Chapter 17: The Cell Cycle

M phase

mitosis (nuclear division)

G₁ phase (gap 1)

interval between mitosis and DNA replication

S phase

DNA replication phase

G₂ phase (gap 2)

mitosis preparation (cell growth, protein synthesis)

cell cycle phases missing in early embryos

growth phases (G₁ and G₂)

major checkpoint between G₁ phase and S phase

START

external regulation signals of START checkpoint

nutrient availability, mating factors, cell size

result of nutrient shortage at START checkpoint

the cell cycle is arrested and the cell enters a resting phase

checkpoint in the late G₁ phase

restriction point

regulation signals of the restriction point

extracellular growth factors

result of a lack of growth factors at the restriction point

the cell enters the isolated G₀ resting stage

PDGF (platelet-derived growth factor)

stimulates skin fibroblasts out of the G₀ stage

function of cell cycle checkpoints

checking for DNA damage

checkpoint at metaphase

spindle assembly checkpoint

function of the spindle assembly checkpoint

stops mitosis at metaphase if the chromosomes are not properly aligned on the spindle

cell cycle phase at which vertebrate oocytes remain arrested for long periods of time

G₂ phase

triggers vertebrate oocyte progression to the M phase

hormonal stimulation

structure of MPF

Cdk1 + cyclin B

hormone responsible for progression from G₂ phase to the M phase

maturation promoting factor (MPF)

protein kinase that regulates the cell cycle and is conserved/recycled

Cdk1 (cyclin-dependent kinase)

protein that regulates Cdk1 activity and is degraded at the end of mitosis

cyclin B

phase in which cyclin B is synthesized and complexed with Cdk1

G₂ phase

result of Cdk1 phosphorylation

Cdk1 is inactivated

result of Cdk1 de-phosphorylation

Cdk1 is activated

Cdk inhibitors

Ink4 and Cip/Kip

phase regulated by Cdk4,6/CycD

G₁ phase to S phase (through the restriction point)

function of Cdk2/CycE

initiates DNA replication in the S phase

phase regulated by Cdk2/CycA

S phase to G₂ phase

molecules that produce Cyclin D

growth factors

formation of the pre-replication complex

MCM hexamers bind to ORC (origin recognition complex)

result of MCM activation

the two DNA strands are pulled apart as replication templates

DNA damage checkpoint for double-strand breaks

ATM

DNA damage checkpoint for single-strand breaks or unreplicated DNA

ATR

checkpoint kinase activated by ATM

Chk2

checkpoint kinase activated by ATR

Chk1

result of Cdk2 inhibition

cell cycle arrest in G₁ and S phases

result of Cdk1 inhibition

cell cycle arrest in G₂

the transcription factor that mediates the G₁ checkpoint in mammalian cells

p53

result of p53 activation

activation of Cdk inhibitor p21, leading to cell cycle arrest

mitosis stage in which chromosomes condense and the mitotic spindle forms

prophase

mitosis stage in which spindle microtubules attach to the kinetochore

prometaphase

mitosis stage in which chromosomes shuffle back and forth to align on the metaphase plate

metaphase

mitosis stage in which the sister chromatids separate and move to opposite poles

anaphase

mitosis stage in which nuclei re-form and the chromosomes decondense

telophase

cell division

cytokinesis

molecules that are activated in a positive-feedback loop to signal entry into the M phase

Cdk1, Aurora, and Polo-like kinases (the mitotic protein kinases)

functions of the mitotic protein kinases

chromatin condensation, nuclear envelope breakdown, fragmentation of Golgi, spindle formation

function of cohesins

maintain links between sister chromatids

function of condensin

links sister chromatids at the centromere only

break down of the nuclear envelope

nuclear membranes fragment → nuclear pore complexes dissociate → nuclear lamina depolymerizes

function of microtubule shrinkage

chromosomes are pulled towards the poles

function of APC/C ubiquitin ligase

regulation of progression to anaphase at the spindle assembly checkpointq

function of the mitotic checkpoint complex (MCC)

inhibits APC/C while kinetochores are unattached

function of separase

degrades cohesin to break the link between sister chromatids

the process that is triggered by inactivation of Cdk1

cytokinesis

contractile ring

structure that cleaves the cell, formed by actin and myosin II filaments