6.1- Cell Cycle

Question 1

What is the cell cycle?

Answer 1

An ordered sequence of events that takes place in cells, resulting in division of the cell, and the formation of two genetically identical daughter cells.

Question 2

Explain interphase.

Answer 2

• cells do not divide continuously.
• it is a long period of growth and carrying out separate functions. Cell spends majority of its time in this phase.
• takes place between devisions. (Not a stage in division).
  During interphase:
• dna replicated + checked for errors in nucleus.
• protein synthesis occurs in cytoplasm.
• mitochondria grow + divide.
• chloroplasts grow + divide (in plant cells).
• normal metabolic processes occur (e.g respiration).

Question 3

What are the 3 stages of interphase?

Answer 3

G1- first growth phase. Proteins from which organelles are synthesised are produced. Organelles replicate. Cell increases in size.

S- synthesis phase. Dna replicated in nucleus.

G2- second growth phase- cell continues to increase in size, energy stores increased. Duplicated dna checked for errors.

Question 4

What is the mitotic phase?

Answer 4

The period of cell devision. Has 2 stages:

1. Mitosis- nucleus divides.
2. Cytokinesis- cytoplasm divides to form 2 cells.

Question 5

What is G0 ?

Answer 5

The phase where the cell leaves the cycle, either temporarily or permanently.
Reasons for this:
1. differentiation- cell becomes specialised to a particular function and is no longer able to divide.
2. DNA may be damaged- a damaged cell cannot divide and enters a period of permanent cell arrest (G0).
3. As you age, the number of cells in your body increases. Growing numbers of senescent cells have been linked to diseases (cancer/arthritis).

Some cell types that enter G0 can be stimulated to go back to cell cycle and divide again. (Eg. Lymphocytes)

Question 6

What must be controlled about the cell cycle?

Answer 6

It is vital to ensure that the cell only divides when:

• it has grown to the right size.
• the replicated dna is error free.
• chromosomes are in their correct positions (during mitosis).

This ensures the fidelity of cell division- that 2 daughter cells are created from the parent cell.

Question 7

What are check points?

Answer 7

The control mechanisms of the cell cycle.

Monitor and verify whether processes at each stage have been completed accurately before progressing to next stage.

Question 8

What is the first checkpoint and its role?

Answer 8

G1 checkpoint:

• checkpoint at the end of G1 phase (before S phase).
• checks for; cell size, nutrients, growth factors, dna damage.
• if these are satisfied it triggers dna replication. If not it enter G0 (resting state).

Question 9

What is the second checkpoint and its role?

Answer 9

G2 checkpoint:

• the checkpoint at the end of G2 phase. Before the start of mitotic phase.
• checks for; cell size, dna replication (without error), dna damage.
• if these are satisfied, cell initiates the molecular processes that signal the start of mitosis.

Question 10

What is the final checkpoint?

Answer 10

Spindle assembly/metaphase checkpoint:

• at the metaphase stage of mitosis.
• checked for chromosomes to be attached to spindle fibres.
• mitosis can’t proceed until this is passed.