8.3 Cell Division

Question 1

What is needed to make a mitotic spindle?

Answer 1

2 microtubule organizing centers

Question 2

What happens in the S phase?

Answer 2

the replication of centrosomes occurs in the S phase. The S phase is also where DNA is replicated

Question 3

How do centrioles replicated in animal cells?

Answer 3

These structures have an interesting replication cycle of their own in which a daughter centriole buds off of the mother centriole at a right angle. It is not entirely clear how this develops.

once the cell has two centrosomes, they push each other apart during the very first stages of mitosis, and that uses microtubule motor proteins.

Question 4

What two things must happen in prophase?

Answer 4

the formation of chromosomes from the disassembled
DNA that’s present in an interphase cell and also the disassembly of the nuclear envelope, which allows the mitotic spindle microtubules to interact with the chromosomes

Question 5

What two proteins help with the formation of chromosomes in prophase?

Answer 5

cohesins and condensins which are motor like proteins and in structure they are hinge shaped dimers that have two ATP dependent binding sites.

Question 6

What is the role of condensin and cohesions in prophase?

Answer 6

Each DNA strand is formed into an individual chromatid by the activity of the condensins. (condense and package chromosomes)

In addition, each pair of chromatids is held together by cohesins. the cohesions are found mostly in
the areas of the chromosomes that will attach to the mitotic spindle microtubules.

Question 7

What happens to proteins as the cell begins entry into mitosis?

Answer 7

Many proteins are phosphorylated during the entry of the cell into mitosis.

The stabilizing MAPs are deactivated by phosphorylation, and the catastrophins are activated. As a consequence, the turnover rate of microtubules is greatly increased in a mitotic cell versus an interphase cell. This is probably because the microtubule cytoskeleton needs to rapidly adapt to the large changes in the shape and function of the mitotic spindle during mitosis.

Question 8

In addition to the breakdown of the interphase microtubule array, what other interphase structures are broken down during prophase?

Answer 8

membrane vesicle structures like the endoplasmic reticulum and the golgi bodies

this probably ensures that at least some parts of each organelle type end up in
each daughter cell.

Question 9

What marks the end of prophase and the beginning of metaphase?

Answer 9

The breakdown of the nuclear envelope is what marks the end of prophase and the beginning of metaphase

Question 10

what happens if you have two dyenins motor proteins that move toward the minus end of the microtubules?

Answer 10

As they do that they are going to bring those minus ends of the microtubules they are moving on together
This is what happens in some organisms that form mitotic spindles without centrosomes, like plant cells. The minus ends get collected together and as a
consequence the plus ends are outward
(Generation of minus end focus)

Question 11

What happens if motor (dyenins) proteins are interacting with microtubules which are attached to opposite poles, not the same pole?

Answer 11

You would get sliding of antiparallel microtubules (multimetric plus end directed motor protein)

Question 12

What are interzonal microtubules?

Answer 12

interacting microtubules that are bridging the distance between the two centrosomes

Question 13

What is an astral microtubule?

Answer 13

microtubules that are splayed out away from the

poles are called astral microtubules.

Question 14

What would happen if you activated dynein-like motors in the interzone microtubules forming mitotic spindle ?

Answer 14

the result would be that the poles would be pulled together as each part of the dimer of dyneins tries to move toward the minus end of the microtubule, which is in the centrosome. On the other hand, if you put kinesin-like motors in that interzonal area, their action as they try to move towards the plus ends of the microtubules would be to push the microtubules apart.

Question 15

Multimeric minus-end directed motors and Multimeric plus-end directed motors do?

Answer 15

Multimeric minus-end directed motors assemble each spindle pole

Multimeric plus-end directed motors separate the spindle poles

Question 16

What is the effect of kinesin motors on the plus end of microtubules?

Answer 16

kinesin motors is going to group the

plus ends of microtubules together as they move.

Question 17

What are kinetochore microtubules?

Answer 17

microtubules which attach to the chromosomes

The microtubule plus ends are embedded in a special part of the chromosome called the kinetochore. These are end-on attachments.

Kinetochores contain minus end dynein.
Kinetochores also capture and stabilize microtubules and move to the metaphase plate.

Question 18

how do chromosomes attach to the microtubules?

Answer 18

to help chromosomes attach to the mitotic spindle, there are dynein motors at the kinetochore which attach to microtubules just the way organelles attach to microtubules with dynein motors

This initial attachment is used to move chromosomes toward the spindle
pole, which is where the minus end of the microtubules are. This brings the chromosome toward
the mitotic pole

as the chromosomes move toward the pole, the density of microtubule plus ends gets higher and higher because that’s where the microtubules are being grown from. The probability of attaching end-on starts to increase. So the initial attachment of chromosomes is usually a lateral attachment which allows the chromosomes to be transported along the microtubule toward the pole

Question 19

Describe the steps of chromosomes attaching to microtubule in five steps:

Answer 19

1) Unattached chromosome at prometaphase
2) kinetochore binds laterally to an astral microtubule
3) Chromosome slides toward spindle pole
4) lateral attachment coverts end-on unipolar attachment; microtubule now called a kinetochore microtubule
5) Free kinetochore captures microtubule from opposite spindle pole to convert unipolar to bipolar attachment.

Dynein motor activity accomplishes this task

Question 20

The rapid movement of a chromosome toward a spindle pole following the initial capture helps
to create end-on attachments of the chromosome to the spindle microtubules. However, this also
moves the chromosome away from the opposite pole and it decreases the chance of encountering
a microtubule from the other pole, and that has to be resolved somehow because chromosomes
need to get attached to both poles. How is this problem solved?

Answer 20

forces move a captured chromosome away from the pole that had captured it

This force is thought to be generated by kinesin-like motor proteins which are attached to chromosome arms and so they are going to move the chromosome back out toward the center of the mitotic spindle. This is called a polar ejection force, and it helps move chromosomes toward the opposite spindle pole so
the microtubules on the other pole can interact with the chromosome and catch it as well.

Without this force it’s likely that chromosomes would end up randomly at the pole that initially captures them, and both chromatids would stay there, which is not a good thing.

Question 21

What happens in metaphase?

Answer 21

the attached chromosomes line up at the middle of the mitotic spindle.

the position of the chromosomes in the middle of the center of the mitotic spindle is actually a balance of pushing and pulling forces that’s basically a tug of war between the two poles.

The metaphase spindle translates this pulling force into information that’s used to ensure that all of the chromosomes are where they’re supposed be before the cell is allowed to go into anaphase. So the metaphase is really a critical decision making point for a mitotic cell.

Question 22

What is the anaphase promoting complex, or APC?

Answer 22

The initial events of anaphase involve the separation of
the chromatin pairs. This is accomplished by APC

APC, is activated by interaction with a cell division cycle protein called CDC 20

Question 23

How does anaphase promoting complex initiate anaphase?

Answer 23

APC leads to the destruction or degradation of a protein called securin. Securin is an inactivator of a protease, so it inactivates separase.

however, the destruction of securin releases separase, which is then activated. The activity of separase cuts the cohesins proteins that link sister chromatids together. This is releases the chromatids so that they can move to the opposite poles.

REMEMBER:
that securin is released and that that leads to the activation of separase, and separase actually cuts up the cohesins.

Question 24

What happens if cohesins aren’t cut up by separase in anaphase?

Answer 24

although the signal to enter anaphase is generated by the mitotic spindle in metaphase, the sister chromatids can’t separate and mitosis is consequently defective.

AKA cell division won’t occur.

and the signals that lead to separation of the cell into two new cells is therefore not generated properly. You end up with a single cell that has all the chromosomes.

Question 25

What happens in the anaphase stage of mitosis?

Answer 25

Anaphase is the stage of mitosis where the chromatids are separated. Chromosomes move to the poles at the end of anaphase (this is known as anaphase A)

the mitotic spindle poles, actually move further apart. That separation of the poles is known as anaphase B

Question 26

What is the difference between anaphase A and B?

Answer 26

The kinetochore microtubules are getting shorter to
accomplish anaphase A,

but the interzonal microtubules are getting longer to make anaphase B happen.

Question 27

Does anaphase A and B happen at the same rate?

Answer 27

Mammalian cells, like the one in this video, perform both anaphase A and anaphase B about equally. However, cells found in different organisms emphasize one or the other of these two mechanisms to varying degrees. So some cells do mostly anaphase A and very little anaphase B and some weird cells do all anaphase B and hardly any anaphase A.

Question 28

How do kinetochore microtubules get shorter during anaphase A ?

Answer 28

by disassembly at both ends of the microtubule.
microtubules are exhibiting some sort of slow
treadmilling behavior in which they are disassembling at the minus end. It’s thought that this movement makes some kind of contribution to the movement of chromosomes toward the poles in anaphase at least in some cells.

This is kind of unusual in that dynamic instability, which we discussed in other lectures, involves both assembly and disassembly only at the plus end of the microtubule.

Question 29

Most of the force that’s used to move chromosomes during anaphase appears to be generated from where?

Answer 29

at the plus end of the microtubules, the kinetochore microtubules

it’s thought that some kinds of a sliding collar of protein is what actually attaches each microtubule to the kinetochore, and that’s shown on the diagrams on this slide. As the kinetochore microtubules disassemble, the splaying out of the filaments, the protofilaments,
is actually what is thought to generate the force that slides the collar down toward the myosins at
the microtubules. that force is generated without using ATPs, so it is an ATP independent force, whereas dynein motor proteins of course use ATP

Question 30

Anaphase B involves the movement of spindle poles away from each other. how does this movement happen?

Answer 30

This movement involves both the movement of multimeric kinesin motors toward the plus ends of the interzonal microtubules, as well as the elongation of the interzonal microtubules themselves

Question 31

What is telophase?

Answer 31

Telophase is the transition from the mitotic state back to the interphase state. The phosphorylation state of proteins that was upregulated during the entry into mitosis is reversed, with both deactivation of mitotic phase kinases and the activation of protein phosphatases playing a role.

Deactivation of kinases allows phosphatases to reduce phosphorylation of proteins (such as MAPs, actin associated proteins, focal adhesions, nuclear laminins etc) in telophase. This in turn allows reformation of the nucleus and reattachment of the cell to it’s substrate.

Question 32

Reassembly of the nuclear lamins around chromosomes following the completion of mitosis, occurs as a consequence of what?

Answer 32

as a consequence of the dephosphorylation of the nuclear lamins. One of the things that’s interesting about this process is that the nuclear envelope forms tightly around the chromosomes initially, it’s not like it forms a big sphere with the chromosomes hanging around inside of it.

Question 33

What is cytokinesis?

Answer 33

the process that completes the division of
the mother cell’s cytoplasm into two daughter cells. Although this generally happens in the
center of the dividing cell and results in two equal sized cells, this is not always the case and
there are many examples of unequal cell divisions which result in daughter cells that are different
sizes.

Question 34

What determines where an animal cell will divide?

Answer 34

the position of the mitotic spindle

displacement of the mitotic spindle to one side of the cell resulted in cytokinesis of only one half of the cell, that was the half that had the mitotic spindle. So this
clearly showed that the mitotic spindle was what determines where a cell is going to divide if its
an animal cell.

Question 35

What is a contractile ring?

Answer 35

In animal cells, cytokinesis is accomplished by the formation of a unique structure called the
contractile ring. The early constriction of the ring results in a formation of a structure on the cell that’s called a cleavage furrow

actin and myosin II filaments make the contractile ring. in the interzonal microtubules

Question 36

What generates the cleavage furrow?

Answer 36

Myosin II activity generates a cleavage furrow and then a
final mid-body of tightly packed microtubules. Asymmetric divisions are dictated by positioning
the spindle to one side of a cell.

Question 37

What creates the midbody structure?

Answer 37

In many animal cells the contractile ring closes down onto the remnants of the mitotic spindle
and it creates a narrow channel of cytoplasm between the two daughter cells containing the
interdigitated interzonal microtubules. That structure is called a midbody.

Question 38

How do plants accomplish cytokinesis?

Answer 38

During interphase the growing plant cell will construct an array of microtubulessurrounding the cell periphery. This is called the preprophase band. This generates some sort ofsignal that causes the cell wall in that region to specialize; so the cell kind of remembers where the preprophase band was. The preprophase band then disassembles but this site is where the
cell ultimately will divide during cytokinesis. At the end of anaphase another structure forms at
this site, which extends through the cell at the plane of cell division. Much like the midbody of animal cells, this structure also contains interdigitating microtubules in actin filaments. This structure is called a phragmoplast and its function is to guide the deposition of vesicles
containing new cell wall components to the site of cytokinesis, where they will fuse and form a
new cell wall. The formation of the cell wall proceeds outward from the middle of the cell, and
the transient detached structure is called a cell plate. Eventually the cell plate will fuse with the rest of the cell wall and it will just be another cell wall.

Question 39

What is a phragmoplast?

Answer 39

in animal cells it is the midbody. it’s function is to guide the deposition of vesicles containing new cell wall components to the site of cytokinesis, where they will fuse and form a new cell wall.

Question 40

Can cell division occur in the absence of cytokinesis? What is an example?

Answer 40

Cell division in the absence of cytokinesis occurs in some developmental events

EX: epithelial cell layer in an early Drosophila larvae need to replicate as quickly as possible, so they don’t bother with cytokinesis at all, they just divide with no separation of the cytoplasm. Only after enough cells have been generated to make an epithelial tissue will they undergo cytokinesis to form the final epithelial layer with discrete cells.

Question 41

Summary of mitosis:

Answer 41

INTERPHASE: two microtubule organizing centers to make mitotic spindle and replicate centrosome

PROPHASE: formation of chromosomes from DNA (condensins and cohesions) and disassembly of Nuclear Envelope.

METAPHASE: attached chromosomes line up at the middle of the mitotic spindle.

ANAPHASE: separation of chromatin pairs (securing activates separase and separase cuts up the cohesins)

ANAPHASE A: minus end force moves chromosomes toward spindle poles. Shortening microtubules occur both at kinetochore and spindle pole

ANAPHASE B: Movement of spindle poles away from each other.Plus end slides microtubules apart. Interzonal microtubules elongate during this process to increase the distance of separation.

TELOPHASE: Transition from mitotic state back to the interphase. Deactivation of kinases allows reduction of phosphorylation of proteins like MAPs, focal adhesions, nuclear lamins in telophase. This in turn allows reformation fo the nucleus and reattachment of the cell to its substrate.

CYTOKINESIS: the process that completes the division of the mother cells cytoplasm into two daughter cells.

Question 42

How do spindle poles become separated in prophase?

Answer 42

by microtubule pushing and plus-edn directed motors (kinesins)

Question 43

Multimeric minus-end directed motors in prophase do what?

Answer 43

Assemble each spindle pole

Question 44

Multimeric plus-end directed motors do what in prometaphase?

Answer 44

separate each spindle pole

Question 45

Astral microtubules do what?

Answer 45

position the spindle in the cell center in prophase

Question 46

What do dynein motors do at the kinetochore?

Answer 46

They attache to microtubules and move chromosomes toward the spindle pole (- end). This brings microtubules toward the mitotic pole.

Question 47

What increases the probability of end on attachment in prometaphase?

Answer 47

dyneins moving the chromosomes toward sspindle pole, which causes the density of microtubule plus ends to get higher b/c that’s where microtubules are being grown.

Initial attachment of chromosomes is lateral attachment. and this allows chromosomes to be transported along microtubule toward the pole using dyneins.

Question 48

What is the polar ejection force?

Answer 48

force that move a captured chromosome away from the pole that captured it. Force is generated by kinesin-like motor proteins and move the chromosome back toward the center of the mitotic spindle