Ch.18 - Cell Division Cycle

Question 1

The euk CC typ incl wh four phases? Briefly summarize.

Answer 1

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.

Question 2

T/F: During all of interphase, gene xcr typ continues.

Answer 2

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.

Question 3

The euk ___________________ is a complex network of regulatory proteins that helps ensure cells repl all DNA/organelles and divide in orderly manner.

Answer 3

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.

Question 4

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?

Answer 4

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.

Question 5

CCCS regulates progression thru CC at wh three main transition points?

Answer 5

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.

Question 6

The core of CCCS is a series of molecular ______ that operate in a defined seq and coord main events of CC, incl _______ and ____________.

Answer 6

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.

Question 7

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.

Answer 7

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.

Question 8

T/F: Protein kinases at core of CCCS are present in proliferating cells thru/o CC.

Answer 8

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).

Question 9

The concen of both cyclin-dep protein kinases (Cdks) and cyclins rises/falls in cyclical fashion during CC.

Answer 9

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.

Question 10

As S cyclin concen ↑ → ______ (concen/activation) of S-Cdk __ (↑/↓) → drives cell into ___ phase.

Answer 10

As S cyclin concen ↑ → activation of S-Cdk ↑ → drives cell into S phase.

Question 11

Diff cyclin–Cdk complexes trigger diff steps of CC. Describe the activity of four cyclin-Cdk complexes, incl when active and effect on CC.

Answer 11

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.

Question 12

T/F: ea cyclin–Cdk complex phos’s diff set of target proteins in cell.

Answer 12

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.

Question 13

Cyclin concens are regulated by ________ and ________. Over the course of CC, concen of ea cyclin rises _______ (abruptly/gradually) then falls _______ (abruptly/gradually).

Answer 13

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).

Question 14

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 ____.

Answer 14

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.

Question 15

Cyclin concens ↑ gradually, but activity of cyclin-Cdk complexes tends to switch on abruptly. Describe the mechanism by wh cyclin-Cdk complexes are activated.

Answer 15

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 P_i’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.

Question 16

Explain how both protein kinases and phosphatases regulate activity of specific cyclin–Cdk complexes → control CC progression.

Answer 16

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.

Question 17

In addition to de/phos, activity of Cdks can also be modulated by binding of _________ proteins, such as ___.

Answer 17

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.

Question 18

Describe the function of p27 in the CCCS.

Answer 18

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.

Question 19

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.

Answer 19

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.

Question 20

Why is the G1 to S phase transition such an imp checkpoint?

Answer 20

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

Question 21

Cdks are stably ________ (in/activated) in G1.

Answer 21

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.

Question 22

Why do you suppose cells have evolved a special G0 phase, rather than just stopping in G1 and not moving on to S phase?

Answer 22

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.

Question 23

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 ___.

Answer 23

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.

Question 24

Escape fr CC arrest or certain non-prolif states reqs accumulation of ______.

Answer 24

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).

Question 25

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.

Answer 25

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.

Question 26

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.

Answer 26

In dephos state, Rb binds/inhibits partic xcr regulators that normally turn on genes reqd for cell prolif; i.e. *negative* control. ## Footnote 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.

Question 27

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.

Answer 27

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.

Question 28

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?

Answer 28

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.

Question 29

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?

Answer 29

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.

Question 30

T/F: cells in G0 retain ability to reassemble CCCS quickly and divide again.

Answer 30

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**.

Question 31

Prep for DNA repl begins early in G1, when proteins/complexes are recruited to __________ along ea chromo.

Answer 31

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.

Question 32

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.

Answer 32

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.

Question 33

Repl initiation signal comes fr wh cyclin-Cdk complex?

Answer 33

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.

Question 34

Besides 'pulling the trigger' that initiates DNA repl, how does S-Cdk help prevent re-repl of DNA?

Answer 34

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.

Question 35

Explain how CCCS can delay entry into M phase if errors occur during DNA repl, or if repl is delayed.

Answer 35

**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.

Question 36

Remarkably, a single protein complex, \_\_\_\_\_, brings about all the intricate rearrangements that occur in early stages of mitosis.

Answer 36

Remarkably, a single protein complex, **M-Cdk**, brings about all the intricate rearrangements that occur in early stages of mitosis.

Question 37

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?

Answer 37

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.

Question 38

When activated at end of G2, M-Cdk activity sharply ↑ via pos feedback and drives cell fr G2 into M phase. Explain.

Answer 38

**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).

Question 39

The hallmark of prophase is duplicated chromos condensing and becoming visible under microscope. What two protein complexes help configure duplicated chromos for segr?

Answer 39

**Cohesins** and **condensins** help configure duplicated chromos for segr.

Question 40

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.

Answer 40

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**.

Question 41

After dupl chromos have condensed, wh two complex cytoskeletal machines assemble in seq to carry out M phase?

Answer 41

**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**.

Question 42

T/F: Cytokinesis begins before mitosis ends.

Answer 42

True Cytokinesis begins before mitosis ends.

Question 43

Describe what is meant by the "centrosome cycle".

Answer 43

**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

Question 44

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.

Answer 44

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.

Question 45

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.

Answer 45

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.

Question 46

During prophase, as mtubs rapidly grow/shrink and explore cell. When do mtubs become 'interpolar mtubs', and when do centrosomes become 'spindle poles'?

Answer 46

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.

Question 47

Prometaphase starts abruptly w disassembly of nuclear envelope (NE). What triggers this dissassembly, what what happens to NE afterwards?

Answer 47

Prometaphase starts abruptly w disassembly of NE, wh **breaks up into small mem vesicles**. * NE breakdown is **triggered by phos** and conseq disassembly of nuclear pore proteins and lamins (ifils).

Question 48

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.

Answer 48

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.

Question 49

Kinetochores tend to attach mtubs fr opp poles, resulting in "bi-orientation" of sister chromatids. Explain why this orientation is signif.

Answer 49

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.

Question 50

T/F: all kinetochores can attach only one mtub ea.

Answer 50

False of mtubs attached to ea kinetochore varies among species: ea human kinetochore binds 20–40 mtubs, whereas yeast kinetochore binds just one.

Question 51

Plants don't have centrosomes. How, then, do plants direct assembly of a functional bipolar spindle?

Answer 51

In cells w/o centrosomes (plants, some animal types) **chromos nucleate mtub assembly** → **motor proteins move/arrange mtubs and chromos into a bipolar spindle**.

Question 52

The start of metaphase is characterized by formation of metaphase plate. Describe how this struc/assembly is formed.

Answer 52

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**.

Question 53

Explain why continuous, balanced instability of mtubs is reqd to maintain metaphase spindle.

Answer 53

Sister chromatids are not simply deposited at metaphase plate, but rather suspended there under tension. This tension serves as a signal—monitored by CCCS—that all chromos are properly attached and ready for segr, i.e. ready for anaphase.

Question 54

Anaphase begins abruptly w breakage of _______ linkages → ea chromatid—now considered a \_\_\_\_\_\_—free to be pulled to spindle pole.

Answer 54

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.

Question 55

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.

Answer 55

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.

Question 56

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.

Answer 56

**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).

Question 57

Unattached chromos send a "stop" signal to CCCS → blocks activation of APC. Describe the mechanism and signif of this negative feedback signal.

Answer 57

Every chromo must be properly attached to spindle before anaphase; monitored by neg feedback signal: * Unattached chromos send a “stop” signal to CCCS → blocks activation of APC → **securin (inhibitory) continue to bind/prevent separase (protease) fr cleaving cohesins that link sister chromatids → sister chromatids remain glued t/g**. * **None of the dupl chromos can be pulled apart until every chromo has been positioned correctly on mitotic spindle**. * "Spindle assembly checkpoint" controls onset of anaphase, as well as exit fr mitosis. * Mechanism not entirly known.

Question 58

Describe the events of telophase.

Answer 58

**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.

Question 59

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?

Answer 59

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.

Question 60

Cytoplasmic division (cytokinesis) typ begins in \_\_\_\_\_\_\_, but not completed until two daughter nuclei have formed in \_\_\_\_\_\_\_\_.

Answer 60

Cytoplasmic division (cytokinesis) typ begins in **anaphase**, but not completed until two daughter nuclei have formed in **telophase**.

Question 61

T/F: cytokinesis deps on both mtubs and mfils.

Answer 61

True ## Footnote Cytokinesis in animal cells deps on a transient struc based on **actin** and **myosin fils**: contractile ring. H/e, both the plane of cleavage and timing of cytokinesis are det by **mitotic spindle** (**mtub**-based).

Question 62

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.

Answer 62

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**.

Question 63

During anaphase, overlapping _______ (astral, interpolar, or kinetochore) mtubs that form central spindle recruit/activate proteins that signal __________ to initiate assembly of contractile ring midway b/w spindle poles.

Answer 63

During anaphase, overlapping **interpolar** mtubs that form central spindle recruit/activate proteins that signal **cell cortex** to initiate assembly of contractile ring midway b/w spindle poles. * Bc these signals originate in anaphase spindle, this mechanism also contribs to *timing* of cytokinesis in late mitosis. * Mechanism by wh mitotic spindle dictates position of cleavage furrow is still uncertain.

Question 64

T/F: furrowing process (cytokinesis) occurs in all-or-nothing fashion.

Answer 64

True ## Footnote Once furrowing process begins, cleavage proceeds even if mitotic spindle is artificially sucked out or depoly'd (e.g. colchicine).

Question 65

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.

Answer 65

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**.

Question 66

CD in animals typ involves large changes in shape and ↓ cell-cell and/or cell-ECM conns. How, then, are new cells incorporated into tissues after CD?

Answer 66

Changes result, in part, fr the reorg of actin/myosin in cell cortex, only one aspect of wh is the assembly of contractile ring. For CD in animal tissue, this cycle of de/attachment presumably enables rearrangement of contacts w adj cells/ECM → new cells produced by CD can be accommodated w/i tissue.

Question 67

Briefly describe how cytokinesis in higher plants differs fr that of animals.

Answer 67

Cytokinesis in plant cells involves formation of new cell wall. 1. New cell wall starts to assemble in cytoplasm b/w two sets of segr chromos at start of **telophase**. 2. **Interpolar** **mtubs** remaining at telophase form the **phragmoplast** and **guide vesicles**, derived fr Golgi, toward equator of spindle. 3. **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. 4. **Pre-existing** **pmem** and mem surrounding **new cell wall fuse**, completely sep daughter cells. 5. Later, cellulose microfibrils are laid down w/i matrix to complete construction of new cell wall.

Question 68

T/F: Organelles such as mito/chloro cannot assemble spont fr individual components.

Answer 68

True ## Footnote Organelles such as mito/chloro cannot assemble spont fr individual components; arise only fr growth/division of preexisting organelles. ER/Golgi also derive fr preexisting organelle fragments.

Question 69

How are various mem-enclosed organelles, like mito/chloro and ER/Golgi, segr during CD?

Answer 69

**Mito/****chloro****are****typ****abundant** and safely inherited if, on average, their #s simply double once per CC. During interphase, ER is continuous w nuclear mem and is organized by mtub cytoskeleton → cell enters M phase → mtubs reorg and release ER → ER typ remains and is divided—along w rest of cell—during cytokinesis. The Golgi fragments during mitosis → fragments assoc w spindle mtubs via motor proteins → xprtd into daughter cells as spindle elongates in anaphase. Other cell components—incl other mem-enclosed organelles, ribosomes, and all soluble proteins—are inherited randomly.

Question 70

In animals, most common form of programmed cell death is \_\_\_\_\_\_\_\_\_.

Answer 70

In animals, most common form of programmed cell death is called **apoptosis** (fr Greek: “falling off,” as leaves fall fr tree).

Question 71

T/F: In adult tissues, cell death typ exactly balances CD.

Answer 71

True **In adult tissues, cell death** **typ** **exactly balances CD**. * Some tissues, e.g. mouse paws and our own hands/feet, are sculpted by apoptosis during embryonic dev. * Apoptosis in other tissues/ orgs isn't as clear: cells simply die when the struc they form is no longer needed.

Question 72

Cells that die as result of acute injury typ swell/burst in a process called _______ → spill contents over adj cells → triggers potentially damaging ______ response

Answer 72

Cells that die as result of acute injury typ swell/burst in a process called **necrosis** → spill contents over adj cells → triggers potentially damaging **inflam** response. * Apoptosis is much quicker/cleaner: no damage to adj cells.

Question 73

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.

Answer 73

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.

Question 74

Apoptosis is mediated by an IC proteolytic cascade; involves family of proteases called \_\_\_\_\_\_\_\_\_.

Answer 74

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.

Question 75

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.

Answer 75

**Activation of caspases**: ## Footnote *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**.

Question 76

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.

Answer 76

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.

Question 77

T/F: activation of apoptosis is typ triggered in all-or-none fashion.

Answer 77

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.

Question 78

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.

Answer 78

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.

Question 79

\_\_\_\_\_ 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.

Answer 79

**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.

Question 80

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.

Answer 80

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. ## Footnote **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.**

Question 81

What types of proteins most commonly act as EC signals that influence cell survival/growth/div?

Answer 81

Most EC signals that influence cell survival/growth/div are either **soluble proteins secreted by other cells** or **proteins bound to** **surface** **of other cells/ECM**.

Question 82

T/F: All EC signals that influence cell survival/growth/div do so in positive fashion, i.e. stim growth.

Answer 82

False ***Most*** **act pos (stim), but some act** **neg****(inhibit) survival, growth, or div.** Positively acting signal proteins can be functionally classified into **three major categories**: * **Survival factors** promote cell survival, largely by suppressing apoptosis. * **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. * **Caution**: cell proliferation refers to an ↑ in cell #s, whereas growth factor refers to EC signal proteins. Note: These categories are **not** **mutually exclusive**; many signals have 1+ function.

Question 83

What are the three major categories of positively acting EC signals that influence cell survival/growth/div?

Answer 83

**Survival factors** promote cell survival, largely by suppressing apoptosis. **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. * Caution: cell proliferation refers to an ↑ in cell #s, whereas growth factor refers to EC signal proteins. These categories are **not mutually exclusive**; many signals have 1+ function.

Question 84

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?

Answer 84

**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.

Question 85

Survival factors typ act by activating cell-surface receptors → receptors activate IC signal paths that normally suppress \_\_\_\_\_\_\_\_\_, typ by regulating _____ proteins.

Answer 85

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**.

Question 86

Mitogens stim CD by promoting entry into ___ phase.

Answer 86

Mitogens stim CD by promoting entry into **S** phase. ## Footnote 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.**

Question 87

Why are platelet-derived growth factor and hepatocyte growth factor classified as *mitogens* rather than *growth factors*?

Answer 87

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.

Question 88

Growth of an organism/organ deps on cell growth as much as CD. H/e, unlike CD, cell growth does not dep on \_\_\_\_\_\_\_\_\_\_\_.

Answer 88

Growth of an organism/organ deps on cell growth as much as CD. H/e, unlike CD, cell growth does not dep on **cell-cycle control system (CCCS)**. * Many animal cells, incl nerve cells and most muscle cells, grow mostly after terminal differentiation, i.e. when CD has permanently stopped.

Question 89

T/F: GFs both ↑ rate of nutrient synth and ↓ rate of degrad.

Answer 89

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.

Question 90

T/F: some EC signals can act as both mitogens *and* growth factors (GFs).

Answer 90

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.

Question 91

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.

Answer 91

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

Question 92

The shortest euk CCs of all—shorter even than those of many bacteria—occur in many early animal embryos. These so-called cleavage divisions take place w/o any signif increase in the weight of the embryo. How can this be? Wh phase of CC would you expect to be most reduced?

Answer 92

**Egg cells of many animals are big and contain stores of enough cell components to last for many CDs**. * The daughter cells that form during the first CDs after fertilization are progressively smaller in size and thus can be formed w/o need for new protein or RNA synth. Whereas normally dividing cells would grow continuously in G1, G2, and S phases, until their size doubled, there is no cell growth in these early cleavage divisions, and both G1 and G2 are virtually absent. As G1 is typ longer than G2 and S phase, **G1 is the most drastically reduced in these divisions**.

Question 93

One of the functions of M-Cdk is to cause a precipitous drop in M-cyclin concen halfway thru M phase. Describe the conseqs of this sudden decrease and suggest possible mechanisms by wh it might occur.

Answer 93

**Loss of M cyclin leads to inactivation of M-Cdk**. As a result, M-Cdk target proteins become dephos by phosphatases, and the cells exit fr mitosis: disassemble mitotic spindle, reassemble NE, decondense chromos, etc. * M cyclin is degraded by ubiquitin-dep destruction in proteasomes, and **activation of M-Cdk leads to activation of APC, wh ubiquitylates M cyclin, but w a substantial delay**.

Question 94

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.

Answer 94

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.

Question 95

An antibody that binds to myosin prevents the movement of myosin molecules along actin filaments. How do you suppose the antibody exerts this effect? What might be the result of injecting this antibody into cells (A) on the movement of chromos at anaphase or (B) on cytokinesis?

Answer 95

Antibodies bind tightly to the antigen (in this case myosin) to wh they were raised. When bound, an antibody can interfere w function of the antigen by preventing it fr interacting properly w other cell components. (A) movement of chromos at anaphase deps on mtubs and their motor proteins and *does not* dep on actin or myosin. Injection of an anti-myosin antibody into a cell will therefore have no effect on chromomovement during anaphase. (B) Cytokinesis, on the other hand, deps on assembly and contraction of a ring of actin and myosin fils, wh forms the cleavage furrow that splits the cell in two. Injection of anti-myosin antibody will therefore block cytokinesis.

Question 96

PDGF is encoded by a gene that can cause cancer when expressed inappropriately. Why do cancers not arise at wounds in which PDGF is released from platelets?

Answer 96

The on-demand, limited release of PDGF at a wound site triggers CD of neighboring cells for a limited amount of time, until the PDGF is degraded. This is diff fr the continuous release of PDGF from mutant cells, where PDGF is made in an uncontrolled way at high levels. Moreover, the mutant cells that make PDGF often express their own PDGF receptor inappropriately, so that they can stim their own prolif, thereby promoting the development of cancer

Question 97

What do you suppose happens in mutant cells that A. cannot degrade M-cyclin? B. always express high levels of p21? C. cannot phosphorylate Rb?

Answer 97

All three types of mutant cells would be unable to divide. The cells: A. would enter mitosis but would not be able to exit mitosis. B. would arrest permanently in G1 bc cyclin–Cdk complexes that act in G1 would be inactivated. * Recall: p53 is activated in response to DNA damage, wh activates xcr regulator that expresses p21, and p21 binds/inhibits cyclins in G1 to allow time to repair damage. C. would not be able to activate xcr of genes reqd for CD bc reqd xcr regulators would be constantly inhibited by unphos Rb.