Protein Control of Cell Division

Question 1

What is the cytoskeleton ?

Answer 1

The cytoskeleton gives mechanical support and shape to cells

It consists of different protein structures including microtubules, which are found in all eukaryotic cells, which extend through the cytoplasm

Eukaryotic cells have a membrane-bound nuecleus (DNA contained within a membrane)

Question 2

What are microtubules ?

Answer 2

Microtubules are hollow cylinders composed of the protein tubulin.

They radiate from the microtubule organising centre (MTOC) or centrosome.

MTOC and centrosome same thing

Question 3

What do microtubules do ?

Answer 3

Microtubules control the movement of membrane-bound organelles and chromosomes and the distribution of organelles.

Question 4

How are microtubules made and broken down ?

Answer 4

Formation and breakdown of microtubules involves polymerisation and depolymerisation of tubulin

Question 5

What is the role of microtubules in cell division ?

Answer 5

-Microtubules form the spindle fibres that are active during cell division and pull chromatids apart

-Cell division requires remodelling of the cytoskeleton and

Question 6

What are the two parts of the cell cycle ?

Answer 6

Interphase and mitotic (M) phase

Question 7

What is interphase ?

Answer 7

The cell grows and DNA is synthesised in preparation for cell division

-Interphase involves growth and DNA synthesis including G1, a growth phase (where proteins and organelles are synthesised); S phase, during which the DNA is replicated; and G2, a further growth phase.

Is an active period of growth. The cell will still continue to grow during S phase

Question 8

What is mitotic (M) phase ?

Answer 8

Mitotic phase involves mitosis and cytokinesis

In mitosis the chromosomal material is separated by the spindle microtubules. This is followed by cytokinesis, in which the cytoplasm is separated into two daughter cells.

Question 9

What are the stages of mitosis ?

Answer 9

Mitosis consists of prophase, metaphase, anaphase and telophase

Question 10

Describe the stages of mitosis ?

Answer 10

Prophase
-DNA condenses into chromosomes each consisting of two sister chromatids. Nuclear membrane breaks down; spindle microtubules extend from the MTOC by polymerisation and attach to chromosomes via their kinetochores in the centromere region.

Metaphase
-Chromosomes are aligned at the metaphase plate (equator of the spindle).

Anaphase
-As spindle microtubules shorten by depolymerisation, sister chromatids are separated, and the chromosomes are pulled to opposite poles.

Telophase
-The chromosomes decondense and nuclear membranes are formed around them.

Cytokineses after

Question 11

What is the mitotic index ?

Answer 11

Used to determine the percentage of cells undergoing mitosis and is determines by the tissue type

P + M + A + T
———————– x 100
Total cells

Where P is the number of cells in prophase

Question 12

What controls the cell cycle ?

Answer 12

Progression through the cell cycle is controlled by checkpoints

Question 13

What are checkpoints ?

Answer 13

Checkpoints are mechanisms within the cell that assess the condition of the cell during the cell cycle and halt progression to the next phase until certain requirements are met.

Question 14

Which proteins are involved in regulating the cell cycle

Answer 14

Cyclin proteins that accumulate during cell growth (interphase) are involved in regulating the cell cycle

Cyclins combine with and activate cyclin-dependent kinases (CDKs). Active cyclin-CDK complexes phosphorylate proteins that regulate progression through the cycle. If sufficient phosphorylation is reached, progression occurs.

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

Question 15

What are the three major checkpoints ?

Answer 15

-G1 checkpoint

-G2 checkpoint

-M checkpoint

Question 16

What is the G1 checkpoint ?

Answer 16

-Occurs towards the end of G1

-Sufficient cell growth and other conditions must have occurred and be satisfied before the cell can enter S phase

Question 17

Which protein regulated the G1 checkpoint ?

Answer 17

-Retinoblastoma (Rb) protein is cell cycle regulatory protein which restricts progression from G1 into S phase, when non-phosphorylated.

-At the G1 checkpoint, retinoblastoma protein (Rb) acts as a tumour suppressor by inhibiting the transcription of genes that code for proteins needed for DNA replication (in S phase, so cell stays in G1)

-Phosphorylation by G1 cyclin-CDK inhibits the retinoblastoma protein (Rb) - This allows transcription of the genes that code for proteins needed for DNA replication so cells progress from G1 to S phase.

Is a cell cycle regulatory regulatory protein which, when non-phosphorylated, acts as a tumour suppressor

Question 18

What is the G2 checkpoint ?

Answer 18

-Occurs at the end of G2

-At the G2 checkpoint, the success of DNA replication and any damage to DNA is assessed.

DNA damage triggers the activation of several proteins including p53 that can;
-Stimulate DNA repair
-Arrest the cell cycle
-Cause cell death (apoptosis - must be carefully controlled in a multicellular organism)

p53 is The Guardian of the Genome

Question 19

What is a metaphase checkpoint

Answer 19

-A metaphase checkpoint controls progression from metaphase to anaphase, and occurs during metaphase.

-At the metaphase checkpoint, progression is halted until the chromosomes are aligned correctly on the metaphase plate and attached to the spindle microtubules.

-Ensures that each daughter cells receives the correct number of chromosomes.

Question 20

What can uncontrolled reductions and increases in the rate of the cell cycle lead to ?

Answer 20

-An uncontrolled reduction in the rate of the cell cycle may result in degenerative disease (such as, Alzheimer’s)

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

Question 21

Which proteins are usually involved in the control of cell growth or division.

Answer 21

A proto-oncogene is a normal gene, usually involved in the control of cell growth or division, which can mutate to form a tumour-promoting oncogene

Question 22

What is apoptosis ?

Answer 22

Programmed cell death

Question 23

What triggers apoptosis ?

Answer 23

Apoptosis is triggered by cell death signals that can be external or internal

Question 24

What is an example of an external death signal

Answer 24

The production of death signal molecules from lymphocytes is an example of an external death signal.

Cells may initiate apoptosis in the absence of growth factors

Question 25

What is an example of an internal death signal

Answer 25

DNA damage is an example of an internal death signal.

Question 26

How do external death signal work ?

Answer 26

External death signal molecules bind to a surface receptor protein and trigger a protein cascade within the cytoplasm

Result in the activation of caspases (types of protease enzyme) that cause the destruction of the cell

Question 27

How does an internal death signal work ?

Answer 27

An internal death signal resulting from DNA damage causes activation of p53 tumour-suppressor protein

Result in the activation of caspases (types of protease enzyme) that cause the destruction of the cell

Question 28

Why is apoptosis important

Answer 28

Apoptosis is essential during development of an organism to remove cells no longer required as development progresses or during metamorphosis