6.1 The Cell Cycle

Question 1

What is the cell cycle?

Answer 1

A process that all body cells in multicellular organisms use to grow and divide. It takes place in a cell, resulting in division of the cell, and the formation of two genetically identical daughter cells

Question 2

What are the two main phases of the cell cycle?

Answer 2

Interphase and M (mitotic) phase

Question 3

Describe what happens at interphase.

What are the three stages of interphase?

Answer 3

Interphase is the period of cell growth and DNA replication. The cell spends majority of its time in this phase.

During interphase:

• DNA is replicated and checked for errors in the nucleus
• protein synthesis occurs in the cytoplasm
• mitochondria grow and divide, increasing in number in the cytoplasm
• chloroplasts grow and divide in plant and algal cytoplasm
• the normal metabolic processes of cell occur (some, including cell respiration also occur throughout cell division)

Three stages of interphase:

• G1 - the first growth phase: proteins from which organelles are synthesised are produced and organelles replicated. The cell increases in size.
• S - synthesis phase: DNA is replicated in the nucleus
• G2 - the second growth phase: the cell continues to increase in size, energy stores are increased and the duplicated DNA is checked for errors

Question 4

State what happens at the mitotic phase

Answer 4

The mitotic phase is the period of cell division
Cell division involve two stages:
- mitosis
- cytokinesis

Question 5

What is the G0 phase? What are reasons a cell enters the G0 phase?

Answer 5

G0 is the name given to the phase when the cell leaves the cycle, either temporarily or permanently
There are a number of reasons for this:
- Differentiation - a cell that becomes specialised to carry out a particular function is no longer able to divide and cannot enter the cycle again

• the DNA of a cell may be damaged, in which case it is no longer viable. A damaged cell can no longer divide and enters a period of permanent cell arrest. The majority of normal cells only divide a limited number of times and eventually become senescent
• as you age, the number of senescent cells increases. Growing number of senescent cells have been linked with many age related diseases, e.g. cancer

A few types of cells that enter G0 can be stimulated to go back into the cell cycle and start dividing agin e.g. lymphocytes in an immune response

Question 6

How is cell cycle controlled?

Answer 6

Checkpoints occur at keep points in the cycle too make sure its ok for the process to continue:
- G1 checkpoint: this checkpoint is at the end of the G1 phase, before entry into the S phase. If the cell satisfies the requirements of this checkpoint it is triggered to begin DNA replication. If not it enters a resting state (G0)

• G2 checkpoint: this checkpoint is at the end of G2 phase, before the start of the mitotic phase. In order for this checkpoint to be passed, the cell has to check a number of factors including whether the DNA has been replicated without error. If this checkpoint is passed, the cell signals the beginning of mitosis
• spindle assembly checkpoint (metaphase checkpoint): the checkpoint is at the point in mitosis where all the chromosomes should be attached to spindles and have aligned. Mitosis cannot proceed until this checkpoint is passed.