Cell Cycle

Question 1

Define the function of the cell cycle

Answer 1

The most basic function of the cell cycle is to duplicate accurately the vast amount of DNA in the chromosomes and then segregate the copies precisely into two genetically identical daughter cells. The most basic function of the CELL is to duplicate. DUPLICATION has to be accurate: especially DNA content Identical copies of the DNA (Chromosomes) passed into two IDENTICAL daughters -Cell cycle is the process of transferring the same genetic material from mother cell to daughter cell -Most basic function is to accurately duplicate the vast amount of DNA

Question 2

Define and explain the phases of the cell cycle

Answer 2

There are two main phases: 1) Interphase: This is the longest period 2) M Phase or mitotic phase: nuclear and cytoplasm divisions. Phases of the cell cycle: 1) Interphase 2) Preprophase (plant cells) 3)Prophase 4) Prometaphase 5) Metaphase 6) Anaphase 7) Telophase 8) Cytokinesis Interphase has 3 separate phases within it: 1) G1 Phase: It is the gap phase between the M and S phases. It contains a checkpoint 2) S Phase: DNA replication 3) G2 Phase: It is the gap phase between the S phase and the M phase. It also contains a checkpoint G phases provide time for the cell to monitor the internal and external environment to ensure that conditions are suitable for the cell to commit to division. The G1 phase is especially important in this respect. Its length can vary greatly depending on external conditions and extracellular signals from other cells. If extracellular conditions are unfavorable, for example, cells delay progress through G1 and may even enter a specialized resting state known as G0 (G zero), in which they can remain for days, weeks, or even years before resuming proliferation. If extracellular conditions are favorable and signals to grow and divide are present, cells in early G1 or G0 progress through a commitment point near the end of G1 known as Start (in yeasts) or the restriction point (in mammalian cells). After passing this point, cells are committed to DNA replication, even if the extracellular signals that stimulate cell growth and division are removed. -The point is that you have the cell cycle and the different phases -The phases are designed with a very specific purpose -Once you enter a phase, the cell doesn’t go back  It is a binary system -Cells receive certain signals to allow them to enter certain phases. -The cells will not make the decision of moving on to the next phase by themselves, they receive signals from the environment to mediate it -Interphase is the longest period. -G phases are Gap Phases: They have an important function in asking if the cell is going to go into the next phase.

Question 3

Define the importance of the G phase

Answer 3

G phases are Gap Phases: They have an important function in asking if the cell is going to go into the next phase. G phases provide time for the cell to monitor the internal and external environment to ensure that conditions are suitable for the cell to commit to division.

Question 4

Define and explain mitosis

Answer 4

-Mitosis is a part of the cell cycle process by which chromosomes in a cell nucleus are separated into two identical sets of chromosomes, each in its own nucleus. -Cells duplicate their genetic material before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA. -A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only then splits into daughter cells. -Cytokinesis also is a final separation of the two cells  Separate the genetic material then the cell -It is a highly organized system from the beginning

Question 5

Phases in the M Phase of the cell cycle

Answer 5

1) Prophase 2) Prometaphase 3) Metaphase 4) Anaphase 5) Telophase 6) Cytokinesis After S phase, chromosome segregation and cell division occur in M phase (M for mitosis), which requires much less time (less than an hour in a mammalian cell). M phase involves a series of dramatic events that begin with nuclear division, or mitosis. Mitosis begins with chromosome condensation: the duplicated DNA strands, packaged into elongated chromosomes, condense into the much more compact chromosomes required for their segregation. The nuclear envelope then breaks down, and the replicated chromosomes, each consisting of a pair of sister chromatids, become attached to the microtubules of the mitotic spindle. As mitosis proceeds, the cell pauses briefly in a state called metaphase, when the chromosomes are aligned at the equator of the mitotic spindle, poised for segregation. The sudden separation of sister chromatids marks the beginning of anaphase, during which the chromosomes move to opposite poles of the spindle, where they decondense and reform intact nuclei. The cell is then pinched in two by cytoplasmic division, or cytokinesis, and cell division is complete When mitosis begins, the chromosomes condense and become visible. the nuclear envelope, which segregates the DNA from the cytoplasm, disintegrates into small vesicles. Microtubules project from opposite ends of the cell, attach to the centromeres, and align the chromosomes centrally within the cell. The microtubules then contract to pull the sister chromatids of each chromosome apart. Sister chromatids at this point are called daughter chromosomes. As the cell elongates, corresponding daughter chromosomes are pulled toward opposite ends of the cell. A new nuclear envelope forms around the separated daughter chromosomes. As mitosis concludes, the cell may begin cytokinesis. In animal cells, a cell membrane pinches inward between the two developing nuclei to produce two new cells. -When the genetic material is duplicated and cells have duplicated material too, the chromosomes line up at the mitotic plate to get it all on one plane and evenly separate. The spindle pulls the material out -There are also spindles that go from the centrosome or aster to the cell membrane to get this leverage for the pulling of DNA -Then you get Telophase, where cleavage furrow forms via myosin and actin. It just keeps closing until the two cells are separated.

Question 6

Prophase

Answer 6

Prophase: -Chromatin condenses. -Centrosomes move to opposite ends of the nucleus. -The centrosomes start to form a framework called the mitotic spindle, that is made of microtubules. -Nucleolus disappears.

Question 7

Prometaphase

Answer 7

Prometaphase: -Nuclear envelope fragmentizes. -Chromosomes become more condensed -A kinetochore is formed at the centromere, the point where the sister chromatids are attached -Microtubules attach at the kinetochores

Question 8

Metaphase

Answer 8

Metaphase: -Chromosomes align on an axis called the metaphase plate. -All of the chromosomes thus go on one plane

Question 9

Anaphase

Answer 9

Anaphase: -Each centromere splits, freeing chromatids. -Each chromatid moves toward a pole. -Cell begins to elongate, caused by microtubules not associated with the kinetochore.

Question 10

Telophase

Answer 10

Telophase: -Formation of nuclear membrane and nucleolus. -Formation of chromatin. -Formation of the cleavage furrow - a shallow groove in the cell near the old metaphase plate created by actin and myosin. -Cytokinesis = division of the cytoplasm.

Question 11

Cytokinesis

Answer 11

Division of the cytoplasm

Question 12

Centromere

Answer 12

The centromere (centro- + -mere) is the part of a chromosome that links sister chromatids. During mitosis, spindle fibers attach to the centromere via the kinetochore.

Question 13

Kinetochore

Answer 13

The kinetochore is the protein structure on chromatids where the spindle fibers attach during cell division to pull sister chromatids apart.

Question 14

Chromatid

Answer 14

Chromatid is one copy of a newly replicated chromosome, which typically is joined to the other copy by a single centromere.

Question 15

Centromere

Answer 15

The centromere is the part of a chromosome that links sister chromatids. During mitosis, spindle fibers attach to the centromere It is a specialized DNA region, in the heterochromatin.

Question 16

Centrosome

Answer 16

the centrosome is an organelle that is the main place where cell microtubules get organized. They occur only in animal cells. The centrosomes with the centriole pairs are duplicated and each one moves to the poles of the cells to form the mitotic spindle which is essentially a network of microtubules that will bind to the centrosome

Question 17

Kinetochore

Answer 17

The kinetochore is the protein structure on chromatids where the spindle fibers attach during cell division to pull sister chromatids apart. -Kinetochore is the centromere with attached spindles and this is important to allow proper separation of genetic material -Each chromosome forms two kinetochores at the centromere, one attached at each chromatid. A kinetochore is a complex protein structure that is analogous to a ring for the microtubule hook; it is the point where microtubules attach themselves to the chromosome. When the spindle grows to sufficient length, kinetochore microtubules begin searching for kinetochores to attach to. A number of nonkinetochore microtubules find and interact with corresponding nonkinetochore microtubules from the opposite centrosome to form the mitotic spindle -It contains the centromere which is a specialized DNA region in the heterochromatin. -The kinetochore contains 12 proteins including H3 histone variant CENP-A -The kinetochore forms around the centromere.

Question 18

Mitotic Spindle

Answer 18

The mitotic spindle is the macromolecular machine that segregates chromosomes to two daughter cells during mitosis. ***The mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis. Assembly of spindle microtubules begins in the centrosome, the microtubule organizing center. The centrosome replicates, forming two centrosomes that migrate to opposite ends of the cell, as spindle microtubules grow out from them. An aster of microtubules extends from each centrosome. -The mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis -The assemble of the spindle begins at the centrosome -The centrosomes replicate and form 2 centrosomes that go to the poles of the cell -Microtubules extend from each centrosome -The spindle includes the centrosomes, the spindle microtubules, and the asters -Some spindle microtubules attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate -In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell -The microtubules shorten by depolymerizing at their kinetochore ends

Question 19

S phase

Answer 19

DNA duplication occurs during S phase (S for synthesis), which requires 10–12 hours and occupies about half of the cell-cycle time in a typical mammalian cell.

Question 20

Cytokinesis

Answer 20

Cytokinesis: The division of the cytoplasm in two. -The final step of the cell cycle. -Begins in anaphase and finishes in telophase, at the end of mitosis. Cytokinesis is the physical process that finally splits the parent cell into two identical daughter cells. During cytokinesis, the cell membrane pinches in at the cell equator, forming a cleft called the cleavage furrow. The position of the furrow depends on the position of the astral and interpolar microtubules during anaphase. It utilizes actin and myosin to pinch the cell into two separate cells. The cytoskeletal filaments play the main role.

Question 21

Bipolar array of microtubules:

Answer 21

Bipolar array of microtubules: + ends oriented outward; - ends oriented to the spindles.

Question 22

Kinetochore microtubules

Answer 22

attached to the centromeres of the chromosomes to pull them to that side or that centrosome.

Question 23

Interpolar microtubules

Answer 23

extend from the spindle pole across the equator. They connect the two centrosomes or asters to again keep things grounded and generate leverage

Question 24

Astral microtubules

Answer 24

extend from the spindle pole to the cell membrane. They ground the spindle pole (centrosome or aster) to the membrane to give it leverage to pull the chromosomes

Question 25

Cleavage Furrow

Answer 25

the cleavage furrow is the indentation of the cell’s surface that begins the progression of cleavage, by which animal cells undergo cytokinesis, the final splitting of the membrane, in the process of cell division. It involves actin and myosin cytoskeleton filaments An indentation on the cell surface starts the division into two cells: cleavage furrow. -Actin and myosin filaments form the contractile ring, which contracts as daughter cells divide. The cleavage furrow forms because of the action of a contractile ring of overlapping actin and myosin filaments. As the actin and myosin filaments move past each other, the contractile ring becomes smaller, akin to pulling a drawstring at the top of a purse. When the ring reaches its smallest point, the cleavage furrow completely bisects the cell at its center, resulting in two separate daughter cells of equal size. -Cleavage furrow is an indentation and occurs when myosin and actin start to contract and form this furrow -By doing this it allows for the separation  without this you end up with a cell with two nucleuses inside it because they never cleaved or separated

Question 26

RhoA

Answer 26

RhoA regulates the contractile ring of actin and myosin! Rho/GTP (active form) is modified by GAPs by converting GTP in GDP, resulting in Rho/GDP (inactive form). Activation of Rho/GDP requires of GEFs that help remove the GDP from Rho. Because there is 10x more GTP than GDP in the cytoplasm , then GTP binds to Rho, resulting in the active form that controls intracellular actin dynamics., actin polymerization. -A GTPase that is a part of the Ras superfamily regulated the contractile ring of actin and myosin to cleave the two cells -Activated RhoA promotes the assembly of actin filaments in the contractile ring -Subsequent stimulation of myosin II filaments promotes contraction of the ring -Rho-GTP is the active form that creates the downstream effects via downstream effectors. GAPs (GTPase activating proteins) dephosphorylate the GTP to the GDP to get Rho-GDP which is inactive. GEFs (Guanine nucleotide exchange factors) exchange the GDP for GTP. Get the active Rho-GTP -There are other GTPases that convert GTP to GDP to not mediate the signal. ***The Rho-GTP activates downstream effectors that ultimately cause the contraction of the actin/myosin ring and thus cause the cytokinesis process

Question 27

G0

Answer 27

G0 is a special part of the cell cycle that when there are no conditions, the cell will go into the G0 resting stage. There are 2 types of G0: One in which you can go back to G1 and one in which the cell is permanently in G0 (senescence). Cells can go into G0 if the conditions are not given to go into G1

Question 28

Define the function of the cell cycle control

Answer 28

The function of the cell cycle control is to ensure that the cell is ready for the next step. In other words, it is to ensure that there are no errors in the cell because it is a binary system, once the cell moves on, it cannot go back. The checkpoints provide an opportunity for the cell to be checked to make sure that the DNA was replicated and the environment is sufficient for cell division. -Ensures events are properly timed. -Ensures events occur in correct order. -Ensures events occur only once per cycle -Feedback. -Now looking at the elements which control the cell cycle -This has to occur in a timely manner -It has to occur in the correct order -It can only occur once per cell cycle -Also it needs feedback loops, whether positive or negative.

Question 29

Define the features of the cell cycle control system

Answer 29

Has the following features: -A clock, or timer, that turns on each event at a specific time, thus providing a fixed amount of time for the completion of each event. -A mechanism for initiating events in the correct order. -A mechanism to ensure that each event is triggered only once per cycle. -Binary (on/off) switches that trigger events in a complete, irreversible fashion. -Robustness: backup mechanisms to ensure that the cycle can work properly even when parts of the system malfunction. -Adaptability, so that the system’s behavior can be modified to suit specific cell types or environmental conditions. Binary (on/off) switches that trigger events in a complete, irreversible fashion. It would clearly be disastrous, for example, if events like chromosome condensation or nuclear envelope breakdown were initiated but not completed -If the events have occurred it will move on to the next phase, if they havent it will not! It is binary because it either does or it doesn’t, there are no middle points -There are also backup mechanisms to ensure the cycle can work properly even when parts of the system malfunction Triggers progression through control points. By analogy with a washing machine, the cell-cycle control system is shown here as a central arm—the controller—that rotates clockwise, triggering essential processes when it reaches specific points on the outer dial. -This is representing the checkpoints -G2 checkpoint: Is all DNA replicated? Is the environment favorable? Allows the cell to move into the mitotic phase. -M checkpoint: Are all chromosomes attached to the spindle? Allows cell to exit metaphase -G1 checkpoint: Is the environment favorable? Allows cell to enter S phase. -It is a binary system and once it goes it can’t go back. Once you enter a certain part of the cycle you can’t go back, you have to go forward –> it is the washer machine system -If the checkpoint is not reached, then it will not go past it! The cells are very well controlled by outside and internal signals.

Question 30

Describe the elements of the cell cycle control

Answer 30

a

Question 31

Define “checkpoints”

Answer 31

Cell-cycle progression can be regulated by intracellular and extracellular signals.

Question 32

Typical Cell Cycle Duration

Answer 32

The duration of these cell cycle phases varies considerably in different kinds of cells. For a typical rapidly proliferating human cell with a: 1) Total cycle time of 24 hours 2) G1 phase might last about 11 hours 3) S phase about 8 hours 4) G2 about 4 hours 5) and M about 1 hour.

Question 33

Determining the length of the total cell cycle

Answer 33

Uses Colchicine which is an inhibitor of microtubules polymerization! The prevents M phase from taking place because we cannot get the formation of the mitotic spindles. -You can add an inhibitor of the M phase, Colchicine, which will inhibit the microtubule polymerization and prevent the M phase from occurring because the spindles cannot form -You add the inhibitor, making most cells stuck in the M-phase because they cannot pull the chromosomes apart! -The first cells to appear in the M phase where probably close to the M phase when the inhibitor was added (beginning of G1) and can count how long it takes to go back to M phase (you will see chromosome formation) -It varies because there are cells that are constantly dividing.

Question 34

Determining the length of the G1/S phase

Answer 34

In this method you can add radioactive Thymidine and synchronize all the cells in the beginning of G1 phase. Then, you add the radioactive thymidine, and calculate the amount present in each cell every hour. -Can add radioactive Thymidine which will be incorporated into the DNA during the S phase when replication takes place. -You can count every hour for 24 hours and see that there is no uptake of T during G1 phase. And the S phase will be where the count starts going up -The G1 phase is from the point of not uptake to the point to where you start to see uptake

Question 35

Determining the length of the G2 phase

Answer 35

-To find out G2, you add radioactive thymidine, and use hydroxy-urea to inhibit DNA replication. -Then you add Colchicine to inhibit the M phase. -The first cells that you start to visualize are the ones that were towards the end of S phase. This first signal is the length of G2 phase because they where at the end of S. Then you can subtract that from the last one to appear to get the length of S phase too

Question 36

Define and explain the methods that can be used to analyze cell cycle

Answer 36

1) Radioactive Thymidine: It can be incorporated in the DNA in S phase 2) Colchicine: Blocks microtubule polymerization and thus M phase 3) Hydroxy-Urea: Inhibits DNA replication 4) DNA fluorescent dyes.

Question 37

Cell Cycle Control System:

Answer 37

1) Can arrest the cell cycle at specific checkpoints. 2) Checkpoints generally operate through negative intracellular signals. 3) Is based on cyclically activated protein kinases. 4) Depends on cyclical proteolysis. 5) Depends on transcriptional regulation.

Question 38

Cell Growth or G1 checkpoint

Answer 38

Cell Growth Checkpoint: -Occurs toward the end of G1. -Checks whether the cell is big enough and has made the proper proteins for S phase. -Cell goes to resting (G0) if not ready to divide. The G1 checkpoint, just before entry into S phase, makes the key decision of whether the cell should divide.

Question 39

G2 Checkpoint

Answer 39

DNA Synthesis Checkpoint: -Occurs during G2 phase. -Checks whether DNA has been replicated correctly. -Cell continues to M if ready. The G2 checkpoint determines if the DNA has been replicated properly.

Question 40

M or Mitotic Spindle Checkpoint

Answer 40

The mitotic spindle checkpoint occurs at the point in metaphase where all the chromosomes should have aligned at the mitotic plate. -Occurs during M phase. -Checks whether mitosis is complete. -Cell divides and cycle repeats if complete.

Question 41

Cyclins

Answer 41

All cyclins are named according to the stage at which they assemble with CDKs. The cyclins are not constant throughout the cell cycle and the cyclins present vary depending on what phase of the cell cycle the cell is in. Cyclin synthesis and breakdown varies by stage — with cell cycle progression dependent on the synthesis of new cyclin molecules. CDKs REMAIN CONSTANT Cyclin degradation is equally important for progression through the cell cycle. Specific enzymes break down cyclins at defined times in the cell cycle. When cyclin levels decrease, the corresponding CDKs become inactive. Cell cycle arrest can occur if cyclins fail to degrade. Some of them are more predominant or play a more significant role throughout different phases of the cell cycle Cyclins are a family of proteins that have no enzymatic activity of their own but activate CDKs by binding to them and are synthesized and degraded as needed.

Question 42

CDK Phosphotases

Answer 42

-There are phosphatases and the phosphatases can 2 two things: 1) Wee1 kinase will add a phosphate to the inhibitory site and this inhibits this complex so it can no longer drive the cell cycle. Influences cell size by inhibiting the entry into mitosis, through inhibiting Cdk1. 2)Cdc25 phosphatase will remove the inhibitory phosphate and activate the complex Cdc25 is a dual-specificity phosphatase: By removing inhibitory phosphate residues from target Cdks, control entry into and progression through various phases of the cell cycle, including mitosis and S (“Synthesis”) phase.

Question 43

Cyclin Dependent Kinases (CDKs)

Answer 43

All CDKs exist in similar amounts throughout the entire cell cycle. CDKs require the presence of cyclins to become active and constitutively present. They are activated by specific phosphorylation and cyclin binding.

THEY PHOSPHORYLATE OTHER PROTEINS WHEN THEY ARE ACTIVATED

Question 44

Cyclin Expression Cycle

Answer 44

Question 45

G1/S Checkpoint with Rb

Answer 45

Two cell cycle kinase complexes, CDK4/6-Cyclin D and CDK2-Cyclin E, work in concert to relieve inhibition of a dynamic transcription complex that contains the retinoblastoma protein (Rb) and E2F.

In G1-phase uncommitted cells, hypo-phosphorylated Rb binds to the E2F-DP1 transcription factors forming an inhibitory complex with HDAC to repress key downstream transcription events.

Commitment to enter S-phase occurs through sequential phosphorylation of Rb by Cyclin D-CDK4/6 and Cyclin E-CDK2 that dissociates the HDAC-repressor complex, permitting transcription of genes required for DNA replication.

• Other elements further control the cell cycle.
• Retinoblastoma protein (Rb Protein)
• The Rb protein, when it is active, inhibits E2F by binding to it. If the Rb is phosphorylated (in 2 sites) then it is inactivated and now the E2F protein is active and can cause DNA synthesis
• There is positive feedback as well in which the active G1/S-Cdk will feedback and further activate E2F.
• You can have a mutation in the Rb protein which renders it unable to bind to E2F and get uncontrollable DNA synthesis or transcription

KNOW FOR EXAM What does it mean to have a gene mutation for Rb for example? What does this cause?

Question 46

DNA Damage Response

Answer 46

Mdm2 is normally associated with p53, causing it to be degraded. However, in response to DNA damage, there is the activation of ATM and ATR kinases which phosphorylate the p53, causing it to be activated and dissociate from Mdm2. Now the p53 enters the nucleus and binds to the p21 regulatory region of the DNA and causes the upregulation of p21 which is a Cdk inhibitor. The p21 binds to Cdks and prevents the cell from moving forward in the cell cycle.

Question 47

Mitosis Checkpoint feedback

Answer 47

• Here there is Cdk1 and M-cyclin which binds to it and activates it. Wee1 will phosphorylate it at the inhibitory site and Cdk-activating kinase will phosphorylate at the activating site.
• An active M-Cdk will then activate the Cdc25 which will cause more activation of M-Cdk à This is one positive loop.
• The other positive loop is that the active M-Cdk goes and inhibits Wee1. Now the M-Cdk cannot become inactivated and thus stays active.
• The Cdks tell the cell that it is ready to move into the next stage of the cell cycle

KNOW THIS FOR EXAM

Question 48

Nocodazole/Vimblastine

Answer 48

This drug acts on microtubules and this the chromosomes will fail to separate. It dissembles microtubules, preventing the formation of a spindle.

Question 49

Cytochalasin D

Answer 49

This drug does not effect mitosis because it is causing the disassembly of actin filaments which are not involved in the formation of the spindle in the mitotic phase. This would prevent the cytokinesis from occurring