Chapter 10. Cell Division

Question 1

What are the functions of cell division?

Answer 1

2 functions

• multicellular organisms: for growth, maintenance and repair of cells and tissues
• unicellular organism: for reproduction

Question 2

how prokaryotic cells reproduce/divide

Answer 2

binary fission

Question 3

How are prokaryotic chromosomes different in structure than eukaryotic chromosomes?

Answer 3

Prokaryotic chromosomes: double-stranded, circular DNA + additional smaller loops of DNA(plasmids)
Eukaryotic chromosomes: condensed into compact chromosomes to fit into the nucleus

Question 4

Understand the following terms in relationship to chromosome compaction:
Histones, Nucleosome, Histone octamer, chromatin fiber

Answer 4

DNA wrapped around histone protein two times (8 of them –> histone octamer)
Nucleosome (section of DNA that is wrapped around core protein) is the fundamental subunit of chromatin fiber

Question 5

Understand the difference between a replicated and an unreplicated chromosome. What is a centromere? How do they relate to the various stages of the cell cycle? Which one consists of sister chromatids? How many DNA molecules are there in an unreplicated chromosome? In a replicated chromosome? What are the roles of cohesins?

Answer 5

Unreplicated chromosome: one DNA (n) - separation happens in mitosis.
Replicated chromosome: two DNA (2n), sister chromatids held together in centromere by cohesin - made in S phase
Centromere: region of DNA where sister chromatids are held together - made during prophase where replicated pair of chromosomes condense and are joined at centromere.
Role of cohesins : holding sister chromatids together.

Question 6

Which phase of the cell cycle is DNA replicated?

Answer 6

S(synthesis) phase

Question 7

Know the main events that occur during the various stages of the cell cycle.

Answer 7

G1 Cell growth
S DNA synthesis
G2 Preparation of mitosis
Mitosis Creating two identical daughter cells

Question 8

What is G0? What does terminal differentiation mean? Understand how some cells continually cycle, some cycle part of the time, and some never cycle. Cells that never cycle are said to be permanently in what stage of the cell cycle?

Answer 8

G0 is point in G1 phase of cell cycle, right before G1 check point, where cells are no longer dividing. Some cells are temporarily resting in G0 whereas some are terminally differentiated.
Terminal differentiation: Different cell types (e.g., neurons, skeletal and heart myocytes, keratinocytes) undergo terminal differentiation, in which acquisition of specialized functions entails definitive withdrawal from the cell cycle. (this is Gap0 stage of cell cycle)

Question 9

What is meant by a cell “check point”? What are the three major checkpoints discussed in class? What conditions are monitored in determining whether the cell can progress through that checkpoint? Know that the cell cycle checkpoints are regulated by both positive and negative regulators. Which would promote movement through the cycle? Which would inhibit?

Answer 9

Cell checkpoint: points in cell cycle where cells are regulated/monitored for normal cell division.
< 3 check points – and what is monitored >
G1 checkpoint – cell size, sufficient nutrient, growth factors, no DNA damage, cell crowding, and attachment to ECM
*Rb, P53, and P21 are negative regulators of G1/S checkpoint. w
G2 Checkpoint – adequate cell size, protein ready, successful chromosome replication
*cyclin(positive regulator)-cdk complex pushes cells to prophase. (Cyclin B binds to Cdk1, activating MPF. Targets of MPF is chromatin condensation, nuclear envelope breakdown, fragmentation of Golgi apparatus, and spindle formation.)
M(metaphase) checkpoint – all chromosomes attached to mitotic spindle
*APC (anaphase promoting complex, also a ubiquitin ligase) is a positive regulator. It destroys MPF by putting ubiquitin to cyclin (destroy cyclin), which inactive the Cdk. - MPF is destroyed because it builds mitotic spindle.
APC also destroys cohesin. APC adds ubiquinone to securin, which activates separase. Securin is broken down by proteosome. Active separase chops up cohesin.

Cyclin-Cdk complex is positive regulator.
Positive regulator –> promote movement
Negative regulator –> inhibit movement through the cell cycle

Question 10

Understand how the G1/S checkpoint is also known as the restriction point. Why is this particular checkpoint critical in terms of regulation of cell numbers?

Answer 10

Because in past the G1/S checkpoint, in G1 phase, DNA will be replicated – and don’t want to replicate the damaged DNA?
- Committed steps – where after G1 checkpoint. Mitosis happens and cells duplicate.

Question 11

What is the role of growth factors(external factor) in regulating G1/s check point?

Answer 11

Growth factors activate Ras protein –> synthesize Cyclin D –> bind to Cdk –> targets Rb which activates E2F gene –> E2F gene turns on genes for DNA synthesis (transcription of S phase)

Question 12

Understand the role of cyclins and cyclin dependent kinases (cdk) in regulating movement through the cell cycle. How did “cyclins” get their name?

Answer 12

Cyclin dependent kinase(Cdk) is normally inactivated, but when cyclin binds to it, it opens/activates the active site of cdk, causing cdk to put phosphate onto target protein. Cyclins pushes through each checkpoint and are destroyed.
Cyclins get their name – how cyclins are synthesized/degraded in stages of the cell cycle.

Question 13

What is MPF? Know which cyclin is part of MPF. How does the concentration of this cyclin change during the various stages of the cell cycle?

Answer 13

MPF is maturation promoting factor (remember it as a mitosis promoting factor). MPF is made of Cdk and cyclin B. Concentration of cyclin B gradually increase and peaks at start of mitosis and breaks goes down. It pushes to mitosis. (G2 checkpoint, pushing to prophase)

Question 14

Know that cdk’s are kinases. Know that cdk must be phosphorylated on a key residue for it be active. What event allows it to be phosphorylated. What is the role of phosphorylation of TARGET proteins in the positive regulation of the cell cycle by these cdk enzymes?

Answer 14

Binding of cyclin to cdk opens up the active site of cdk, which binds to another kinase, and that kinase adds phosphate to the cdk – this phosphorylates the target protein. The role = push to next step of cell cycle.

Question 15

How does the MPF activity relate to movement into prophase?

Answer 15

MPF activity targets the proteins that push to mitosis and put cell into prophase.

Question 16

What are some of the targets of MPF (i.e. condensin, laminin, MAPs(microtubule associated proteins), etc.)? How does activation of these targets relate to events that would move a cell into prophase/prometaphase?

Answer 16

Condensin (chromatin condensation), laminins (nuclear envelope breakdown), nuclear pore complex, inner nuclear complex, Golgi matrix protein(fragmentation of Golgi apparatus), and centrosome and microtubule-associated proteins (spindle formation).
- They condense chromatin, breakdown the nuclear envelope, and form spindles. (–> what happens in prophase in getting ready of separation)

Question 17

What is the metaphase checkpoint? Understand briefly the role of APC (anaphase-promoting complex) and ubiquitylation in the destruction of cyclinB and securin. How does this relate to progression through the metaphase checkpoint? How does this promote anaphase?

Answer 17

Metaphase checkpoint: checkpoint from metaphase to anaphase, ensuring that cell is ready to divide.
Anaphase promoting complex (APC, ubiquitin ligase)
1) Destroys the MPF
2) Destroys cohesin holding sister chromatids
–> activated when chromosomes are lined up. It attaches ubiquitin units on cyclin. Lead to its destruction

Question 18

What is a proteosome? How does this relate to ubitquitin?

Answer 18

Proteosome: protein complex that works in recycling of proteins to amino acids that had been tagged by ubiquitin
- When proteins get ubiquitinated (added ubitquitin), it gets sent to proteosome to be broken down

Question 19

What is cohesin? Understand how the activity of APC is related to moving a cell into anaphase. How does this relate to securin and separase?

Answer 19

Cohesin: what holds sister chromatids together (cohesin – hold sister chromatids together in the centromere. Centromere is the region where DNA is pinched. They are also in the chromatid.) APC is a ubiquitin ligase – it destroys MPF and cohesin. Separase is normally inactivated by securin but APC adds Ub to securin –> breaking securin down. Active separase chop up cohesin and chromatids separate.

Question 20

Why is the G1/S checkpoint so important to regulate? What is the cell “committed” to do once it passes the G1/S checkpoint?

Answer 20

G1/s checkpoint is important to regulate because once cells pass G1 checkpoint, cells are committed to undergo mitosis –> important that faulty DNA doesn’t get duplicated reproduced.

Question 21

What is a tumor suppressor gene? What is a proto-oncogene? Which one is a positive regulator and which is a negative regulator of the cell cycle? Also know whether mutations would need to activate or inactivate each of these types of genes to be related to cancer

Answer 21

Tumor suppressor gene: negative regulator, “break”. Mutation in this gene =inactivated, (“no break”)
Proto-oncogene: Proto-oncogenes are normal genes that code for positive cell cycle regulators. positive regulator “gas” . Mutation in this gene = activated, always on/cannot turn off (becomes oncogene “too much gas”)

Question 22

What is the role of growth factors in the regulation of the G1/S checkpoint? Which cyclin takes part in the G1/S checkpoint and what effect does growth factor signaling have on this cyclin?

Answer 22

Growth factors activate Ras protein which synthesizes cyclin D.

Question 23

What is Ras? What is its relationship to growth factor signaling? What is its function? Is it a positive or negative regulator? Know that it is a proto-oncogene. How does it get turned into an oncogene?

Answer 23

Ras: DNA gene. It makes Ras protein. Ras is activated by growth factors.

• Function: synthesis of Cyclin D
• Positive regulator. Proto-oncogene
• It gets turned to oncogene when there’s a mutation in ras protein(it is constitutively active) resulting in a tumor formation

Question 24

Understand the role of Rb and E2F in the regulation of the G1/S checkpoint. Which one is a positive regulator and which is a negative regulator?

Answer 24

Rb is one of the target of Cdk/cyclin D complex.
Rb(in its active form) is phosphorylated, becoming inactive. Inactive E2F becomes activated –> E2F turns on DNA synthesis gene (transcription of S phase)
Rb –> negative regulator (break on a cell cycle, keep the E2F inactive)
E2F –> positive regulator

Question 25

22. How do growth factors regulate Rb? What complexes phosphorylates Rb? What is Rb’s relationship with E2F? What is the function of E2F? How does phosphorylation this Rb relate to movement into S phase? Is Rb a positive or negative regulator of the cell cycle? Know that Rb is a tumor suppressor gene.

Answer 25

Growth factor work by inhibiting negative regulator(Rb). Activated growth factor receptor –> activated Cdk –> phosphorylation of Rb. Cyclin D/Cdk complex phosphorylates Rb. Rb keeps E2F inactivated. E2F function : DNA synthesis –> movement into S phase.
Rb is a tumor suppressor gene, a “break”

Question 26

Understand the role of p53 function in causing cell cycle arrest at the G1/S checkpoint. How does DNA damage relate to p53 stability and transcription of p21? What is p21? When does p53 lead to apoptosis? Know that p53 is a tumor suppressor gene. Is p53 considered a positive or negative regulator of the cell cycle?

Answer 26

P53 is activated when there’s any DNA damage. DNA damage activates protein kinase that phosphorylates p53, stabilizing and activating it. P53 binds to p21 gene –> translation and transcription –> p21 protein (Cdk inhibitor. Inactivate cyclin-cdk complex). P53 is a tumor suppressor gene.
P53 lead to apoptosis when there is no DNA damage.

Question 27

24. What is meant by contact inhibition (or density-dependent inhibition)? What is anchorage dependence? What is the relationship of cancer cells to both of these concepts?

Answer 27

Contact inhibition: cells pulled out of cell cycle when they come into contact on all sides
Anchorage dependence: don’t want the cell that is not anchored to anything. Quality control. It needs to be anchored to ECM. All cell need to be attached to ECM – in order to be able to respond to positive signal.
Cancer cells –> cells keep dividing on all sides regardless of contact or anchorage

Question 28

Understand some of the internal or external mechanisms that signal whether progression through a cell cycle checkpoint is appropriate or not? (i.e., growth factors, cell size, DNA damage, mitotic spindle completed, contact inhibition, etc.)

Answer 28

Internal 
-	Cell size
-	DNA damage
-	Mitotic spindle
External 
-	Growth factors
-	Contact inhibition
-	Hypoxia (external factor. Blood on the outside.)

Question 29

What is interphase?

Answer 29

the interval between the end of one mitotic or meiotic division and the beginning of another.: (G1, S, and G2 phase)

Question 30

What happens in Prophase

Answer 30

: chromosome compaction, nuclear envelope starts breaking down, disassemble microtubule & build mitotic spindle

Question 31

What happens in pro-metaphase?

Answer 31

: (in late prophase) nuclear envelope gone, microtubule attach with chromosomes, start moving into center
Starts abruptly with the breakdown of the nuclear envelope. Chromosomes can now attach to spindle microtubules via kinetochores and undergo active movement.
- Sister chromatids develop a protein kinetochore in the centromere region

Question 32

What happens in metaphase?

Answer 32

: cell elongation, sister chromatids remain attached by cohesion proteins, chromosomes line up along metaphase plate

Question 33

What happens in Anaphase?

Answer 33

: pull apart sister chromatids by losing tubulin dimer in the plus end–> now chromosomes, cell elongation

Question 34

What happens in Telophase?

Answer 34

chromosomes reach opposite poles and begin to decondense/unravel, nuclear envelope start forming, nucleosome appear, spindles depolymerize into tubulin monomers that will form cytoskeletal components for the daughter cells

Question 35

What is a centrosome?

Answer 35

Centrosome is a microtubule organizing center. It consists of two centrioles.

Question 36

How many centrosomes are normally in a cell? When are the centrosomes duplicated? Why do they have to be duplicated in terms of cell division?

Answer 36

• one centrosomes normally in a cell, it is duplicated in S phase of interphase.
• they are duplicated so that microtubule can attach to each side of chromosomes and separate (for them to attach at metaphase & separate during anaphase).

Question 37

Understand the behavior of centrosomes during each stage of mitosis.

Answer 37

Centrosome

1) prophase: outside the nucleus, mitotic spindle assembles between the two centrosomes and start moving to opposite poles
2) metaphase: centrosomes at the other polar ends start attaching microtubules to chromosome’s centromere.
3) anaphase: centrosome move chromosomes to center by kinetochore microtubule, motor protein
4) telophase: mitotic spindle disassembles into tubulin monomers that will form cytoskeletal components for the daughter cells

Question 38

How do you tell the difference between prophase and prometaphase? Between metaphase and anaphase?

Answer 38

Prophase – still has nuclear envelope, no connection between chromosomes and centrosome
Prometaphase – complete breakdown of nuclear envelope. As sister chromatids develop kinetochore, sister chromatids can now can make spindle microtubule attach with chromosomes and start moving it to the center.

Metaphase – chromosomes lined up at the metaphase plate
Anaphase – sister chromatids separated, now called chromosomes

Question 39

What is the mitotic spindle? What is a kinetochore?

Answer 39

Mitotic spindle: cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells.
Kinetochore: kinetochore is the disc-shaped protein complex of the chromosome that allows spindle fibers to attach during the cell division.

Question 40

What is the centrosome’s relationship to the spindle?

Answer 40

Mitotic spindles are formed out of centrosome.

Question 41

Know the difference between kinetochore microtubules, interpolar microtubules, and aster microtubules. What role do each play in cell division?

Answer 41

Kinetochore microtubule: microtubules that attach to kinetochore.
- in cell division, motor protein in the kinetochore shortens the kinetochore microtubule pulling each sister chromatids towards opposite poll.
Interpolar microtubule: microtubule that overlaps with other side, move using motor proteins to elongate the cell
Aster microtubule: microtubule that spread around, it pushes back on plasma membrane using motor protein.

Question 42

5. How do the addition and subtraction of tubulin dimers relate to the function of microtubules during cell division - (especially during prometaphase and anaphase)? Which end (plus or minus) of the microtubule do you see the most activity in terms of addition or subtraction of dimers? How does the addition or subtraction of tubulin at this end affect the movement of the chromosomes?

Answer 42

during prometaphase, by addition and subtraction of tubulin dimers, it moves the chromosome to the center. In addition of dimers, rapid addition on plus end. In subtraction, rapid loss on plus end. Slow depolymerization on both ends.

Question 43

Understand the relationship between motor proteins associated with kinetochore microtubules and interpolar microtubules and how this relates to the events of prometaphase and anaphase.

Answer 43

Interpolar microtubules - outward push on spindle poles
Kinetochore microtubules
- Prometaphase. Addition and subtraction of dimer to place chromosomes on metaphase plate
- Anaphase. Loss of tubulin dimers on both sides pulling it towards opposite pole.

Question 44

During mitosis which stage do the sister chromatids become daughter chromosomes?

Answer 44

Anaphase

Question 45

How does the cell elongate during anaphase and telophase?

Answer 45

During Anaphase, cohesin broken down, interpolar microtubules push outward and interpolar microtubules pull outwards.

Question 46

9. What is cytokinesis?

Answer 46

Cytokinesis is separation of cytoplasm between two daughter cells.

Question 47

How does cytokinesis differ in plant cells and animal cells? What is a cleavage furrow and a cell plate? What is the role of microfilaments during cleavage furrow activity? What do the secretory vesicles used during cytokinesis in plants contain?

Answer 47

In animal cells, actin and myosin filaments form contractile ring create cleavage furrow.
As plant cells have rigid cell wall, vesicles form cell plate between two sides. Modified Golgi vesicles that have enzyme start to fuse together to make plasma membrane. Enzyme in those vesicles make cell plate. (contain lipids, proteins and carbohydrates needed for the formation of a new cell boundary)

Question 48

What is the difference between somatic cells and gametes?

Answer 48

Somatic cells : bodily cells, diploid(2n)

Gametes : used for sexual reproduction, haploid(n)

Question 49

11. What is the overall purpose of meiosis? What is the main purpose of meiosis I? What is the main purpose of meiosis II?

Answer 49

Purpose of meiosis is to create genetically different offspring to be used in sexual reproduction.

• Meiosis I, separate homologous chromosome pair
• Meiosis II, separate sister chromatids

Question 50

What are homologous chromosomes? Understand the concept of diploid and haploid. How does this relate to homologous chromosomes? How does this relate to gametes vs. somatic cells?

Answer 50

Homologous chromosomes: set of duplicated pair of maternal and paternal chromosomes.

Question 51

. How many chromosomes does a human somatic cell contain? How many does a human gamete contain?

Answer 51

Somatic cells - 46 chromosomes

Gametes - 23 chromosomes

Question 52

What is Synaptonemal complex?

Answer 52

: structure that form between homologous pair of chromosomes at Prophase I

Question 53

What is Tetrad?

Answer 53

homologous chromosome bind firmly together to form tetrad.

Question 54

What is chiasmata?

Answer 54

point where crossover occurs.

Question 55

What is “crossing over”

Answer 55

at chiasmata, occur during prophase I

Question 56

What is Random assortment?

Answer 56

occurs in Metaphase I. Getting either maternal or paternal on different poles.

Question 57

Understand how crossing over contributes to genetic recombination. Understand how random assortment contributes to genetic recombination. Understand how these two mechanisms generate genetic diversity.

Answer 57

Crossing over contributes to genetic variation as it creates new combination of gene not found in either parent.
Random assortment creates genetic variation by creating unique combination of paternal and maternal genes.

Question 58

How to predict the total number of possible outcomes after meiosis (just take random assortment into account - assume no crossing over occurs).

Answer 58

How many different combinations are possible = 2^n

n=haploid number.

Question 59

How are the events of mitosis and meiosis different from each other? How are they similar? How are the outcomes of these processes different? Which events are unique to meiosis (as compared to mitosis)? What stage(s) of meiosis do these events occur?

Answer 59

Mitosis 
-	One division
-	creates 2 genetically identical daughter cells.
-	somatic cells (2n)
Meiosis 
-	2 divisions (Meiosis I and Meiosis II)
-	creates four genetically dissimilar offspring. 
-	For sexual reproduction, gametes (n)

Events unique to meiosis

• Separation of homologous pair of chromosomes in meiosis I.
• Random assortment in metaphase I
• Cross-over at prophase I