U1 KA5 - PROTEIN CONTROL OF CELL DIVSION - 2) cell Cycle And Control Of Cell Cycle

Question 1

What is cell division / what does it allow / what is cell cycle

Answer 1

• cell division allows organisms to grow and develop , to replace dead cells and repair tissue
• cell cycle is continual process , it regulates the growth and replacement of genetically identical cells throughout the life of the organism

Question 2

What does the cell cycle consist of

Answer 2

The two parts
- interphase -cell contents are built up during interphase - consist of G1,S,G2 phases
- mitotic phase -composed of mitosis which is division of nucleus and cytokineses - division of cytoplasm to form two cells
Whole cell cycle is G1, S,G2 , M

Question 3

Why would you expect the cell cycle of a eukaryote to be more complex than that of a prokaryotic cell

Answer 3

• Prokaryote genetic information is stored as a circular chromosome or as plasmids.
• Eukaryote genetic information is packaged as multiple chromosomes – this means cell division is more complex.

Question 4

Cell cycle - Which phase lasts longer

Answer 4

Interphase lasts much longer than the mitotic phase (look at diagrams in notes)

Question 5

What happens when you look at a group of cells in light microscopy (what can you see in terms of cell division)

Answer 5

• when we look at a group of cells by light microscopy , only a small proportion of them are in the mitotic phase
• most appear to be doing nothing - interphase - however this is far from reality : interphase is a very active period of growth :during interphase protein synthesis takes place , cytoplasmic organelles are synthesised , the cell grows and replicates chromosomes

Question 6

Interphase and it’s three sub phases

Answer 6

Interphase is a period of cell growth and DNA replication

1) G1 - is the first GROWTH stage - the cell makes new proteins and copies of organelles (proteins and organelles are synthesised)
2) S phase - during S phase the DNA is replicated in preparation for mitosis
3) G2 - the second GROWTH stage- again the cell makes new proteins and copies of organelles in preparation for mitosis

Question 7

What comes after G2 and how many phases are there

Answer 7

At the end of G2 , cells enter the mitotic phase which is divided into two stages

1) mitosis - when the nucleus and its contents divide (chromosomal material separated by spindle microtubules )
2) cytokinesis - the separation of the cytoplasm into daughter cells

Question 8

What is mitosis and what are the phases

Answer 8

• in mitosis the chromosomal material is separated by the spindle microtubules.
  Mitosis consist of prophase , metaphase , anaphase and telophase. (PMAT)

Question 9

Explain prophase

Answer 9

• DNA condenses into chromosomes , each consisting of two sister chromatids (DNA replication has already occurred)
• nuclear membrane breaks down (to facilitate formation of two nuclei ) : (microtubule start to disassemble and form spindle fibres ) spindle microtubules extend from the MTOC by polymerisation and attach to chromosomes by their kinetichores in the centromere region

Question 10

Explain metaphase

Answer 10

• chromosomes are aligned at the metaphase plate /equator of the spindle (metaphase plate describes the arrangement of the chromosomes at the equator of the cell
  Movement is achieved by polymerisation and depolymerisation of tubulin dimers into the microtubules forming spindle fibres

Question 11

Explain anaphase

Answer 11

• anaphase is a rapid phase
• as spindle microtubules shorten by depolymerisation , sister chromatids are separated and the chromosomes are pulled to opposite ends of the poles (now chromatids are called chromosomes in their own right )

Question 12

Explain telophase

Answer 12

• the chromosomes decondense and nucleur membranes are formed around them
• cytokinesis also occurs during this period - this involves separation of the cytoplasm into two daughter cells

Question 13

Why do microtubules play an important role in cell division

Answer 13

• the spindle fibres allow both the alignment of the chromosomes at the metaphase plate and the separation of chromatids to opposite poles
• the spindle fibres also play an important role in the formation of daughter nuclei , allowing the separated chromatids to group at the opposite poles of the cell

Question 14

Importance of cell cycle :

• the cell cycle has many complex events that have to work perfectly do produce new __________ _____.
• those new __________ _____ have to be produced at the correct rate and in correct locations to allow regulated _____ and ______

Answer 14

The cell cycle has many complex events that all have to work perfectly to produce new daughter cells. Those new daughter cells have to be produced at the correct rate and in the correct locations to allow regulated growth and repair.

Question 15

Importance of cell cycle control : uncontrolled reduction and uncontrolled increase in rate
Of cell cycle
- how can tumours form (talk about the gene :

Answer 15

• an uncontrolled reduction in the rate of the cell cycle may result in degenerative disease. In a degenerative disease , such as Alzheimer’s or Parkinson’s , insufficient replacement cells are being formed for normal tissue function.
• an uncontrolled increase in the rate of cell cycle may result in tumour formation. Uncontrolled growth can result in cancer. 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 16

How is progression through the cell cycle regulated

Answer 16

• progression through the cycle is regulated by checkpoints at G1, G2 and metaphase.

Question 17

What are checkpoints

Answer 17

• there are checkpoints at various stages within the cell cycle , this is where stop and start signals regulate the cycle. Checkpoints are mechanism within the cell that asses the condition of the cell during the cell cycle and halt progression to the next phase until certain requirements are met.

Question 18

Describe G1 checkpoint

• when is it
• what is it monitoring and controlling
• what proteins are involved in regulating the cycle and how

Answer 18

• the G1 checkpoint is near the end of G1.
• here the cell size is monitored - there has to be sufficient cell mass to form two daughter cells and other conditions must be satisfied for it to enter S phase
• this checkpoint controls entry to the S phase
• cyclin proteins that accumulate during cell growth are involved in regulating the cell cycle : cyclin proteins combine with AND activate cyclin dependent kinases (CDKS)
• active CDK complexes phosphorylate proteins that regulate progression through the cycle .
• if a sufficient threshold of phosphorylation is reached the cell cycle moves on to the next stage , if an insufficient threshold is reached the cell is held at a checkpoint.

Question 19

G1 checkpoint - retinoblastoma protein - what does this do

Answer 19

• 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
• the non phosphorylate form of rb restricts progression from G1 to S by acting as a tumour supressor and binding to a transcription factor therefore preventing transcription of certain genes (that code for proteins needed for DNA replication) required for S phase to begin
• the phosphorylation by G1 cyclin CDK of Rb , inhibits the activity of Rb(it can no longer bind to transcription factor) this allows transcription of genes that code for proteins needed for DNA replication allowing cells to progress from G1 to S

Question 20

G2 protein checkpoint

- what is assessed

Answer 20

• at the G2 checkpoint the success of DNA replication and any damage to DNA is assessed
• dna damage triggers activation of proteins that stimulate DNA repair /arrest cell cycle/cause cell death
• a protein activated as a result of DNA damage is p53
• from this checkpoint cells can progress to M phase

Question 21

p53

• what is it
• what is its function
• upon recognising damage to DNA of the cell, what cellular responses can p53 bring

Answer 21

• p53 is is one of the proteins activated as a result of DNA damage
• p53 has been described as “guardian of genome” as its role is to maintain a functional genome
• response of p53:
  + can activate DNA repair proteins to repair DNA damage
  + arrest cell cycle at G1 checkpoint , which means 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 22

p53 and cancer

Answer 22

• the p53 has been found to be missing or faulty in about 50% of cancerous cells - whuch is a reason why cells can divide without being halted at the G1 checkpoint

Question 23

Metaphase checkpoint

• what does it control
• what does it ensure

Answer 23

• a metaphase checkpoint controls progression from metaphase to anaphase. At the metaphase checkpoint progression is halted until the chromosome are aligned correctly on the metaphase plate and attached to the spindle microtubules
• this checkpoint ensures that each daughter cell receives the correct number of chromosomes

Question 24

What is a proto oncogene , and what happens when it mutates to a tumour promoting oncogene

Answer 24

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. These oncogenes cause the cell to divide in an uncontrolled and unregulated manner , for example a cell may continue to grow and divide despite the absence of growth signals
- mutation of just one proto oncogene is enough to cause unregulated growth