18.2 Regulators of Cell Cycle Progression

Question 1

what were the 3 experiments that led to the discovery of maturation promotion factor (MPF)

Answer 1

1. frog oocyte
2. yeast genetics
3. sea urchin embryos

Question 2

summarize the findings of the frog oocyte experiment

Answer 2

frog oocytes could be induced to enter M phase by microinjection of cytoplasm from oocytes that have already been hormonally stimulated → a cytoplasmic factor present in hormonally stimulated oocytes was enough to trigger progression from G2 → M

Question 3

in the frog oocyte experiment, what was the factor that was found to induce progression from G2 to M phase

Answer 3

maturation promotion factor (MPF)

Question 4

what led to the conclusion that MPF is a general regulator of transition from G2 to M

Answer 4

MPF was found to be present in somatic cells, where it induces entry in to M phase

Question 5

summarize the findings of the yeast genetics experiment

Answer 5

temperature-sensitive cdc28 mutants were found to be arrested at the START regulatory point; cdc28 protein was required to progress past START

Question 6

cdc genes (cdc28 in S. cerevisiae, cdc2 in S. pombe) were found to encode (), which is a conserved cell cycle regulator in all eukaryotes

Answer 6

a protein kinase called Cdk1

Question 7

summarize the findings of the sea urchin embryo experiments

Answer 7

Hunt and colleagues identified 2 proteins (cyclin A and cyclin B) that accumulate throughout sea urchin embryo interphase but are rapidly degraded at the end of each mitosis → suggests that cyclins have a role in inducing mitosis

Question 8

further findings of sea urchin embryo experiment

Answer 8

• cyclin A was analogous to MPF
• MPF was composed of a regulatory cyclin B subunit and a catalytic Cdk1 subunit

Question 9

further findings of the sea urchin embryo experiment found that () was analogous to MPF

Answer 9

cyclin A

Question 10

further findings of the sea urchin embryo experiment found that MPF was composed of (1) and (2)

Answer 10

1. regulatory cyclin B subunit
2. catalytic Cdk1 subunit

Question 11

in association with Cdk1, drive G2 → M transition

Answer 11

mitotic B-type cyclins

Question 12

in assocation with Cdk1, control passage through START

Answer 12

G1 cyclins (Clns)

Question 13

other () - association with Cdk1 is required for progression through S phase

Answer 13

B-type cyclins

Question 14

cell cycles of higher eukaryotes are also controlled by multiple Cdk1-related protein kinases, known as ()

Answer 14

Cdks (cyclin-dependent kinases)

Question 15

activity of Cdks is regulated by binding of inhibitory proteins called ()

Answer 15

Cdk inhibitors (CKIs)

Question 16

in mammalian cells, there are 2 CKI families: ()

Answer 16

Ink4 and Cip/Kip

Question 17

provide a link between growth factor signaling and cell cycle progression

Answer 17

D-type cyclins

Question 18

explain how D-type cyclins link growth factor signaling and cell cycle progression

Answer 18

• growth factors can induce the synthesis of cyclin D through the Ras/Raf/MEK/ERK pathway
• D cyclins are rapidly degraded by APC/C ubiquitin ligase → in the absence of growth factors, D cyclins have low concentrations in the cell
• mutations in this mechanism can lead to cancer

Question 19

The () protein is a substrate protein of Cdk4,6/cyclin D comlpex

Answer 19

Rb

Question 20

The Rb protein is a substrate protein of ()

Answer 20

Cdk4,6/cyclin D complex

Question 21

the Rb protein is a prototype ()

Answer 21

tumor suppressor gene

Question 22

inactivation of () gene leads to tumor development

Answer 22

tumor suppressor

Question 23

proteins encoded by tumor suppressor genes act as breaks that slow down cell cycle progression via ()

Answer 23

regulation of transcription

Question 24

control of Rb by () couples the regulation of gene expression to the availability of growth factors in G1

Answer 24

Cdk4,6/cycD phosphorylation

Question 25

Rb activity is regulated by changes in phosphorylation → Rb is phosphorylated by Cdk4,6/cycD complexes as cells pass through the ()

Answer 25

G1 restriction point

Question 26

underphosphorylated Rb (present in G0 or early G1) binds to members of the () → regulates expression of genes involved in cell cycle progression

Answer 26

E2F family of transcription factors

Question 27

explain how the E2F family of transcription factors regulates expression of genes involved in cell cycle progression

Answer 27

• cyclin E is expressed by E2F transcription factors
• E2F binds to target sequence regardless of whether Rb is present or not

Question 28

Rb acts as a repressor → Rb/E2F complex suppresses transcription of ()

Answer 28

E2F-regulated genes

Question 29

how does Rb phosphorylation lead to activation of E2F-regulated genes

Answer 29

Rb phosphorylation leads to dissociation of Rb/E2F complexes; Rb no longer acts as a suppressor

Question 30

progression through G1 restriction point and entry into S phase is mediated by activation of ()

Answer 30

Cdk2/cyclin E complexes

Question 31

high levels of (1) and (2) drive progression through S and G2

Answer 31

1. cyclins
2. Cdk2 activity

Question 32

Cdk2/cycE activity is inhibited in G0 or early G1 by the CKI () (part of the Cip/Kip family)

Answer 32

p27

Question 33

Cdk2 inhibition by p27 is relieved by several mechanisms as cells progress through G1:

Answer 33

• growth factor signaling reduces both transcription and translation of p27
• Cdk2 activation brings about the complete degradation of p27 by phosphorylating it → p27 becomes a target for ubiquitylation
• positive autoregulation further activates Cdk2/cycE, which also phosphorylates and inactivates APC/C ubiquitin ligase → prevents cycE degradation

Question 34

Cdk2/cycE complexes initiate S phase by activating DNA synthesis at ()

Answer 34

replication origins

Question 35

it is important that once a segment of DNA has been replicated in the S phase, control mechanisms must ()

Answer 35

prevent the reinitiation of DNA replication until the cell cycle has been completed

Question 36

DNA replication is initiated by the activity of ()

Answer 36

MCM helicase proteins

Question 37

MCM helicase binds to () found at replication origins during G1

Answer 37

origin recognition complex (ORC) proteins

Question 38

MCM/ORC complex remains inactive as a () throughout G1 until it is activated by the action of Cdk2/cycE (as the cell enters the S phase)

Answer 38

pre-replication complex

Question 39

how does Cdk2/cycE activate the MCM/ORC complex

Answer 39

Cdk2/cycE phophorylates activating proteins that are recruited to the MCM/ORC complex

Question 40

APC/C inhibition (by Cdk2/cycE activation) leads to activation of () (protein kinase), which directly phosphorylates MCM

Answer 40

DDK

Question 41

() during S, G2, and M phases prevents MCM proteins from re-associating with replication origins

Answer 41

high activity of Cdks

Question 42

cell cycle arrest at DNA damage checkpoints is mediated by () (protein kinases) → recognize damaged DNAs

Answer 42

ATM and ATR

Question 43

ATR is activated by ()

Answer 43

single-stranded or unreplicated DNA

Question 44

ATM is activated by ()

Answer 44

double-strand breaks

Question 45

ATR and ATM phosphorylate and activate (), respectively

Answer 45

Chk1 and Chk2 (checkpoint kinases)

Question 46

Chk1/Chk2 activations both inhibit/induce degradation of () via phosphorylation

Answer 46

Cdc25

Question 47

Chk1/Chk2 activations both inhibit/induce degradation of Cdc25 via ()

Answer 47

phosphorylation

Question 48

Cdc25 inhibition leads to inhibition of (1) and (2)

Answer 48

1. Cdk2 (cell cycle arrest in G1 and S)
2. Cdk1 (cell cycle arrest in G2)

Question 49

in mammalian cells, arrest is also mediated by the (), which is phosphorylated by both ATM and Chk2

Answer 49

p53 transcription factor protein

Question 50

increased p53 levels lead to induction of CKI (), which inhibits complexes of Cdk2 with cycE or cycA

Answer 50

p21