Chapter 14 and 15: Mitosis and Cell Cycle

Question 1

What are the forces in Anaphase B that move sister chromatids and poles further apart?

Answer 1

B) Dyneins attached to the plasma membrane pull the poles apart, forcing the sister chromatids, attached to their respective poles, further apart.

C) A combination of shortening kinetochores fibers and motor proteins cause the sister chromatids to move further apart and closer to their respective poles, which do not change

Question 2

How does centrosome reproduce?

Answer 2

Centrosomes reproduce during the S phase about the time DNA is replicated.

Cdk2/cyclin E takes it out and cyclin A act as regulators of centrosome reproduction and are active during S phase when both DNA and centrosomes are replicated. Procentrioles form next to the mother and daughter centrioles from the pericentriolar material

Question 3

Cohesins help the cell remember the gene expression pattern that existed before the cell divided by marking transcription factor binding, helping the transcription factors find their correct places. True or false?

Answer 3

True
The cohesins are a very important protein of the cell. It binds with the main sister chromatids and helps in correct alignment during the cell division and during the anaphase if the cell cycle. Since cellular material disintegrated during the division so the information must be passed on to the cell for reformation. This is provided by cohesins.

Question 4

List the different Ubiquitination events that occur over the cell cycle. Give their names and roles and regulatory mechanism(s).

Answer 4

-Cyclin Degradation: regulation of CDK-Cyclin complexes. CDKs activate cyclin ubiquitin ligases that destroy cyclin and inactivate CDK.
-CKI Degradation: Reactivating the cell cycle. CKIs are degraded by SCF complexes and ubiquitin ligases, activating CDK-Cyclin complexes.
-Metaphase/Anaphase Transition: regulation of sister chromatid separation. Anaphase Promoting Complex APC and cdc20 activator ubiquitinates securin. Separase is then freed from securin and cleaves cohesin at the centromere, allowing sister chromatids to separate.

Question 5

To prevent cells with damaged DNA from passing through the G1 checkpoint, what are ATR or ATM doing on Chk1 or Chk2?, which then modulate protein phosphatase Cdc25

Answer 5

ATR and ATM are protein kinases that sense and initiate repairs when DNA is damaged. ATM and ATR also alert the cell to arrest the cell cycle until the DNA is repaired.

ATM and ATR do this by phosphorylating Chk1 and Chk2 which then phosphorylate Cdc25. Cdc25 is an important protein involved in the activation of Cdk2/cyclin E so when phosphorylated Cdc25 is degraded, Cdk2/cyclin E is never activated and the cell cannot pass the G1/S checkpoint

Question 6

What are the events of prophase?

Answer 6

During prophase, Cdk1/cyclin B accumulates in the nucleus. Phosphorylation of numerous proteins by Cdk1/cyclin B causes the chromosomes to condense and the nuclear envelope to fall apart. Once the nuclear envelope is gone, and the centrosomes are oriented at opposite poles of the cell, the centrosomes send out microtubules and form the mitotic spindle

Question 7

what are Chk1 or Chk2 doing on Cdc25?

Answer 7

Chk1 and Chk2 are kinases that phosphorylate cdc25. Phosphorylated cdc25 is degraded which leads the inactivity of Cdk2/cyclin E and arrest of the cell cycle while the DNA is repaired.

Question 8

What is the function of the anaphase-promoting complex?

Answer 8

The anaphase-promoting complex (APC) is a ubiquitin ligase that leads to the degradation of several proteins such as securin, cyclin B, and Cdc20. Degradation of the these proteins allows the cell to enter anaphase and complete mitosis.

Regulation of sister chromatid separation. Anaphase Promoting Complex APC and cdc20 activator ubiquitinates securin. Separase is then freed from securin and cleaves cohesin at the centromere, allowing sister chromatids to separate.

Question 9

How many kinetochores are there in a human cell at mitosis?

Answer 9

There is a total of 46 chromosomes, 46 centromeres, and 92 kinetochores.

Question 10

What are astral microtubule? What is their origin?

Answer 10

Astral microtubules originate at the centrosome and radiate out in all directions forming an aster. Astral microtubules orient the centrosomes within the cell which also orients the mitotic spindle. They also move the centrosomes away from each other during anaphase B.

They originate in the poles and extend outward in all directions. They orient the spindle and also move the centrosomes away from each other during anaphase.

Question 11

What is the mitotic spindle?

Answer 11

It consists of the poles which contain 2 centrioles in the centrosome, microtubules, MT-dependent motors, and kinetochores.

The mitotic spindle is composed of the two centrosomes, the microtubules that extend out towards the chromosomes, and the motor proteins that generate pushing/pulling forces

Question 12

How does cdc25 affect commitment to Mitosis?

Answer 12

Cdc25 is a phosphatase that removes the inhibitory P on CDK1/cyclin b(mitotic cdk), therefore activating M-CDK/cyclin b which promotes mitosis.

cdc25 phosphatase, enters the nucleus, where it activates cyclin B/ CDK1. This breaks down the membrane surrounding the nucleus causing the nuclear envelope to break down and is the beginning of prometaphase stage of mitosis.

Question 13

Does cdc25 expression oscillate during cell cycle?

Answer 13

Yes it does. Because it is a phosphatase that acts on CDK1/cyclin b which is only expressed during the G2 phase to promote mitosis.

Cdc is also expressed during G1 and activates Cdk2/cyclin E.

Question 14

How the ORI (origin of replication) being licensed to fire?

Answer 14

ORI proteins located at the replication sites of DNA. Licensing to fire comes from Cdc6 and Cdt1 which are proteins that are part of the pre-Replication Complex. Once the whole RC is assembled and in place, replication is ready to fire

Question 15

How is the position of the spindle used to determine the formation of the contractile ring?

Answer 15

The position of the spindle determines where the mid body forms, which in turn defines the site where the contractile ring assembles. If ring forms in wrong place them the cell could end up with two nuclei and wouldn’t be able to divide again.

Question 16

Initiation of DNA synthesis and replication of centrioles are tightly coordinated events. Briefly explain how this coordination is achieved.

Answer 16

DNA synthesis and centriole replication are coordinated because they are controlled by the same cdk/cyclin complexes; Cdk2/cyclin E, and Cdk2/cyclin A.

Question 17

The signal for the spindle assembly checkpoint comes from the spindle poles. True or false?

Answer 17

False
The signal comes from the kinetochores. Kinetochores not under enough tension release the “wait anaphase” signal (Mad2 and BubR1). This signal delays the activation of APC cdc20 which is responsible the degradation of the cohesins holding the sister chromatids together.

Question 18

What are the critical events for M phase to occur in animal cells?

Answer 18

Cdk1/cyclin B is activated, all the DNA is replicated correctly, and there are the proper nutrients and growth factors available to the cell.

Question 19

What is the pre-Replication Complex (RC)?

Answer 19

The pre-RC is assembled during G1 after the checkpoint, but never when CDK-cyclin is high. The ORC (origin recognition complex) is bound to the ORI (origin of replication), then the licensing proteins cdc6 and cdt1 bind to ORC, then MCM helicase binds, completing the Pre-RC.

Question 20

Hypothesize the early and late outcome of a cell without a functional Retinoblastoma protein. How oncologist classify this protein?

Answer 20

Retinoblastoma (Rb) is a tumor suppressor protein that binds and inhibits the transcription factor E2F. E2F stimulates the genes that transcribe cyclin E which binds with Cdk2 and initiates S phase. When Rb is active, mitosis cannot start.
If a cell did not have a functional Rb protein then E2F would not be inhibited and cyclin E levels would remain high and there would be no restrictions on a cell to enter S phase and begin DNA replication.

Certain extracellular factors such as ‘tumor necrosis factor-a’ can detect the lack of Rb and trigger apoptosis in the cell. A late outcome would possibly just be referring to developing cancerous cells from lack of regulation.

Question 21

What are centromeres? What are their structure and roles?

Answer 21

The centromere is a specialized region on the chromosome that contains the kinetochores. They are condensed heterochromatin composed of highly repetitive DNA sequences that bind to a unique set of proteins.

The centromere is a region of the chromosome specially designed to associate with kinetochores. The centromere has numerous repeating DNA sequences which interact with modified forms of the protein histone which tightly bind the DNA. Other proteins (CENPs and MCAK) are part of the centromere and facilitate kinetochore formation and microtubule binding

Question 22

After the nuclear envelope breaks down, microtubules gain access to the chromosomes and, every so often, a randomly probing microtubule connects with a kinetochore and captures the chromosome. True or false?

Answer 22

True.
Microtubule-Kinetochore connection is a “search and capture” process.

Question 23

What type of molecular sensors a cell possesses to respond to the late outcome of the absence of functional Retinoblastoma protein?

Answer 23

Certain extracellular factors such as ‘tumor necrosis factor-a’ can detect the lack of Rb and trigger apoptosis in the cell.

Question 24

What is congression (that really important part of metaphase) during mitosis?

Answer 24

Congression is the lining up of chromosomes into the mitotic plate/spindle equator. Congression is formed through pulling forces, leaving a zero net force on the chromosomes

Question 25

Chromosomes are pushed away from the poles (polar winds) by chromokinesins and spindle microtubule interactions. True or false?

Answer 25

True.
Chromokinesins are located on the surface of the chromosome arms and interact with spindle microtubules. These are NOT the same microtubules interacting with the kinetochore. These chromokinesin/microtubule interactions push the chromosomal arms away from the pole. This force stops mono-oriented chromosomes from being pulled all the way to one pole

Question 26

list the Cdks-Cyclins complexes present at each phase of the cell cycle? On the same diagram show the cell cycle checkpoints

Answer 26

G1- CDK4/cyclin d
CDK2/cyclin e
G1 checkpoint
S- CDK2/cyclin a
G2/M- CDK1/cyclin b
G2 checkpoint

G1: Cdk4 and cyclin D promote progression through G1 towards S phase. Cyclin D is encoded by a delayed early response gene (p. 698) which is activated by transcription factors are made when growth factors stimulate the cell to leave G0. Cdk and cyclin E initiate beginning of S phase and replication
S Phase: Cdk2 and cyclin A initiate and maintain DNA replication
Mitosis: Cdk1 binds with cyclin B in the nucleus and phosphorylates certain substrates. Cell will not enter mitosis without this complex being activated. Cyclin B degrades during G1.

Question 27

Each kinetochore fiber exerts a poleward-pulling force that is proportional to its length (until the pull forces from each pole are equal). True or false?

Answer 27

True.
Longer microtubules exert a stronger pulling force on the kinetochore. In this way, chromosomes are arranged in the mitotic plate where net tension on the kinetochore is zero.

Question 28

Stable, bipolar attachment of sister chromatids is assessed by kinetochore passenger proteins and the MCAK motor (that depolymerizes microtubules) so that the SAC is not passed too soon. True or false?

Answer 28

True.
SAC activation inhibits the action of APC, the protein complex that degrades securin which inhibits separase which breaks the cohesins holding the sister chromatids together. SAC is activated when errors occur in the microtubule/kinetochore coupling process.The MCAK destabilizes microtubules when the chromosome is under merolitic or syntelic conditions, freeing the kinetochore to bind with the correct microtubule.

Question 29

A cell in quiescence is?

Answer 29

In a state of non-division.
in a a temporary and reversible withdrawal from the cell cycle (G0)

In a non-dividing state. When signals indicating that a cell should proliferate become limiting.
Quiescence (G0) is a reversible state.

Question 30

CENP-E protein in an unattached kinetochore can slide along an adjacent, attached kinetochore fiber toward a centered chromosome to help the unattached chromosome get closer to the center. This may help shorten the distance for the microtubules searching for an unattached kinetochore. True or false?

Answer 30

True.
When a chromosome is mono-oriented, meaning that only one of its sister chromatids is attached to the mitotic spindle, CENP-E can interact with a neighboring bi-oriented chromosome. This interaction allows CENP-E to generate pulling forces and move toward the mitotic plate or spindle equator.

By moving toward the mitotic plate, the mono-oriented chromosome increases its proximity to the other opposing spindle microtubule. This increases the likelihood of the unattached kinetochore of the mono-oriented chromosome binding to the opposing microtubule, leading to proper bi-orientation and alignment of the chromosome along the metaphase plate.

The interaction of CENP-E with neighboring chromosomes contributes to the dynamic and coordinated movements of chromosomes during mitosis, ultimately ensuring accurate chromosome segregation.

Question 31

What is the phenotype of a cell that loses the function of Wee kinase, the inhibitory kinase of Cdk1?

Answer 31

The cell enters mitosis prematurely

Question 32

What is the role of the spindle assembly checkpoint (SAC)?

Answer 32

To verify correct attachment of microtubule and kinetochore.
It ensures that every kinetochore is properly attached to the spindle.

To delay sister chromatid separation, that is, the onset of anaphase, until each and every kinetochore is properly attached to spindle MTs.
The more appropriate name is kinetochore attachment checkpoint

Question 33

What prevents an origin of replication from being used more than once per cell cycle?

Answer 33

To prevent an origin of replication from being used more than once per cell cycle, a regulatory mechanism known as “licensing” is in place. The licensing mechanism ensures that DNA replication occurs only once per cell cycle by controlling the initiation of replication at each origin.

The s-cdk phosphorylates cdc-6 so that it dissociates from the ORC. The MCMs can no longer bind with the absence of cdc 6. This results in the prevention of the pre replication site from refiring. Also, cyclin a needs to be present to activate the CDK so it can phosphorylate cdc6, which allows the origin to fire.

Question 34

The isolation of conditional cell cycle mutations in yeast helped identify many of the key proteins involved in cell cycle control. Many of these mutations were given the name cdc (cell division cycle) and are temperature sensitive. Hypothesize about the outcomes of A cdc temperature sensitive mutant.

Answer 34

Temperature sensitive mutants will experience arrest of the cycle when transferred to different temperature environment.

When a temperature-sensitive mutant is shifted to a higher or restrictive temperature, it leads to a loss of protein function or altered protein structure due to the temperature-induced conformational changes. This can result in the malfunctioning or inactivation of essential proteins involved in cell cycle progression.

Question 35

There are several mechanisms for regulating Cdk activity. List three distinct mechanisms of Cdk activation or inhibition.

Answer 35

Three mechanisms of Cdk (cyclin-dependent kinase) regulation:

Cyclin Binding: Cyclins bind to Cdks, activating them for cell cycle progression.

Phosphorylation: Phosphorylation activates or inhibits Cdks, depending on the context and specific sites.

Cdk Inhibitors: Proteins called CKIs directly inhibit Cdk activity to control cell cycle timing and progression.

These mechanisms collectively contribute to the precise regulation of Cdk activity and orchestrate the progression of the cell cycle. By modulating Cdk activity, cells can tightly control cell cycle events and maintain genomic stability.

Question 36

Explain why cells that lack functional p53 are more prone to accumulate mutations in their DNA?

Answer 36

P53 is a checkpoint gene that restrains the cell cycle when DNA is damaged. When damaged DNA is allowed to replicate, more and more replication errors occur and mutations can accumulate.

Question 37

What is one function of the Cdk1-cyclin B complex in mammalian cells?

Answer 37

Phosphorylate lamin protein and trigger nuclear envelope disassembly

Question 38

Which of the following statements about the anaphase-promoting complex

Question 39

which one of the following functions is not performed by a motor protein within the mitotic spindle

Answer 39

elongate microtubules at the (-) end

Question 40

What is the role of the spindle assembly

Answer 40

it ensures that every kinetochore is properly attached to the spindle

Question 41

After nuclear envelopes breakdown when centrosomes separate to form the spindle separation of the centrosomes occur through

Answer 41

A) pulling of the astral microtubules by dynein anchored at the plasma membrane.

B) pushing apart parallel overlapping microtubules of the same polarity by the kinesin.

Question 42

the following statements refer to cyclins, which is true?

Answer 42

3. cyclins are ubiquitinated and degraded at specific phases of the cell cycle.
4. cyclins bind to Cdks

Question 43

which of the following statements about cdc25 is true?

Answer 43

2. it is a protein phosphatase that promotes entry into mitosis by removing an inhibitory phosphate on Cdk1
3. it is the rate limiting step for entry into mitosis

Question 44

Kinetochores control the transition from metaphase to anaphase. why is this statement true?

Answer 44

Anaphase will only begin if all the kinetochore in the cell have an attachment to the spindle

Question 45

Which of the following statements about Cdk-1 and cycling B are true?

Answer 45

cyclin levels oscillate throughout the cell cycle, peaking in mitosis, but Cdk-1 levels remain constant.

Question 46

Which of the follow are mechanisms triggered by growth factors to stimulate cell proliferation and re-entry into the cell cycle.

Answer 46

1. activation of gene expression, including expression of G1 cyclin.
2. Activation of ubiquitin mediated destruction of Cdk inhibitors
3. inactivation of the tumor suppressor protein Rb

Question 47

Which one of the following functions is not performed by a motor proteins within the mitotic spindle?

Answer 47

elongate microtubules at their (-) ends

Question 48

During prometaphase, chromosomes can be observed moving first towards one pole and then towards another, finally coming to the midpoint between the poles at metaphase. Which of the following statements best describes the event that accounts for this movement?

Answer 48

One of the two sister kinetochores is captured by astral MT from one pole and begin to move towards it, the other sister kinetochore is then captured by astral MT form the opposite pole. The coordinated forces from opposite poles position the chromosomes at the midpoint.

Question 49

Kinetochores control the transition from metaphase to anaphase. Why is this statement true?

Answer 49

Anaphase will only begin if all of the kinetochores in the cell have an attachment to the spindle.