Protein Control of Cell Division

Question 1

What does a eukaryotic cell have?

Answer 1

It has a network of proteins extending throughout the cytoplasm, known as the cytoskeleton.

Question 2

What is important to note about the cytoskeleton?

Answer 2

The cytoskeleton is anchored to proteins in the plasma membrane and is dynamic in nature, constantly breaking down and re-forming.

Question 3

What are the functions of the cytoskeleton?

Answer 3

• provide mechanical support so that the cell maintains its shape;
• provide anchorage for many organelles and some enzymes;
• enable the whole cell to move;
• enable organelles within the cell to move.

Question 4

What is the cytoskeleton is made up of?

Answer 4

The cytoskeleton is made up of different types of protein including microfilaments, intermediate filaments and microtubules.

Question 5

What are microfilaments?

Answer 5

Microfilaments are the smallest filaments of the cytoskeleton and include the protein actin.

Question 6

What are intermediate filaments?

Answer 6

Intermediate filaments are slightly larger than microfilaments and have a similar role in maintaining cell shape.

Question 7

What are microtubules?

Answer 7

Microtubules are hollow, straight cylinders composed of globular proteins called tubulins. The microtubule itself is made up of alternating dimers of α and β tubulin. Microtubules govern the location and movement of membrane-bound organelles and other cell components.

Question 8

Where are microtubules found?

Answer 8

Microtubules are found in all eukaryotic cells

Question 9

Where do microtubules originate from?

Answer 9

originate from the centrosome (microtubule organising centre (MTOC)).

Question 10

Where is the centrosome found?

Answer 10

The centrosome is found near the nucleus and contains centrioles, which are the site of microtubule synthesis within the centrosome.

Question 11

What do microtubules play an important role in?

Answer 11

Microtubules play an important role in cell division as this process requires remodelling of the cell’s cytoskeleton. Microtubules also form the spindle fibres, which are active during cell division.

Question 12

What is the cell cycle?

Answer 12

The reproduction of cells, from when the cell is produced by division of the mother cell until the new cell itself divides.

Question 13

What are the 2 parts of the cell cycle?

Answer 13

1. Interphase.

2. The mitotic phase (M Phase)

Question 14

What are the 3 sub-phases interphase is broken up into?

Answer 14

G1, S & G2

Question 15

What is the G1 phase?

Answer 15

G1 is the first ‘gap’ phase; it is a growth period where proteins and organelles are synthesised.

Question 16

What is the S phase?

Answer 16

During S phase the cell continues to grow and copies its chromosomes in preparation for mitosis. (DNA Replication)

Question 17

What is the G2 phase?

Answer 17

The final phase is G2 (the second ‘gap’ phase); this is another growth period during which proteins and organelles are synthesised. This is followed by a checkpoint.

Question 18

What is the M phase split up into?

Answer 18

Mitosis - when the nucleus and its contents divide.

Cytokinesis - the separation of the cytoplasm into daughter cells.

Question 19

What can an uncontrolled reduction in the rate of the cell cycle result in? Give examples.

Answer 19

An uncontrolled reduction in the rate of the cell cycle may result in degenerative disease. e.g. Alzheimer’s disease, heart disease, osteoporosis, muscular dystrophy.

Question 20

What can an uncontrolled increase in the rate of the cell cycle result in?

Answer 20

An uncontrolled increase in the rate of the cell cycle may result in tumour formation.

Question 21

What are the 4 phases of mitosis?

Answer 21

Prophase
Metaphase
Anaphase
Telophase

Question 22

Describe what happens in prophase.

Answer 22

• Chromatin starts to condense into discrete chromosomes. Each duplicated chromosome appears as two identical sister chromatids joined at the centromere.
• In the cytoplasm, the mitotic spindle begins to form between the two centrosomes. This process pushes the centrosomes away from each other along the surface of the nucleus.

Question 23

Describe what happens in metaphase.

Answer 23

• In the initial stage (prometaphase), the nuclear membrane breaks up. Bundles of microtubules extend from each pole toward the equator of the cell, and attach to the centromere of each chromatid.
• As the cell enters metaphase, the centrosomes are now at opposite poles of the cell.
• The chromosomes line up on the metaphase plate in the middle of the cell. The term metaphase plate is used to describe the arrangement of the chromosomes at the equator of the cell.
• Each of the sister chromatids in each chromosome is attached to microtubules from opposite ends of the parent cell at a point called the kinetochore.

Question 24

Describe what happens in anaphase.

Answer 24

• The paired centromeres of each chromosome separate and the chromatids begin moving apart as the spindle shortens. The poles also move further apart. Each chromatid can now be called a chromosome.
• By the end of anaphase, the two poles of the cell each have an identical and complete set of chromosomes.

Question 25

Describe what happens in telophase.

Answer 25

• The cell lengthens during telophase and a nuclear membrane forms around each set of chromosomes. The chromosomes start to uncoil.
• Cytokinesis also takes place during this period, which involves the separation of the cytoplasm into two daughter cells.

Question 26

Why must the cell cycle be controlled?

Answer 26

to ensure that events in the cell cycle proceed in the correct order and that each event is completed before the next starts.

Question 27

What do checkpoints do?

Answer 27

Checkpoints register internal and external cell signals that report whether crucial processes up to that point have been completed successfully or not, and whether the cycle should proceed.

Question 28

What are the three major checkpoints?

Answer 28

G1 checkpoint
G2 checkpoint
M checkpoint

Question 29

Describe G1 checkpoint.

Answer 29

occurs towards the end of G1; sufficient cell growth must have occurred. If sufficiently large to allow division, the cell enters S phase and DNA replication begin.

Question 30

Describe G2 checkpoint.

Answer 30

occurs at the end of G2; DNA replication must be completed for the cell cycle to continue.

Question 31

Describe M checkpoint.

Answer 31

occurs during metaphase and controls entry to anaphase. The M checkpoint checks that chromosomes are aligned correctly on the metaphase plate and, therefore, ensures that each daughter cell receives the correct number of chromosomes.

Question 32

In mammalian cells, what is said to be the most important checkpoint?

Answer 32

G1

Question 33

What are the two possible outcomes from the G1 chekpoint?

Answer 33

If a cell receives the ‘go ahead’ signal at this checkpoint, it will usually complete the cycle and divide.
If not, it will exit the cycle and switch to a non-dividing state called the G0 phase. At any one time, most somatic cells are in the G0 phase.

Question 34

What is important to note during the G0 phase?

Answer 34

the cell is not dividing or preparing to divide; it is known as a quiescent (non-proliferating) phase.

Question 35

What does the control of the cell cycle involve?

Answer 35

many regulatory proteins

Question 36

What happens as cell size increases during G1?

Answer 36

cyclin proteins accumulate and combine with kinases to form regulatory protein molecules which are known as cyclin-dependent kinases (CDKs).

Question 37

What do CDKs cause?

Answer 37

CDKs cause the phosphorylation of proteins that stimulate the cell cycle.

Question 38

What happens if a sufficient threshold of phosphorylation is reached?

Answer 38

If a sufficient threshold of phosphorylation is reached, the cell cycle moves on to the next stage.

Question 39

What happens if an insufficient threshold of phosphorylation is reached?

Answer 39

If an insufficient threshold is reached, the cell is held at a checkpoint

Question 40

What is Retinoblastoma (Rb)?

Answer 40

Retinoblastoma (Rb) is a cell cycle regulatory protein.

Question 41

Describe the function of the non-phosphorylated form of Rb.

Answer 41

The non-phosphorylated form of Rb restricts progression from G1 phase into S phase. It acts by binding to a transcription factor, therefore preventing transcription of certain genes required for S phase to begin; thus the cell remains in G1.

Question 42

Describe the function of phosphorylated Rb.

Answer 42

When a cell is about to enter S phase, the G1 CDKs phosphorylate the retinoblastoma protein. Phosphorylation inhibits the activity of Rb, meaning that it can no longer bind the transcription factor. The transcription factor is released and brings about transcription of the genes that are required to initiate DNA replication, allowing the cell to enter S phase.

Question 43

What can cell cycle checkpoints asses?

Answer 43

Cell cycle checkpoints can assess damage to a cell’s DNA and prevent it from continuing the cell cycle.

Question 44

What does DNA damage trigger?

Answer 44

DNA damage triggers the activation of several proteins that can stimulate DNA repair, arrest the cell cycle or cause cell death.

Question 45

Name a protein acitvated as a result of DNA damage.

Answer 45

p53.

Question 46

What is the p53 protein able to do? (3)

Answer 46

• it can activate DNA repair proteins to repair the DNA damage;
• it can arrest the cell cycle at the G1 checkpoint, which means that the cell cycle halts at this point - this can allow DNA repair proteins time to recognise and fix the DNA damage, so the cell can restart the cell cycle;
• if the DNA damage is too severe, it can initiate apoptosis (programmed cell death).

Question 47

What is apoptosis triggered by?

Answer 47

Apoptosis is triggered by cell death signals which result in the activation of DNAases and a variety of proteinases (collectively known as caspases).

Question 48

What do DNAases do?

Answer 48

DNAases catalyse the breakdown of DNA by hydrolysing phosphodiester bonds of the backbone.

Question 49

What do proteinases do?

Answer 49

bring about degradation of cellular proteins

Question 50

What happens to the cell fragments produced by apoptosis?

Answer 50

The cell fragments produced by apoptosis (known as apoptotic bodies) are engulfed and destroyed by phagocytes.

Question 51

What are the 2 types of cell death signals?

Answer 51

extrinsic and intrinsic

Question 52

Explain extrinsic signals (give example).

Answer 52

Cell death signals may originate outwith the cell (extrinsic signals), for example from lymphocytes. Cytotoxic T-lymphocytes express a death activator ligand on their surface called Fas. When a cell death ligand such as Fas binds to its surface receptor protein on a target cell, it activates a protein cascade that produces active caspases. The activated caspases then bring about apoptosis.

Question 53

Explain intrinsic signals (give example).

Answer 53

Death signals may also originate within the cell (intrinsic signals). For example, as a result of DNA damage, the presence of p53 protein can activate a caspase cascade. In the absence of cell growth factors, cells may also initiate apoptosis.