A4. Cell Division - Mitosis

Question 1

The Cell Cycle - in 4 stages

Figure 1: Stages of the cell cycle.

Answer 1

Interphase:

• GAP PHASE 1
  cell grows and new organelles and proteins are made
• SYNTHESIS
  cell replicates its DNA, ready to divide by mitosis
• GAP PHASE 2
  cell keeps growing and proteins needed for cell division are made

MITOSIS
(the cycle starts and ends here)

Question 2

Interphase (3 things)

Answer 2

• During interphase the cell carries out normal functions, but also prepares to divide.
• The cell’s DNA is unravelled and replicated, to double its genetic content.
• The organelles are also replicated so it has spare ones, and its ATP content is increased (ATP provides the energy needed for cell division).

Question 3

Mitosis

What happens?
Why is it needed?

Answer 3

In mitosis, a parent cell divides to produce two genetically identical daughter cells (they contain an exact copy of the DNA of the parent cell). Mitosis is needed for the growth of multicellular organisms (like us) and for repairing damaged tissues.

Question 4

The structure of chromosomes in mitosis (5 points)

Answer 4

• As mitosis begins, the chromosomes are made of two strands joined in the middle by a centromere.
• The separate strands are called chromatids.
• Two strands on the same chromosome are called sister chromatids.
• There are two strands because each chromosome has already made an identical copy of itself during interphase.
• When mitosis is over, the chromatids end up as one-strand chromosomes in the new daughter cells.

Question 5

1. Prophase (3 points)

Answer 5

1. Prophase

• The chromosomes condense, getting shorter and fatter.
• Tiny bundles of protein called centrioles start moving to opposite ends of the cell, forming a spindle across it.
• The nuclear envelope breaks down and chromosomes lie free in the cytoplasm.

Question 6

2. Metaphase (2 points)

Answer 6

2. Metaphase

• The chromosomes (each with two chromatids) line up along the equator of the cell
• They become attached to the spindle by their centromere.

Question 7

3. Anaphase (2 points)

Answer 7

3. Anaphase

• The centromeres divide, separating each pair of sister chromatids.
• The spindles contract, pulling chromatids to opposite poles (ends) of the spindle, centromere first.

Question 8

4. Telophase (4 points)

Answer 8

• The chromatids reach the opposite poles on the spindle.
• They uncoil and become long and thin again and become chromosomes again.
• A nuclear envelope forms around each group of chromosomes, so there are now two nuclei.
• Division of the cytoplasm (cytokinesis, which starts in anaphase) finishes in telophase. There are now two daughter cells that are genetically identical to the original cell and to each other. Mitosis is finished and each daughter cell starts the interphase part of the cell cycle to get ready for the next round of mitosis.

Question 9

A scientist observes a section of growing tissue under the microscope. He counts 100 cells undergoing mitosis. Of those, 10 cells are in metaphase. One complete cell cycle of the tissue lasts 15 hours. How long do the cells spend in metaphase? Give your answer in minutes.

• The scientist has observed that 10 out of 100 cells are in metaphase. This suggests that the proportion of time the cells spend in metaphase must be 10/100th of the cell cycle.
• You’re told that the cell cycle in these cells lasts 15 hours.
  So the cells spend:

Answer 9

10/100 x 900 = 90 minutes in metaphase.

Question 10

Cancer treatments
Some treatments for cancer are designed to control the rate of ____ __________in __________ _____by disrupting the cell cycle. This kills the tumour cells. These treatments don’t distinguish tumour cells from normal cells though - they also kill normal body cells that are dividing. However, tumour cells divide much more frequently than normal cells, so the treatments are more likely to kill tumour cells.

Answer 10

Some treatments for cancer are designed to control the rate of cell division in tumour cells by disrupting the cell cycle. This kills the tumour cells. These treatments don’t distinguish tumour cells from normal cells though - they also kill normal body cells that are dividing. However, tumour cells divide much more frequently than normal cells, so the treatments are more likely to kill tumour cells.

Question 11

Some cell cycle targets of cancer treatments include:

Answer 11

• G1 (cell growth and protein production)
  Some chemical drugs (chemotherapy) prevent the synthesis of enzymes needed for DNA replication. If these aren’t produced, the cell is unable to enter the synthesis phase (S), disrupting the cell cycle and forcing the cell to kill itself.
• S phase (DNA replication)
  Radiation and some drugs damage DNA. At several points in the cell cycle, the DNA in the cell is checked for damage. If severe DNA damage is detected, the cell will kill itself-preventing further tumour growth.