Exam 4 Lecture 2: Cell Cycle II

Question 1

Draw the mitotic spindle to explain what the following structures do:
Astral Microtubules

Kinetochore Microtubules

Interpolar Microtubules

Answer 1

ALL microtubule minus ends originate within the centrosomes

Astral microtubules: grow away from pole and spindle itself, anchoring spindle to cell membrane

Kinetochore microtubules: bind to sister chromatid DNA

Interpolar microtubules: overlap with each other in the middle, allowing for attachment points for motors

Question 2

What is the focus of the spindle pole?

AKA what structure is at the poles?

Answer 2

The centrosome is the focus of the spindle pores

ALL microtubule minus ends emenate emenate from this structure

Within a centrosome, you have two centrioles

Question 3

Centriole Replication:
Centriole Replication occurs during ____ phase

The two centrioles seperate, and then duplicate during ___ phase

At the beginning of ___ phase the complex splits in two and the two pairs seperate

The microtubules nucleate on the mother’s only to form an ____

Answer 3

Centriole Replication occurs during G1 phase

They then seperate.

Each mother centriole replicates to form a daughter during S phase (now you have four)

Then at the beginning of M phase, the complex splits in two and the two pairs seperate

The microtubules nucleate on the mother’s only to form an aster.

Question 4

Microtubule Motors Organize the Spindle:

Explain the role of dyenin and kinesin

Answer 4

Motor proteins kinesin and dynein organize tubules into a bipolar array, and centrosomes form spindle poles.

The kinesins walk towards plus end, pull apart stuff overlapping in the middle

Dynenin walk towards minus end, help pull the entrie thing apart… dynenin also bound to cell membrane

Question 5

Explain how the chromosomes segregate in Anaphase A and Anaphase B

Answer 5

Anaphase A: where the sister chromatids seperate, drivien by microtubule depolymerization and flux

Anaphase B: drags spindle pulls further away to make adequate room for cleavage furrow, this is driven by motors (dynenin specifically)

Question 6

Motor proteins ____ and ____ organize the spindle by cross-linking microtubules and generating force that focuses the poles:

_______ do antiparallel cross-linking

________ do outward push

Answer 6

Motor proteins kinesin and dyenin organize the spindle by cross-linking microtubules and generating force that focuses the poles.

Antiparallel cross-linking by kinesin-5

Outward push by kinesin-4,10

Question 7

The ____ is the structural attachment point for the microtubules

Answer 7

The kinetochore is a structural attachment point for the microtubules

The kinetochore microtubules connect to the kinetochore that is on the centromere region of the chromosome

Question 8

Explain centrosome vs centromere

Answer 8

Centrosome: two centrioles, anchors microtubules at the poles to form the spindle

Centromere: heterochromatin region on sister chromatids that serves as the anchor site for kinetochore MTs

Question 9

Explain how the chromosomes even become attached to the microtubules

Answer 9

In late prophase, the nuclear envelope starts to dissapear, and then in early prometaphase the kinetochore MTs laterally attach to the chromosomes

The lateral attachment and movement is rapidly converted to an end-on attachment so that by metaphase they are all lined up appropriately.

Question 10

The microtubules attach to the correct side of the kinetochore by ______

Increased _____ solidifies the attachment, while decreased _____ destabilizes it.

The most stable attachment is when ____

Answer 10

Microtubules attach to the correct side of the kinetochore by trial and error.

Increased tension solidifies the attachment, while decreased tension destabilizes it.

The most stable attachment is when they are being pulled in opposite directions.

Question 11

What force stabilizes microtubule attachment to the sister chromatids?

Answer 11

TENSION stabilizes MT attachment into the chromosomes.

Question 12

What best describes a centromere?

Answer 12

Centromere: heterochromatin that holds sister chromatids together, anchor site for kinetochore MTs

Question 13

What are the three major forces driving chromosome positioning?

Answer 13

Three Major Forces driving Chromosome Positioning:

1. Depolymerization of tubulin at the plus and and ratcheting of the kinetochore attachment
2. Microtubule flux (treadmilling)
3. Polar ejection force

Question 14

Explain the depolymerizaton of microtubules near the kinetochore

Answer 14

Depolymerization of microtubules at the kinetochore tends to move the chromosomes towards the spindle (away from the middle)

LOOK at the picture

Question 15

Explain “microtubule flux”

Depolymerization near the ____ is balanced by polymerization near the ____, resulting in treadmilling.

Answer 15

Microtubule Flux:

Depolymerization near the spindle is balanced by polymerization near the kinetochore, resulting in treadmilling of the microtubule

Question 16

Explain “polar ejection force”:

______ motors attached to the arms of the chromatids push the chromosomes away from the poles

Answer 16

Kinesin motors attached to the arms of sister chromatids push the chromosomes away from the poles

The Kinesins walk down an interpolar microtubule to bring the chromosome toward the middle plate

This force pushes chromosomes AWAY from the spindle pole

Question 17

Cytokinesis:

_____ & _____ make up the contractile ring

Answer 17

ACTIN AND MYOSIN make up the contractile ring

Myosin sits directly ontop of the actin

Question 18

What is the midbody?

Answer 18

Midbody is the bridge between the two cells pulling apart during cytokinesis

Question 19

_____ double ring forms at the midbody during abcission

Answer 19

Septin double ring forms at the midbody during abcission

Question 20

The contractile ring is regulated by ______
The active form of it (_______) binds to ____, which promote the assembly of actin filaments.

It also binds to _____, which phosphorylate the myosin light chain, thereby activating myosin.

Answer 20

The contractile ring is regulated by GTPase RhoA

The active GTP bound form of RhoA binds to formins, and promotes the assembly of the actin filaments.

It also binds to Rho-activated kinases, which result in the phosphorylation of the myosin light chains, thereby activating myosin.

Question 21

Explain the RhoA activation cascade:

In order to get the contractile ring to happen, which thing is most important to localize?

Answer 21

Localize the Rho GEF at the metaphase plate

Question 22

How does the cleavage furrow know where to form?

Name the three possible models of contractile ring positioning

Answer 22

Three models for how the cell knows where to position contractile ring:

1. Astral stimulation model (astral MTs carry furrow inducing signals to cortex)
2. Central Spindle Stimulation Model (the spindle midzone generates cleavage inducing signals)
3. Astral Relaxation (signal is strong at poles, weak at equator)

Question 23

Mitosis vs Meiosis:

Mitosis occurs all the time in _____ cells, replacing cells that die and allowing growing and remodeling of tissues. Daughter cells are identical, and are ____ (full chromosomal complement, 2 copies of each chromosome)

Meiosis only occurs in ___ cells, and produces ____ These only have one copy of each chromosome

Answer 23

Mitosis: occurs in somatic cells all the time, daughter cells are identical to parent and are diploid (full chromosomal complement - 2 copies of each chromosome)

Meiosis: only occurs in germ cells, and produces haploid gamates

Question 24

When does recombination occur?

What are the two advantageous things about recombination?

Answer 24

Recombination occyrs during Meiosis 1 (metaphase)

Two advantageous things about recomb:

1. ability to get genetic variability
2. acts as a crosslink to keep the chromosome pair together so that the cell can divide correctly

Question 25

For asymmetric cell division, the cell components have to be _______

Answer 25

For asymmetric cell division, the cell components have to be segregated.

Question 26

What is the term for when multiple nuclei share the same cytoplasm, aka it is “mitosis without cytokenesis”?

Answer 26

Syncytium: when multiple nuclei share the same cytoplasm

Question 27

What proteins stimulate the start of the cell cycle?

Answer 27

Mitogens bind to receptors that activate a signal transduction cascade, ending in expression of genes for growth and division.

These division stimulating signals are balanced by inhibitory regulators so that cell division doesn’t get out of control.

Question 28

DNA Damage Arrests the Cell Cycle:

Activation of kinases results in activation of ___, a regulatory protein that promotes the expression of ____ (which promites inhibits Cdk’s).

If ____ doesn’t work cells will continue to divide despite DNA damage.

Answer 28

Activation of kinases results in activation of p53, a regulatory protein that regulates the expression of p21, which inhibits Cdk’s.

If p53 doesn’t work, cells will divide anyway despite DNA damage.

Question 29

What is the name for the gene that inhibits muscle cell growth?

Answer 29

Myostatin is the gene that inhibits muscle cell growh

That giant muscular bull had a mutation in his myostatin gene

Question 30

The crossover point of recombination is called ____

Answer 30

Crossover point of recombination: chiasma

Question 31

Which meiosis looks more like mitosis?

Answer 31

Meiosis II looks like mitosis