Unit 1- Cell Cycle

Question 1

What is the cell cycle made up of?

Answer 1

Interphase and mitosis

Question 2

What is the order and the three stages of interphase?

Answer 2

G1, S, G2

Question 3

Describe G1 stage

Answer 3

This is the first stage of growth and the cell makes new proteins and copies of the organelles.

Question 4

Describe S stage

Answer 4

This is when DNA replication occurs.

Question 5

Describe G2 stage

Answer 5

This is the second growth period of the cell and again the cell makes more proteins and copies the organelles in preparation for mitosis.

Question 6

Describe mitosis and cytokinesis

Answer 6

Prophase: This is when the chromosomes condense and coil up, becoming visible in the nucleus as two joined sister chromatids. Spindle fibres form and attach to centromeres.
Metaphase: Chromosomes become aligned along the metaphase plate at the equator of the cell.
Anaphase: This involves the disassembly of the kinetochore end of the spindle fibre. This pulls the sister chromatids apart.
Telophase: The separated chromosomes are pulled by the spindle fibres to opposite poles to form daughter nuclei. The chromosomes start to uncoil and a nuclear membrane is made again.
Cytokinesis: This is the division of the cytoplasm to form two daughter cells.

Question 7

Why is it important to be able to control the cell cycle?

Answer 7

An uncontrolled increase could result in tumour formation and an uncontrolled decrease would result in degenerative diseases such as Alzheimers or Parkinsons’.

Question 8

Describe the G1 checkpoint

Answer 8

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. This checkpoint controls entry to the S phase.

Question 9

Describe the G2 checkpoint

Answer 9

The G2 checkpoint is at the end of G2. This assesses the success of DNA replication to make sure each daughter cell can receive a complete copy of the DNA. This checkpoint controls entry to mitosis.

Question 10

Describe the M checkpoint

Answer 10

The M checkpoint is during metaphase. This monitors the chromosome alignment to ensure each daughter cell receives one chromatid from each chromosome. This checkpoint controls entry to anaphase. Thus, it triggers the exit from mitosis and the start of cytokinesis.

Question 11

Explain how cyclin dependent kinases help regulate the cell cycle

Answer 11

As the cell size increases during G1, cyclin proteins accumulate and combine with regulatory proteins called cyclin-dependent kinases (CDKs). The binding of cyclin to CDK results in the activation of CDKs. Active CDKs cause the phosphorylation of target proteins that stimulate the cell cycle. If a sufficient threshold level of phosphorylation is reached, the checkpoint is passed with a ‘go ahead’ and the cell cycle moves on to the next stage.

Question 12

Explain the role of the retinoblastoma protein in regulation of the cell cycle

Answer 12

The retinoblastoma (Rb) protein is a transcription factor inhibitor. These transcription factors are involved in the production of proteins that are required for DNA replication in S phase. With low levels of CDK activity, Rb binds to the transcription factor E2F, stopping the genes required to enter the S phase from being transcribed. If the activity of CDK reaches a sufficient threshold (where Rb has been repeatedly phosphorylated 14 times) then the Rb protein can no longer bind to the transcription factors and releases them to promote the transcription of genes required for DNA replication in the S phase.

Question 13

Explain the role of the p53 protein

Answer 13

p53 protein is a transcription factor that can stimulate DNA repair, arrest the cell cycle or trigger cell death.
If DNA damage has occurred p53 causes the expression of genes that stimulate DNA repair and arrest the cell cycle. If DNA repair is successful then the cell cycle can continue once again. If the damaged DNA cannot be repaired, then p53 triggers apoptosis of the cell.

Question 14

Define apoptosis

Answer 14

Programmed cell death.

Question 15

What is apoptosis triggered by?

Answer 15

Cell death signals which can come from inside or outside the cell.

Question 16

Give an example of a cell death signal from outside the cell.

Answer 16

A lymphocyte that has detected abnormal behavior of the cell.

Question 17

Give an example of a cell death signal from inside the cell.

Answer 17

Presence of the p53 protein as a result of DNA damage or as a result of the absence of cell growth factors.

Question 18

Describe the caspase cascade.

Answer 18

• This involves a series of post translational modifications to proteins that are already present in the cytoplasm. This allows a rapid response to occur even in circumstances when the cell’s nucleus is out of action.
• The post translational modifications involve cleavage and subunits are removed so they can reassemble to form active caspases.
• The cascade involves inactive caspases becoming activated.
• Each initiator caspase at the beginning of the cascade activates several executioner caspases.
• These act as DNAase, proteinases and enzymes that destroy keratin and actin filaments.
• Each executioner caspase degrades over 600 cellular components and this growing cascade destroys the cell.
• Once the cellular components have been destroyed, the cell can no longer function. The lack of intact cytoskeleton means that the cell breaks into many small vesicle fragments. These are rapidly engulfed and digested by phagocytic white blood cells.

Question 19

Describe the structure and role of spindle fibres in cell division

Answer 19

Cell division requires the remodelling of the cytoskeleton. Spindle fibres are made of microtubules made of tubulin and they are hollow straight rods. During cell division they radiate from the centrosome and attached to centromeres. The spindle fibres contract and shorten separating the chromatids.

Question 20

Describe the structure and function of actin microfilaments in the cell’s cytoskeleton

Answer 20

They are polymers of globular protein actin consisting of two strands wrapped around each other. Actin microfilaments are responsible for cellular movement.

Question 21

Describe the structure and function of intermediate filaments in the cell’s cytoskeleton

Answer 21

They are formed from fibrous protein and are rope like in structure consisting of 2 monomers wrapped around each other. They provide tremendous mechanical strength.

Question 22

Describe the structure and function of microtubules in the cell’s cytoskeleton

Answer 22

They are hollow straight rods that are polymers of a dimer made from alpha and beta tubulin. They govern the location and movement of membrane bound organelle and other cell components. Cell division requires the remodelling of the cell’s microtubules.

Question 23

What is the G0 phase?

Answer 23

Cells enter this when a go-ahead signal is not reached at the G1 checkpoint, this is a non-dividing stage. In this phase cyclin proteins are not produces so the cell has left the cell cycle and is said to resting. The cell is still active but is no longer involved in cell division. It may return to the cell cycle if conditions change.