LECTURE 18 - THE CELL CYCLE

define the cell cycle

the orderly sequence of events by which a cell duplicates itself

how is the cell cycle linked to cancer

if the cell cycle becomes uncontrolled, then cancer results

what are the 5 parts of the cell cycle

Gap 0

Gap 1

Synthesis

Gap 2

Mitosis

describe Gap 0

it is when the cell has left the cycle and stopped dividing

what phase are most body cells in

Gap 0

describe Gap 1

the cell increases in size

checkpoint ensures everything is ready for synthesis

describe Synthesis

DNA replication occurs

describe Gap 2

control mechanisms ensure everything is ready to enter mitosis

describe Mitosis

cell growth stops at this stage

cellular energy is focused on division

a checkpoint in metaphase ensures the cell is ready to complete division

describe flow cytometry

a fluorescent DNA intercalating dye is used to visualise cells

describe BrdU staining

thymidine analogue is incorporated into DNA during synthesis, and this can be visualised with immunofluorescent staining

explain the G1/S checkpoints

checks:

• is the environment favourable

• cell size, nutrients, DNA damage, growth factors

explain the Intra-S checkpoint

checks:

• DNA damage

• DNA replication

explain the G2/M checkpoint

checks:

• is all DNA replicated

• is the environment favourable

• are all chromosomes attached to the spindle

what does CDK stand for

cyclin dependent kinase

describe a CDK

a serine threonine kinase found in all eukaryotes

what is a cyclin

the regulatory subunit of a CDK

list which cyclin & CDK is expressed in each cell cycle phase

G1 → CDK 4 - cyclin D

G1/S → CDK 2 - cyclin E

S → CDK 2 - cyclin A

G2 → CDK 2 - cyclin A

M → CDK 1 - cyclin B

what does each cyclin letter stand for

D - deciding/dithering

E - executing

A - actioning

B - bisecting

why are cyclin short lived

because they are quickly degraded once a cell has progressed through a stage they function in

what genes induce cyclin D expression

c-fos

c-myc

c-jun

explain CDK regulation in early G1

• CDK 4- cyclin D monophosphorylates retinoblastoma (Rb)

• activated Rb then prevents E2F dependent recruitment of RNA pol II to promoter of G1/S genes

what is E2F

a transcriptional inhibitor

explain CDK regulation in G1/S

• CDK 2 - cyclin E is stimulated if sufficient nutrients available

• CDK 2 - cyclin E hyperphopshorylates Rb, inactivating it

• Rb dissociates from E2F

• free E2F recruits RNA pol II to promoters

explain CDK regulation in S-phase

• pre-replication complex can form on DNA replication sites in G1

• DNA replication will only occur in S phase

• CDK 2 - cyclin A dependent phosphorylation of the pre-DNA replication complex will initiate S phase

explain what happens if something goes wrong in S phase

• checkpoint kinases Chk 1 and Chk 2 are activated

• Chk 1 and 2 phosphorylate p53, causing it to dissociate from Mdm2

• p53 activates the expression of CDK inhibitor p21

• p21 binds to and inhibits CDK 2 - cyclin E and CDK 2 - cyclin A

• this inactivates G1, S etc

• cell cycle progression halts

• Cdc25 is also phosphorylated by Chk 1 and 2, inhibiting it. Therefore wee kinase I left to phosphorylate and inhibit CDK 1 so mitosis cannot be activated

how does p53 act in absence of replicative stress

it interacts with Mdm2 and is targeted for degradation

explain regulation of G2/M

CDK 1 - cyclin B is needed for mitotic spindle formation

wee kinase I phosphorylates and inactivates CDK 1 - cyclin B → inhibits mitosis

Cdc25 phosphatase removes gamma P from CDK 1, activating CDK 1 - cyclin B → stimulates mitosis

what makes up mitotic spindles

astral microtubules

kinetochore microtubules

interpolar microtubules

describe astral microtubules

radiate away from poles to contact the cell cortex to position the spindle

describe kinetochore microtubules

attach sister chromatid pairs to the kinetochore located at the centromere

describe interpolar microtubules

radiate towards the spindle and aid in chromosome corregation

what does spindle assembly and function depend on

kinesin and dynein motor proteins

describe kinesin 5

function at midzone

moves - to +

pushes poles apart

describe kinesin 14

moves from + to -

pulls poles together

describe kinesins 4 and 10

move from - to +

push chromatid from poles to midzone

describe dynein

moves from + to -

pulls pole to cell cortex

what must be active for cells to progress through G2/M

Cdc25

what are the 2 major outcomes of CDK 1 - cyclin B activation

anaphase promoting complex activation (APC)

mitotic spindle formation

what is separase

a protease bound to securin

what is the APC

a ubiquitin ligase

explain the APC function

it polyubiquitinates securin, targeting it for degradation

as securin is degraded, separase is freed

what is the role of separase

to cleave the cohesin complex holding sister chromatids together

explain what happens in cytokinesis

contractile ring of actin and myosin II at cleavage furrow forms

actin and mysoin II generate force to separate cell into 2