mtubs recog/bind chromos at kinetochores: protein complexes that assemble on centromere of ea condensed chromo during late prophase.
- Ea dupl chromo has two kinetochores—one on ea sister chromatid—wh face in opp directions, and thus tend to attach mtubs fr opp poles: “bi-orientation” → generates tension on kinetochores → tension acts as signal that mtubs are properly attached and kinetochores/chromos ready to be sep (during anaphase).
- Kinetochores are not preassigned to one spindle pole; mtubs appear to attach kinetochores w/i reach.
- Kinetochores recog special DNA seq at ea centromere; if this seq is altered, kinetochores fail to assemble → mitosis fails.
Kinetochores tend to attach mtubs fr opp poles, resulting in “bi-orientation” of sister chromatids. Explain why this orientation is signif.
Bi-orientation of sister chromatids generates tension on kinetochores, wh acts as signal that mtubs are properly attached and kinetochores/chromos ready to be segr.
- CCCS monitors this tension to ensure correct chromo attachment → acts as IC signal that cell ready to progress fr prometaphase to anaphase.
T/F: all kinetochores can attach only one mtub ea.
False
of mtubs attached to ea kinetochore varies among species: ea human kinetochore binds 20–40 mtubs, whereas yeast kinetochore binds just one.
Plants don’t have centrosomes. How, then, do plants direct assembly of a functional bipolar spindle?
In cells w/o centrosomes (plants, some animal types) chromos nucleate mtub assembly → motor proteins move/arrange mtubs and chromos into a bipolar spindle.
The start of metaphase is characterized by formation of metaphase plate. Describe how this struc/assembly is formed.
During prometaphase, the dupl chromos—attached to mitotic spindle—begin to jerk in various directions, then align at spindle equator, halfway b/w spindle poles → form metaphase plate.
Anaphase begins abruptly w breakage of _______ linkages → ea chromatid—now considered a ______—free to be pulled to spindle pole.
Anaphase begins abruptly w breakage of cohesin linkages that hold sister chromatids t/g → ea chromatid—now considered a chromo—free to be pulled to spindle pole.
Cohesin linkages are destroyed by _______ (protease), wh is held inactive by _______ (inhibitory protein) before anaphase begins. At start of anaphase, _____ ubiquitylates this inhibitory protein, wh is subseq degraded → allows the protease to sever cohesin linkages.
Cohesin linkages are destroyed by separase (protease), wh is held inactive by securin (inhibitory protein) before anaphase begins. At start of anaphase, APC ubiquitylates this inhibitory protein, wh is subseq degraded → allows separase to sever cohesin linkages.
- Recall: anaphase-promoting complex (APC) - same protein complex that marks M and S cyclins for degrad → Cdk returns to inactive state. APC activity causes rapid ↓ in cyclin concens and helps drive transitions.
During anaphase—after APC ubiquitylates securin and enables separase to cleave cohesin linkages b/w sister chromatids—chromos are pulled to spindle poles, all at about same speed.
Describe the two indep, nearly simult processes that drive this movement.
Anaphase A - kinetochore mtubs shorten (depoly) at both ends → attached chromos pulled toward opp poles.
Anaphase B - spindle poles themselves move apart via motor proteins acting on interpolar and aster mtubs → further segr chromos.
- Kinesins act on long, overlapping interpolar mtubs → elongates/slides mtubs fr opp poles past one/an at equator → push spindle poles apart.
- Dyneins act on outward-pointing astral mtubs at ea pole → pull spindle poles apart, toward cell cortex (whr dyneins are anchored).
Describe the events of telophase.
Telophase - mitotic spindle disassembles and NE reassembles around ea group of chromos (complete set at ea pole) to form daughter nuclei.
- Vesicles of nuclear mem first cluster around individual chromos, then fuse to re-form NE; nuclear pore proteins and lamins that were phos’d during prometaphase are now dephos → reassemble/re-form NE/lamina.
- Once NE has re-formed, pores pump in nuclear proteins → nucleus expands → condensed chromos decondense into interphase state → gene xcr can resume; mitosis complete.
Consider the events that lead to formation of new nucleus at telophase. How do nuclear/cytosolic proteins become properly re-sorted so new nucleus contains nuclear proteins but not cytosolic proteins?
New NE reassembles on surface of chromos. The close apposition of NE to chromos prevents cytosolic proteins fr being trapped b/w chromos and NE.
Nuclear proteins are then selectively imported thru nuclear pores, causing nucleus to expand while maintaining its characteristic protein composition.
The first visible sign of cytokinesis in animal cells is a puckering and furrowing of pmem during anaphase. Furrowing invariably occurs in a plane that runs perpendicular to long axis of mitotic spindle. Explain why this is imp.
Furrowing invariably occurs in a plane that runs perpendicular to long axis of mitotic spindle, wh ensures that cleavage furrow cuts b/w ea group of segr chromos → ea daughter cell receives identical and complete set of GM.
T/F: furrowing process (cytokinesis) occurs in all-or-nothing fashion.
True
Once furrowing process begins, cleavage proceeds even if mitotic spindle is artificially sucked out or depoly’d (e.g. colchicine).
Contractile ring (in animals) is primarily composed of overlapping array of actin (mfils) and myosin fils. Describe where/when contractile ring forms, as well as its mechanism of action.
Contractile ring assembles at anaphase; attached to mem-assoc proteins (cytoplasmic-side).
Once assembled, contractile ring exerts force generated by sliding of actin against myosin.
Unlike stable assoc of actin/myosin in muscle fibers, contractile ring is transient:
- assembles to carry out cytokinesis, then gradually becomes smaller as cytokinesis progresses, and disassembles completely after cleavage.
Briefly describe how cytokinesis in higher plants differs fr that of animals.
Cytokinesis in plant cells involves formation of new cell wall.
- New cell wall starts to assemble in cytoplasm b/w two sets of segr chromos at start of telophase.
- Interpolar mtubs remaining at telophase form the phragmoplast and guide vesicles, derived fr Golgi, toward equator of spindle.
- Vesicles, wh are filled w cell-wall material (polysacchs, glycoproteins), fuse to form growing new cell wall that grows outward (via mtubs) to reach pmem and original cell wall.
- Pre-existing pmem and mem surrounding new cell wall fuse, completely sep daughter cells.
- Later, cellulose microfibrils are laid down w/i matrix to complete construction of new cell wall.
Apoptosis is much quicker and cleaner than necrosis in that there is no damage to adj cells. Describe what happens during apoptosis, e.g. cytoskeleton, NE, nuclear DNA, and cell-surface.
Most imp, cell surface is altered such that it immediately attracts phagocytic cells: typ macrophages that engulf apoptotic cell before it spills contents.
- Rapid removal of dying cell avoids damaging conseqs of necrosis; also allows organic components to be recycled.
Cytoskeleton collapses, NE disassembles, and nuclear DNA fragments.
Apoptotic cell may dev irregular bulges (“blebs”) on surface; but ultimately shrinks/condenses.
Apoptosis is mediated by an IC proteolytic cascade; involves family of proteases called _________.
Apoptosis is mediated by an IC proteolytic cascade; involves family of proteases called caspases.
- Caspases - “suicide protease”; synthd as inactive precursors—procaspases—wh are activated in response to signals that induce apoptosis.
- Similar mechanism in most animal cells.
Caspases are “suicide protease” that are synthd as inactive precursors—procaspases—wh are activated in response to signals that induce apoptosis. Describe this mechanism of activation of function of caspases in cell.
Activation of caspases:
Two cleaved fragments fr ea of two procaspases assoc to form an active caspase, wh is formed fr two small and two large subunits; the two ‘prodomains’ are typ discarded.
Initiator caspase cleaves/activates many downstream executioner caspases.
Some executioner caspases activate other executioners → kick off an amplifying, proteolytic cascade.
Other executioner caspases dismember key proteins, e.g. one executioner caspase targets lamins wh form NE → cleavage causes irreversible breakdown of nuclear lamina → allows nucleases to enter and break down DNA.
Cell dismantles itself quickly and cleanly, and its corpse is rapidly taken up and digested by another cell.
Two cleaved fragments fr ea of two ___________ assoc to form an active caspase. _______ caspase then cleaves and activates many downstream _________ caspases.
Some of these activate other ________ caspases and trigger an amplifying, proteolytic cascade.
Alt, some of these downstream caspases dismember key proteins, such as _______ wh form NE → irreversible breakdown of nuclear lamina → allows _______ to enter and break down DNA.
Two cleaved fragments fr ea of two procaspases assoc to form an active caspase. Initiator caspase then cleaves and activates many downstream executioner caspases.
Some executioners activate other executioners and trigger an amplifying, proteolytic cascade.
Alt, some executioner caspases dismember key proteins, e.g. lamins wh form NE → irreversible breakdown of nuclear lamina → allows nucleases to enter and break down DNA.
T/F: activation of apoptosis is typ triggered in all-or-none fashion.
True
Activation of apoptosis, like entry into new CC stage, is typ triggered in all-or-none fashion; must be tightly controlled.
- Caspase cascade is not only destructive and self-amplifying but also irreversible: once cell reaches critical point along path to destruction, it cannot turn back.
All nucleated animal cells contain inactive procaspases lying in wait for signal to destroy cell. These procaspases are tightly regulated by _____ family of IC proteins.
All nucleated animal cells contain inactive procaspases lying in wait for signal to destroy cell . These procaspases are tightly regulated by Bcl2 family of IC proteins.
- Some Bcl2 proteins promote caspase activation and cell death; others inhibit these processes.
_____ and _____ are the two most imp death-inducing Bcl2 proteins. They are activated in response to ________ → induce release of an electron-xprt protein (_________) fr mito into cytosol → stim apoptosis.
Other Bcl2 proteins, such as _____, prevent these death-inducing Bcl2 proteins fr releasing electron xprtr → inhibit apoptosis.
Bax and Bak are the two most imp death-inducing Bcl2 proteins. They are activated in response to DNA damage/injury → induce release of an electron- xprt protein (cytochrome c) fr mito into cytosol → stim apoptosis.
Other Bcl2 proteins, such as Bcl2 itself, prevent these death-inducing Bcl2 proteins fr releasing electron xprtr → inhibit apoptosis.
EC signals may impact activity of Bcl2 proteins in target cell → stim apoptosis. Other signals activate a set of cell-surface receptors called ____________ → stim apoptosis more directly.
EC signals may impact activity of Bcl2 proteins in target cell → stim apoptosis. Other signals activate a set of cell-surface receptors called death-receptors → stim apoptosis more directly.
Fas - well-studied death receptor; present on many mammalian cell types; activated by mem-bound protein (Fas ligand) that is present on surface of specialized immune cells called killer lymphocytes.
- Killer cells help regulate immune responses by inducing apoptosis in other unwanted/needed immune cells.
- Binding of Fas ligand to receptor triggers assembly of a death-inducing signaling complex (DISC), wh incl specific initiator procaspases (procaspase-8 or -10) that cleave/activate one/an → activate executioner procaspases → launch caspase proteolytic cascade → cell death.
Many types of nerve cells are produced in excess during NS dev → compete for limited amounts of survival factors (secreted by target cells). What does this suggest about animal cell’s dependence on EC signals?
Cells that receive enough survival factor live, others die by apoptosis.
- # of surviving nerve cells is automatically adjusted to match # of cells w wh they conn.
- Similar sys of dependence in other tissues, both during dev/adulthood.
Survival factors typ act by activating cell-surface receptors → receptors activate IC signal paths that normally suppress _________, typ by regulating _____ proteins.
Survival factors typ act by activating cell-surface receptors → receptors activate IC signal paths that normally suppress apoptosis, typ by regulating Bcl2 proteins.
- Some survival factors ↑ concen of Bcl2 itself, wh normally suppresses apoptosis.
Recall: Bax/Bak - two most imp death-inducing Bcl2 proteins; activated in response to DNA damage/injury → induce release of cyt c fr mito into cytosol → stim apoptosis. Other Bcl2 proteins, incl Bcl2 itself, prevent Bax/Bak fr releasing cyt c → inhibit apoptosis.
Mitogens stim CD by promoting entry into ___ phase.
Mitogens stim CD by promoting entry into S phase.
Most mitogens are secreted signal proteins, such as platelet-derived growth factor (PDGF) and hepatocyte growth factor (HGF).
Mitogens bind cell-surface receptors (e.g. ECRs & RTKs) → initiate various IC signal paths (e.g. Ras-GEF, Ras, MAP-kinase module) → stim CD.
Recall: MAP-Kinase paths act mainly by releasing molecular ‘brakes’ that block transition fr G1 to S phase.
Why are platelet-derived growth factor and hepatocyte growth factor classified as mitogens rather than growth factors?
Positively acting signal proteins are functionally classified into three major categories:
- Survival factors promote cell survival, largely by suppressing apoptosis, e.g. via Bcl2 proteins.
- Mitogens stim CD, primarily by overcoming IC ‘braking’ mechanisms that tend to block progression thru CC.
- Growth factors stim cell growth (↑ cell size/mass) by promoting synth and inhibiting degrad of proteins/macros.
Not mutually exclusive; many signals have 1+ function; i.e. PDGF, as a mitogen, primarily stims CD in response to injury by overcoming the molecular “brakes” that block transition fr G1 to S phase.
T/F: GFs both ↑ rate of nutrient synth and ↓ rate of degrad.
True
GFs both ↑ rate of nutrient synth and ↓ rate of degrad.
Like most survival factors/mitogens, most EC GFs bind cell-surface receptors → activate IC signal paths → nutrients (proteins/macros)accumulate.
- Recall: mitogens stim CD by overcoming molecular “brakes”, while GFs stim growth by promoting synth/inhibiting degrad. H/e, not mutually exclusive functional classifications.
T/F: some EC signals can act as both mitogens and growth factors (GFs).
True
Some EC signals, incl PDGF, can act as both GFs and mitogens: stim both cell growth and progression thru CC.
- Such EC signal proteins help maintain approp cell size during prolif.
Some EC signal proteins inhibit cell survival/div/growth. For example, _______ normally inhibits growth/prolif of precursor cells (__________) that fuse to form skeletal muscle cells during mammalian dev.
Some EC signal proteins inhibit cell survival/div/growth. For example, myostatin normally inhibits growth/ prolif of precursor cells (myoblasts) that fuse to form skeletal muscle cells during mammalian dev.
- When gene encoding myostatin is deleted → both #/size of muscle cells ↑ → muscles grow several times larger than normal.
- E.g. cattle bred for large muscles turned out to have mutations in gene encoding myostatin
The polar movement of chromos during anaphase A is assoc w mtub shortening. In partic, mtubs depoly at ends at wh they are attached to kinetochores.
Explain how mtub can shorten and generate force yet remain firmly attached to the chromo.
Chromo movement appears to be driven by kinetochore proteins that cling to sides of depolymerizing mtub. These proteins frequently detach fr—and reattach to— the kinetochore mtub.
As tubulin subunits continue to dissoc, the kinetochore must slide poleward to maintain its grip on the retreating end of the shrinking mtub.
