Lecture 16

Question 1

what is triggered by cell cycle control system

Answer 1

The essential processes of the cell cycle—such as DNA replication, mitosis, and cytokinesis

Question 2

give an analogy for control of cell cycle

Answer 2

By analogy with a washing machine, the cell-cycle control system is shown here as a central arm— the controller—that rotates clockwise, triggering essential processes when it reaches specific points on the outer dial

Question 3

describe the controller

Answer 3

regulated cell cycle

Question 4

what happens when enter s

Answer 4

trigger dna rep machinery
replicate dna

Question 5

what happens when enter m

Answer 5

trigger mitosis machinery
assemble mitotic spindle

Question 6

what happens when exit m

Answer 6

trigger anaphase and process to cytokinesis
complete cell division

Question 7

what causes control system to arrest cycle at specific checkpoints

Answer 7

information about the
completion of cell-cycle
events, as well as signals
from the environment

Question 8

describe g1 checkpoint

Answer 8

is environment favourable

Question 9

describe g2 checkpoint

Answer 9

is environment favorable
is all dna replicated

Question 10

describe metaphase checkpoint

Answer 10

are all chromosomes attached to spindle

Question 11

what are the 2 key components of cell cycle control system

Answer 11

A complex of cyclin with Cdk acts as a protein kinase to trigger specific cell-cycle events. Without cyclin, Cdk is inactive
cyclins make kinases specific

Question 12

describe core of cell cycle control system

Answer 12

Cdk associates successively with different cyclins to trigger the different events of the cycle
Cdk activity is usually terminated by cyclin degradation
For simplicity, only the cyclins that act in S phase (S-cyclin) and M phase (M-cyclin) are shown, and they interact with a single Cdk; as indicated, the resulting cyclin–Cdk complexes are referred to as S-Cdk and M-Cdk, respectively

Question 13

how many cdks do humans and yeast have

Answer 13

humans = many cdks and many cyclins
yeast = one cdk and many cyclins

Question 14

name steps of cdk activation

Answer 14

inactive
partly active
fully active

Question 15

describe inactive cdk

Answer 15

In the inactive state, without cyclin bound, the active
site is blocked by a region of the protein called the T-loop
kinase needs adp and hydrolyzes atp and phosphorylates substrate

Question 16

describe partially active cdk

Answer 16

The binding of cyclin
causes the T-loop to move out of the active site, resulting in partial activation of the Cdk2
cyclin and causes loop to open up = changes and opens

Question 17

describe fully active cdk

Answer 17

Phosphorylation of Cdk2 (by CAK) at a threonine residue in the T-loop further activates the enzyme by changing the shape of the T-loop improving the ability of the enzyme to bind its protein substrates
CAK - now = active site fully exposed
now can phosphorylate

Question 18

how is cdk inhibited

Answer 18

by wee1 kinase adding inhibitory phosphate
The active cyclin–Cdk complex is turned off when the kinase Wee1 phosphorylates two closely spaced sites above the active site.

Question 19

what is cdc25

Answer 19

Removal of these phosphates by the phosphatase Cdc25 results in activation of the cyclin–Cdk complex
takes inhibitory phosphate away and makes active again

Question 20

describe inhibition of cylcin-cdk complex by cki

Answer 20

p27 binds to both the cyclin and Cdk in the complex, distorting the active
site of the Cdk. It also inserts into the ATP- binding site, further inhibiting the
enzyme activity
inhibits - clamps, binds the cyclin cdk complex with phosphate

Question 21

describe control of proteolysis by scf - gen

Answer 21

ck = phosphorylated then scf can bind - adds poly ub (with help of E1 and E2) = degradation of cki in proteasome

Question 22

describe control of proteolysis by scf - formal

Answer 22

The phosphorylation of a target protein, such as the CKI shown, allows the protein to be recognized by SCF, which is constitutively active. With the help of two additional proteins called E1 and E2, SCF serves as a ubiquitin ligase that transfers multiple ubiquitin molecules onto the CKI protein. The ubiquitylated CKI protein is then immediately recognized and degraded in a proteasome.

Question 22

describe control of proteolysis by apc - gen

Answer 22

apc activated by cdc20 and with help of E1 and E2 = adds polyub = degraded mcyclin

Question 22

describe control of proteolysis by apc - formal

Answer 22

M- cyclin ubiquitylation is performed by APC, which is activated in late mitosis by the addition of an activating subunit to the complex. Both SCF and APC contain binding sites that recognize specific amino acid sequences of the target protein.

Question 23

describe cell fusion experiment s and g1

Answer 23

The results show that S-phase cytoplasm contains factors that drive a G1 nucleus directly into DNA synthesis
makes G cells go to s phase, something must promote s phase

Question 24

describe cell fusion experiment s and g2

Answer 24

G2 nucleus, having already replicated its DNA, is refractory to these factors - stays in their phases

Question 25

describe cell fusion experiment g1 and g2

Answer 25

Fusion of a G2 cell with a G1 cell does not drive the G1 nucleus into DNA synthesis
cytoplasmic factors for DNA replication that were present in the S-phase cell disappear when the cell moves from S phase into G2

Question 26

describe initiation of dna rep once per cell cycle- formal

Answer 26

The ORC remains associated with a replication origin throughout the cell cycle
In early G1, Cdc6 associates with ORC
Aided by Cdc6, Mcm ring complexes then assemble on the adjacent DNA = resulting in the formation of the pre-replicative
complex
The S-Cdk (with assistance from another protein kinase, not shown) then triggers origin firing, assembling DNA polymerase and other replication proteins and activating the Mcm protein rings to migrate along DNA strands as DNA
helicases
The S-Cdk also blocks rereplication by causing the dissociation of Cdc6 from origins, its degradation, and the export of all excess Mcm out of the nucleus.
Cdc6 and Mcm cannot return to reset an ORC-containing origin for another round of DNA replication until M-Cdk has been inactivated at the end of mitosis

Question 27

describe initiation of dna rep once per cell cycle- gen

Answer 27

origin replication complex bonds to origin of rep - many origins, dna replicated from many points
cdc6 binds to orc
mcm also binds and pre replicative complex - pre rc
scdk triggers s phase - triggers replication fork
degradation of phosphorylated cdc6 degraded, assembling only goes after s cdk does this
orc phosphorylated, moves along and triggers rep fork
helicases that unwind dna and dna pol to make new dna= now moves and starts rep fork and continues till next fork
orc stays phosphorylated = nothing can happen during g2
keep activity of cyclins up so complex stays = resistant to new rep - nothing cna happen

Question 28

describe activation of mcdk - gen

Answer 28

mcyclin= partially active but need cdk activating kinase
mcyclin binds cdk1, cdk activating kinase adds phosphate, and wee1 inhibitory kinase adds inhibitory phosphate
but active cdc25 = takes away inhibitory phosphate = active mcdk
inactive cdc25 = phosphatase, activated by positive feedback of active mcdk
active mcdk inhibits wee1 = cdk inhibitory kinase

Question 29

describe activation of mcdk -formal

Answer 29

Cdk1 associates with M-cyclin as the levels of M-cyclin gradually rise
The resulting M-Cdk complex is phosphorylated on an activating site by the Cdk-activating kinase (CAK) and on a pair of inhibitory sites by the Wee1 kinase
The resulting inactive M- Cdk complex is then activated at the end of G2 by the phosphatase Cdc25
Cdc25 is stimulated in part by Polo kinase, which is not shown for simplicity
Cdc25 is further stimulated by active M-Cdk, resulting in positive feedback = feedback is enhanced by the ability of M-Cdk to inhibit Wee1.

Question 30

describe levels of types of cyclins during cell cycle

Answer 30

The concentrations of the three major cyclin types oscillate during the cell cycle while the concentrations of Cdks (not shown) do not change and exceed cyclin amounts
In late G1, rising G1/s-­‐cyclin levels lead to the formation of G1/S-­‐Cdk complexes that trigger progression through the start transition. S-­‐Cdk complexes form at the start of S phase and trigger DNA replication, as well as some early mitotic events
M-­‐Cdk complexes form during G2 but are held in an inactive state; they are activated at the end of G2 and trigger entry into mitosis at the G2/M transition. A separate regulatory protein complex, the ApC/C (degrades m cyclin), initiates the metaphase-­‐to-­‐anaphase transition (no more cdk activity here)

Question 31

are the checkpoint needed for cell cycle

Answer 31

noooooooooo
not needed to complete cell cycle
it would just be bad since there would be issues with it

Question 32

job of mcdk and scdk

Answer 32

mcdk=trigger mitosis machinery
scdk = trigger dna rep machinery