1.6 Cell Division

Question 1

What is mitosis?

Answer 1

Mitosis is the division of the nucleus into two genetically identical daughter nuclei. Before mitosis can occur the DNA in the nucleus must be replicated. This happens during interphase. Each chromosome is replicated and then there are 2 identical chromosomes forming a chromatid.

Question 2

What is a chromatid?

Answer 2

The name for when a chromosome has replicated and is in a pair of two that are identical.

Question 3

What is the order of mitosis?

Answer 3

Interphase
Prophase
Metophase
Anaphase
Telophase

Question 4

What happens during interphase?

Answer 4

It is the active part of the cell cycle. Metabolic reactions occur. But the main part is that DNA replication in the nucleus and protein synthesis in the cytoplasm happen. The numbers of mitochondria and cytoplasm increases ready for division.

It consists of 3 phases.
G1 - Cellular contents except the DNA are replicated e.g. organelles.
S - Each of the chromosomes is duplicated.
G2 - This is the checking phase, it checks all proteins and DNA that are needed have been replicated.

Question 5

What happens in prophase?

Answer 5

• The chromosomes supercoil, they condense in length. It occurs by repeatedly coiling the DNA molecule to make the chromosome shorter and wider. Histone proteins help and enzymes are also involved.
• The nucleolus breaks down
• Microtubules grow from structures called microtubule organising centres. to form a spindle shaped array that links the poles of the cell.
• At the end the nuclear membrane breaks down

Question 6

What happens in anaphase?

Answer 6

• Microtubules continue to grow and attach themselves to centromeres on each chromosome. The two attachment points on opposite sides of each centromere allow the chromatids of a chromosome to attach to microtubules from different poles.
• The microtubules are all put under tension to test whether the attachment is correct. This happens by shortening of the microtubules at the centromere. If the attachment is correct, the chromosomes remain on the equator of the cell.

Question 7

What happens in metaphase?

Answer 7

-Each centromere divides, allowing the pairs of sister chromatids to separate. The spindle microtubules pull them rapidly towards the poles of the cell. One of each chromatid is pulled towards each pole this is why they are genetically identical.

Question 8

What happens in telophase?

Answer 8

• The chromatids have reached the poles and are now called chromosomes. At each pole the chromosomes are pulled into a tight group near the MTOC and a nuclear membrane reforms around them.
• The chromosomes uncoil and a nucleolus is formed.

Question 9

How do you work out mitotic index?

Answer 9

Mitotic index = number of cells in mitosis/number of cells overall.

Question 10

What is cytokinesis?

Answer 10

Cytokinesis occurs after mitosis and is different in plant and animal cells.

In animal cells the plasma membrane is pulled inwards around the equator of the cell to form a cleavage furrow. This is accomplished using a ring of contractile protein immediately inside the plasma membrane at the equator. The proteins are actin and myosin and are similar to the proteins that cause contraction in muscles. When the cleavage furrow reaches the centre the cell is pinched apart by two daughter cells.

In animal cells vesicles are moved to the equator where they fuse to form a membrane that is connected to the membrane of the existing cell. The next stage is for pectins and other substances to be brought in vesicles and deposited by exocytosis between the two new cell membranes. This forms the middle lamella that will link the new cell walls. Then they both bring cellulose to the equator and deposit it by exocytosis and as a result each cell builds its own cell wall next to the equator.

Question 11

What are cyclins?

Answer 11

Cyclins are a group of proteins. They ensure tasks are performed at the correct time and that the cell only moves on to the next stage when it is appropriate.

Question 12

How do cyclins work?

Answer 12

These proteins bind to cyclin dependent kinases which are enzymes. Then these enzymes become active and attach phosphate groups to other proteins in the cell so they are activated and carry out the tasks.

Question 13

How were cyclins discovered?

Answer 13

It was accidental. It was during an experiment looking at the protein synthesis in sea urchin eggs Tim Hunt discovered a protein that increased in concentration after fertilisation then decreased in concentration unlike other proteins which just increased. Further investigation showed that it increased for a period of time and then decreased every time.

Question 14

What are tumours?

Answer 14

Tumours are abnormal groups of cells that develop at any stage of life in any part of the body. They divide uncontrollably and are not controlled by the mechanisms that control the cell cycle.
Tumours that do not move are called benign tumours. Other tumours cells can become detached and move elsewhere in the body and create secondary tumours. These are called malignant tumours.

Question 15

What causes tumours?

Answer 15

Chemicals and agents that cause cancer are called carcinogens. There are various types:

• Virus’s
• Chemical mutagens
• High energy radiation - xrays/short wave ultraviolet

Question 16

What are oncogenes?

Answer 16

Mutations are random changes to the base sequence of genes. Most genes do not cause cancer if they mutate. The few genes that do are called oncogenes. In a normal cell they are involved in the control of the cell cycle and cell division. This is why mutations in them can result in uncontrolled cell division and therefore tumour formation.

Question 17

What are mutations?

Answer 17

Mutations are random changes to the base sequence of genes. Most genes do not cause cancer if they mutate. The few genes that do are called oncogenes.

Question 18

What is metastatis?

Answer 18

The movement of cells from a primary tumour to set up secondary tumours in other parts of the body.