1.6 Cell division

Question 1

The cell cycle

Answer 1

1. Interphase: G1, S, G2
2. M phase: mitosis and cytokinesis

Question 2

Interphase

Answer 2

• Preparation for division
• The most active and the longest stage of cell division
• Cells will spend most of their life within this stage undergoing common cell processes such as metabolism, endocytosis, exocytosis and using and obtaining nutrients.
• Interphase has three phases: G1 (growth phase 1), S (synthesis), and G2 (growth phase 2)

Question 3

G1 - growth phase 1

Interphase

Answer 3

• Location: cytoplasm
• Cell enlargement
• Cell doubles its organelles
• Protein synthesis for DNA synthesis (in next stage)
• Accumulates materials needed for DNA synthesis

Question 4

S (synthesis)

Interphase

Answer 4

• Location: nucleus
• The amount of DNA doubles as DNA replication takes place
• At the beginning each chromosome is composed of one DNA molecule called a chromatid. At the end of this stage each chromosome consists of two identical DNA molecules, two sister chromatids.

Question 5

G2 (growth phase 2)

Interphase

Answer 5

• Location: cytoplasm
• Protein synthesis occurs to produce the proteins needed for cell division, such as microtubule proteins that will make up a mitotic spindle.

Question 6

Cyclins

Answer 6

• A family of proteins that control the progression of cells through the cell cycle.
• Cells cannot progress to the next stage of the cell cycle unless a specific cyclin reaches a certain concentration.
• Cyclins bind to enzymes called cyclin-dependant kinases (CDKs) and activate them.
• They attach to other proteins in the cell which triggers them to become active and carry out tasks specific to one of the phases of the cell cycle.
• Cyclins –> cyclin-dependant kinasas –> other proteins

Question 7

Cyclin D

Answer 7

• Triggers cells to move grom G0 to G1 to S phase.

Question 8

Cyclin E

Answer 8

• S phase
• Prepares cell for DNA replication

Question 9

Cyclin A

Answer 9

• S phase –> G2 phase
• Activates DNA repication inside the nucleus

Question 10

Cyclin B

Answer 10

• Mitosis
• Essential for the formation of mitotic spindles and the alignment of sister chromatids

Question 11

Mitosis

Answer 11

A type of cell divison that results in two identical daughter cells (as they have an identical daughter nuclei).

Question 12

Identical daughter cells

Mitosis

Answer 12

• During mitosis the replicated chromosomes (sister chromatids) separate and move to opposite poles of the cell, thus providing the same genetic material at each of these locations.
• When the chromosomes are at the poles of the cell the cytoplasm divides to form the two daughter cells, that both have the same genetic material as the parent cell.

Question 13

Prophase

First stage of mitosis

Answer 13

• The DNA (which is loosely packed in the nucleus as unravelled chromatin) supercoils (the DNA condenses into chromosomes - so that it’s easily segregated)
• The nuclear envelope breaks down into vesicles
• Spindle fibers form
• The centrioles (absent from plant cells) move to opposite poles.

In eukaryotes proteins called histones help with the supercoiling.

Question 14

Metaphase

Second stage of mitosis

Answer 14

• The spindle fibers align to their centromeres
• The sister chromatids align at the equatorial plate

Question 15

Anaphase

Third stage of mitosis

Answer 15

• The sister chromatids are separated and pulled to opposite poles by the spindle fibers

Question 16

Telophase

Last stage of mitosis

Answer 16

• The daughter chromosomes have reached the poles
• The spindle fibers desintegrate
• A nuclear envelope starts to reform at each pole
• The cell elongates in preparation for cytokinesis
• In some cases, the cell membrane can be seen pinching inwardly (the beginning of cytokinesis).

Question 17

Mitotic index

Answer 17

The ratio of the number of cells in a population undergoing mitosis (prophase, metaphase, anaphase or telophase) to the total number of visible cells.
- It indicates how many cells in a tissue are dividing at a given time
- In a tumour where cell division is uncontrolled, the mitotic index is higher than in a normal tissue
- Therefore, we can use the mitotic index to predict how quickly a cancer could spread and the likely outcome a treatment like chemotherapy would have in reducing cell proliferation.

Mitotic index = number of cells in mitosis/total number of cells

Question 18

Cytokinesis

Answer 18

The division of the cytoplasm between the two daughter cells after mitosis (though it often starts in telophase).

Question 19

Cytokinesis in animal cells

Answer 19

• A ring of protein located beneath the plasma membrane at the equator pulls the membrane inwards forming a cleavage furrow.
• When the furrow reaches the center, the cell is pinched apart into two daughter cells.

Question 20

Cytokinesis in plants

Answer 20

• The Golgi apparatus produces vesicles which move to the region of the disk.
• More vesicles arrive and they fuse together to form a cell plate.
• The cell plate extends across the whole equator and becomes the plasma membranes of the two daughter cells.
• Each daughter cell will bring cellulose to the equator and deposit it by exocytosis so each cell builds its own cell wall.

Question 21

Tumourigenesis

Answer 21

The formation of a tumour (or several of them), which is defined as a mass of cells that divide uncontrollably.
There are two types of tumours: benign and malignant.

Not all tumours lead to cancer. Cancer is caused by a malignant tumour.

Question 22

Benign tumours

Answer 22

• Usually localised
• Does not spead to other parts of the body (they are primary tumours).
• Most benign tumours respond well to treatment.

Question 23

Malignant tumours

Answer 23

• A cancerous growth that is often resistant to treatment.
• It may spread to other parts of the body and sometimes recur after it has been removed.
  Primary tumours become deatached and move elsewhere in the body to develop secondary tumours - metastasis

Question 24

Mutation

Answer 24

• A change in the DNA sequence of an organism.
• A change in the base sequence of a some genes can result in tumour formation.
• Not all gene mutations lead to uncontrolled cell division.

Question 25

Mutagens

Answer 25

Agents that cause gene mutations.
- High-energy radiation such as X-rays
- Short-wave ultraviolet light
- Certain chemicals
- Some viruses such as hepatitis B

Note: mutagens that lead to the formation of cancer are caller carcinogens (e.g. dioxin, asbestos)

Question 26

Oncogene

Answer 26

A gene that has undergone a mutation that will contribute to the development of a tumour.
- In their normal non mutated state, oncogenes are termed proto-oncogens. These assist in the regulation of cell division.
- This is why mutations in them can result in uncontrolled cell division and therefore tumour formation.

Remember: not all genes cause cancer if they mutate, only oncogenes.

Question 27

Metastasis

Answer 27

The movement of cells from a primary tumour to other parts of the body where they develop into secondary tumours.
Steps:
1. Cancerous cells deatatch from the primary tumour.
2. Some cancerous cells gain the ability to penetrate the walls of lymph or blood vessels and so circulate around the body
3. The circulating cancerous cells invade tissues at different locations and develop, by uncontrolled cell division, into secondary tumours.

Question 28

Smoking and cancer

Answer 28

• There is a positive correlation between cigarette smoking and the death rate due to cancer.
• The more cigarettes smoked per day the higher the death rate due to cancer.
• Finding that there is a positive correlation between smoking and cancer does not prove that smoking causes cancer.
• However, in this case the causal links are well established. Cigarette smoke contains many different chemicals and twenty of these have been shown in experiments to cause lung tumours.
• There is also evidence that at least forty other chemicals in cigarette smoke are carcinogenic (cancer causing). This leaves little doubt that smoking is a cause of cancer.