Chapter 8 Mitosis and the Cell Cycle

how long does the cell cycle take for mamallian liver cells?

more than one year

two sections of cell division

the miotic phase and interphase

cells spend more time in

interphase

Subsections of interphase

G0, G1, S, and G2

G0 phase

an offshoot from the cell cycle after division

-cells n this phase can choose not to divide but can also reenter the cell cycle

which types of cells remain in G0 and do not re-enter the cell cycle?

neurons

G1 Phase

the first growth phase

S phase

The synthesis phase of the cell cycle; the portion of interphase during which DNA is replicated.

G2 phase

The second growth phase of the cell cycle, consisting of the portion of interphase after DNA synthesis occurs.

-serves as a verification step to check everything was duplicated properly

M phase

when the cell divides

Where are the 3 checkpoints in the cell cycle?

G1, G2, M

What can G1 and G2 checkpoint do?

check if the cell is large enough (only G1)

-if environment is right

-if there is any damage to the DNA

Heterokaryon

formed by fusing a M phase cell with an interphase cell

-DNA from interphase begins to condense, meaning that the M phase is capable of inducing condensation

-used to be called the MPF

MPF

a miner of cyclin and cyclin dependent Kinase

-CDK is a serine +a theronine so it can phosphorylate either on other proteins

how does active CDK control the cell cycle

by phosphorylating other proteins

why are they called cyclins

because the cyclin part of the dimer goes through a cycle of synthesis and degradation

-every new cell gets new cyclins

the core of the cell cycle control system (CDK)

1. cyclins are made and degraded by proteosomes cyclically

2.CDK protein levels remain constant throughout the cycle

3.Presence/absence of cyclin determines whether or not CDK becomes activated (cyclin is needed to phosphorylate things)

G1 cyclins

Cyclin D (CDK4 or 6)

also Cyclin E

cyclin E

control the G1/S checkpoints and controls movement to the S phase

S cyclins

also Cyclin E and Cyclin A (CDK2)

M cyclins

Only Cyclin A using CDK1

Cyclin A

control the G2/M checkpoint. They remain active until destroyed by APC

Cyclin B

Prevalent during M phase

-drives nuclear envelope breakdown and condensation of chromosomes

cyclin expression cycle

Varying levels of different cyclins in each stage

Activity of CDKs APC/C in each phase

the values of each cyclin will change, but the overall amount of cyclin is constant, specific cyclins only around when they have to be

what controls the signaling cascade that determines proliferation or apoptosis?

CDK2 and CDK4 (E and D)

step one of the cell cycle

the cell is in G1 phase, DNA prereplication complexes begin to assemble (multiple per chromosome)

-DNA replication will start from these places

step 2 of the cell cycle

G1 cyclin CDK inactivates CDH1

step 3 cell cycle

G1 cyclin CDK activates the S phase but they remain inactive until activation of a nearby inhibitor by cyclin CDK

step 4 of the cell cycle

G1 cyclin CDK phosphorylate the inhibitor, activating the S phase cyclins

-also signals for ubiquinone to be added by SCF ub ligase

step 5 cell cycle

once SCF ubiquinates the S phase inhibitor, it gets degraded by the proteasome

step 6 cell cycle

the cell enters S phase once SCF is degraded

-the cell will then progress to G2 after DNA replication

step 7 cell cycle

in G2, miotic Clyclin CDKs are activated, the cell enters mitosis and during metaphase, all chromosomes line up at the center of the cell

step 8 cell cycle

during metaphase, chromosomes attach to the microtubules

-when anaphase begins, APC-CDC20 activates to degrade the securin holding the chromosomes together. They separate and move to the cell poles

step 9

after anaphase, another proteosome will degrade any M cyclins left over and the cell will proceed to telophase and into cytokinesis

RB

retinoblastoma

-connects to transcription factor E2F and when activated by Cyclin D CDK, it phosphorylates RB leading to E2F release, which can bind to genes and determine gene expression

if a mutation occurs in the RB protein, what can happen

a tumor may develop in the eye and become cancerous

Retinoblastoma is always diagnosed before the age of 3 because

a child with this mutation cannot make RB proteins

Does RB mutation halt the cell cycle?

no because regardless, E2F is still being made however cell division will become uncontrolled

which transcription factor is always active?

E2F

S phase cyclin are only needed during S phase but can begin to accumulate before. Inactivation of this is done by

Sic1

how does Sic1 become removed for S phase cyclin activation

it gets phosphorylated by G1 cylins and becomes a target for ubiquitone and get degraded

-once degraded, S cyclins are activated

once an active S phase cyclin is present

it phosphorylates several components causing conformational changes, allowing for proteins to detach from the replication orgin which is what must occur for DNA to unwind

-phosphates are added back after, directly onto the components allowing for DNA replication to begin

unreplicated DNA checkpoint

before G2; prevents mitosis before completion of synthesis

spindle assembly checkpoint

check for chromosome attachment to spindle before mitosis

-if not, MAD2 prevents separation of the chromosomes until ready

segregation checkpoint

occurs during mitosis before telophase

-if the chromosome is not at the poles, CdC14 prevents movement to telophase

if any issues occur at the checkpoints,

P53 comes in and halts cell replication to prevent further damage

how is P53 important in cancer?

stopping damaged DNA from being replicated stops replication of cancer cells

the four stages of mitosis

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

Summary of interphase

chromosomes are duplicated, cohesion via cohesins and centrosomes are duplicated

-during G2, DNA duplication occurs

-during S phase, chromosomes are replicated

prophase

spindle poles migrate to the poles of the cell

-chromosome condensation via condensin 1 and 2 begins

-some cohesin is removed, some keep the chromatids together until anaphase

-the kinectochore assembles on the centromere

where is the location for kinetochore assembly and where sister chromatids sit when conjoined?

the centromere

-acts as the attachment point for microtubules during cell division

prometaphase

chromosomes are captured by microtubules

-the nuclear envelope begins to break down

metaphase

chromosomes attached to their spindles are lined up at the metaphase plateq

anaphase

chromatids are separated and pulled to opposite poles via shortening of the kinetochore tubules

process for separating chromatids during anaphase

APC/C ubiquirin ligase degrades securin and activates the separase protease

-activated separase eats the rest of the cohesin and the chromatids can separate freely

telophase

the nuclear envelope begins to reform

-chromosomes decondense and the mitotic spindle disappears

cytokinesis

divides the cytoplasm into two separate cells

cytokinesis in animal cells is done via

cleavage furrow by the contractile ring in actin remodeling

cytokinesis in plant cells is done via

a cell plate

mitotic apparatus

The collective term for all the spindle fibers that form during mitosis

the mitotic apparatus is made up of

the spindle and astral microtubules

three types of microtubules

astral, polar, kinetochore

kinectochore microtubules

Attach to the kinectochore on the centromere of each chromosome

(direct interaction)

chromosome biorientation

where kinectochores attach to microtubules on the opposite spindles during mitosis in order to ensure equal separation of chromatids

if too many chromatids were attached to a spindle

a cell could end up with an extra chromosome or without one

ex: down syndrome

polar microtubules

interact with other polar microtubules to push poles apart

astral microtubules

form tufts at the end of the mitotic apparatus to hold spindle poles in place

what are the two ways chromosomes are captured in prometaphase?

end capture or side capture

end capture

when a microtubule by chance comes into contact with the chromosome as it grows, it can capture it

-very random, does not always occur

side capture

if the microtubule does not make it to the kinetochore region, kinetochore proteins can interact to bring them to the microtubule

chromosome congression

alignment of chromosomes at metaphase plate

steps to chromosome congression

1. kinetochores attach to the microtubule using end or side capture

2. if end, the chromosome is drawn to the spindle pole by a dynein-dynactin motor protein complex (walks to the - end)

3. the microtubule on the opposite pole picks up the other chromatid, allowing for bi-orientation

congression

-if bi-oriented chromosomes are attached to several microtubules, they locate to a central point between them

following congression, chromosomes experience

a "tug of war" until all but one kinetochore tubule shorten (oscillation)

on the shortening sides of a oscillated chromatid,

kinesin 13 stimulates disassembly at the + end

the dynein-dynactin motor moves the chromosome

toward the spindle pole (-)

on the lengthening side of the oscillated chromatid

kinesin 7 maintains its connection and grows the tubule

NCD80

a sleeve like protein complex form around the KMT (kinetochore microtubule) and attach to the kinetochore of the chromosome

how many NCD80 per chromosome

2

the site of dynein or kinesin motor cargo binding

Ncd80

mechanism of Ncd80

it is pulled by the dynein dynactin motor while the MT depolymerizes behind to make space for chromosomal movement

-also connects the microtubule to the chromosome

CEN-PA

a special histone exclusively found at the kinetochore region of chromosomes

-marks the kinetochore for the mitotic apparatus

-associated with the inner kinetochore

CPC

chromosomal passenger complex

-contains aurora B kinase

Aurora B kinase

phosphorylates Ndc80 at the kinetochore in the presence of low tension, releasing these attachments

when biorientation occurs, kinetochore regions bind to opposing microtubules, causing tension and a change in confirmation, causing pulling away from the CPC. Because of this:

Aurora B can no longer phosphorylate those proteins, resulting in a strong attachment of ncd80 and the microtubule

-this tension increases stabillity

if chromosomes are organized wrong

weak association and Aurora B kills Ncd80

cohesin is degraded by

separase

securin is an inhibitor of ______. Securin is degraded by

separase, anaphase ubiquitin ligase and APC/C and cdc20

once securin is degraded, what is activated

separase

separase degrades the _____ holding chromatids together

Cohesin

Anaphase A and anaphase b occur

simultaneously

the force from anaphase A comes from

the kinetochore tubules interacting with the chromosomes

the force from anaphase B comes from

the polar microtubules interacting with eachother at the spindle

anaphase A

rapid shrinking of the microtubules at the + end, leading to depolymerization

-bc of this, the tubules shrink towards the spindle pole and the sister chromatids are pulled there as well

anaphase B

polar microtubules push against eachother causing the poles to move apart, resulting in the elongation of the spindle

-astral tubules help as well

kinesin motors

attached to 2 different polar microtubules and the heads walk toward the +

-this movement causes a pushing in the opposite direction, allowing for polymerization of the new MT as they stretch

spindle elongation mechanism in anaphase b

using polar and astral mt aided by kinesins and dynein's

polar MT in spindle elongation

in the overlapping regions of polar MT, kinesins are present, and the heads walk toward the + end of the mT

-this causes pushing and elongation

astral MT in spindle elongation

they interact with dynein's bound to the plasma membrane

-their heads walk to the - end and pull, causing elongation