The Cell Cycle and Mitosis

The cells cycle begins when…

two new cells are formed by division of a parent cell

The cell cycle ends when…

a cell divides

M phase

when the cells actually divide, first the nucleus then the cytoplasm

G1 phase

major decision is made, whether a cell will divide again

G0

cells waiting for a signal to divide

Cells that exit the cell cycle are…

said to undergo terminal differentiation

S Phase

• DNA is replicated, cell is now 4N

• centrosomes separate and each grows a new centriole

• cell is committed to either divide or die

G2 phase

• less variable than G1

• last chance for cell to opt out of mitosis → DNA is checked for damage

• DNA begins to condense into familiar pair of sister chromatids for each chromosome

When in G2, what is the only way out of mitosis?

apoptosis

Prophase

• individual chromosomes become visible

• nuclear envelope disappears

• MRs forming mitotic spindle

Interphase

The centrosomes (MTOCs) begin to migrate away from each other

• as they move, they act as nucleation sites for MRs destined to form the mitotic spindle

• aster forms near each centriole

aster

dense starburts of MTs

What is prometaphase marked by?

• the fragmentation of the membranes of the nuclear envelope

• centrosomes complete their movement to opposite sides of nucleus

• spindle MTs contact the condensed chromosomes

• MRs attach to chromosomes at the centromere region (kinetochores)

Metaphase

• chromosomes align at metaphase plate

• agents that interfere with spindle function are used to arrest cells

• the sister chromatids of each chromosome are being actively tugged toward opposite poles

Anaphase

• shortest phase

• in A → chromosomes are pulled toward spindle poles as kinetochore MTs get shorter

• in B → spindle poles themselves move away from each other as polar MRs lengthen

Telophase

• the daughter chromosomes arrive at poles of the spindle

• chromosomes uncoil into interphase chromatine

• nucleoli reappear + nuclear envelopes re-form

• cytokinesis also takes place

shortly after S phase, what happens to centromeres?

kinetochore proteins begin to assemble on them

During prometaphase, + ends of spindle MTs bind…

with the kinetochores associated with each chromatid

Forces exerted by these kinetochore microtubules gradually…

move chromosomes toward the center of the cell

The inner kinetochore contains…

proteins that bind to centromeric DNA

The outer kinetochore contains what?

Proteins that attach to the plus ends of microtubules

fibrous corona

becomes visible when a kinetochore is not attached

the microtubule-containing apparatus responsible for…

separation of chromatids into daughter cells is the mitotic spindle

Kinetochore microtubules

connect the kinetochores of sister chromatids to the centrosomes

polar microtubules

interact with microtubules from the opposite pole of the cell

Astral microtubules

are shorter and from the asters at each pole

• some interact with proteins lining the plasma membrane

Anaphase A

sister chromatids separate at the centromere, but all microtubule associations remain

Anaphase B

polar microtubules dissociate from each other, but kinetochore MTs stay attached

What is driven by kinesins (catastrophins)?

chromosomes are moved, kinetochores first, toward the spindle poles during anaphase A

One kinesin-like motor is at the plus end of the MT embedded in the…

kinetochore, and moves the chromosome as it “chews up” the MT

One kinesin-like motor is located at the minus end of the MTs, embedded in the…

spindle pole, and induces depolymerization, “reeling in” the microtubules and the attached chromosomes

Bipolar kinesin motors

they bind to overlapping polar MTs from opposite spindle poles, forcing the spindle poles away from each other

Cytoplasmic dynein is associated with…

astral microtubules that are connected to the cell cortex

cell cortex

layer of actin microfilaments lining the inner surface of the plasma membrane

The dynein moves toward the minus ends of the…

microtubules and appears to move the spindle toward the cortex

After chromosomes have separated…

cytokinesis divides the cytoplasm in two

When does cytokinesis start?

late anaphase or early telophase

Certain cell types undergo many rounds of nuclear division without…

cytokinesis, forming a syncytium (multinucleate cell)

what initiates cleavage?

the cleavage furrow, it continues to deepen until opposite surfaces make contact and split the cell in two

What does the furrow pass through?

spindle equator

Cleavage depends on a bundle of actin microfilaments…

(the contractile ring) that forms just below the plasma membrane in early anaphase

As cytokinesis ends, the connection between the two cells narrows to a thin stalk…

(midbody), which persists for some time until abscission, the final separation of the two daughter cells