1.6 (Cell Division)

Question 1

Why do cells divide?

Answer 1

Growth
Asexual reproduction
Tissue repair
Embryonic development

Question 2

What is the cell cycle?

Answer 2

The series of events through which cells pass to divide and create two identical daughter cells.

Question 3

Describe interphase.

Answer 3

G1 (Gap 1):
Increase the volume of cytoplasm
Organelles produced
Proteins synthesised
S (Synthesis):
DNA replicated
G2 (Gap 2):
Increase the volume of cytoplasm
Organelles produced
Proteins synthesised

Question 4

What happens during interphase?

Answer 4

Metabolic reactions- (e.g. respiration to produce ATP) are necessary for the life of the cell
Protein synthesis - proteins and enzymes are necessary to allow cell grow
Organelles numbers are increased to first support the enlarged cell
DNA is replicated to ensure a second copy is available to enable mitosis

Question 5

Describe prophase.

Answer 5

• The centrosomes move to opposite poles of the cell and spindle fibres begin to form between them
• DNA supercoils- chromatin condenses and becomes sister chromatids, which are visible under a light microscope
• The nuclear membrane is broken down and disappears

Question 6

Describe metaphase.

Answer 6

• Spindle fibres from each of the two centrosomes attach to the centromere of each pair of sister chromatids
• Contraction of the microtubule spindle fibres cause the sister chromatids to line up along the centre of the cell.

Question 7

Describe anaphase.

Answer 7

• Continued contraction of the microtubule spindle fibres cause the separation of the sister chromatids
• The chromatids are now referred to as chromosomes
• Chromosomes move to the opposite poles of the cell

Question 8

Describe telophase.

Answer 8

• The chromosomes uncoil de-condense to chromatin (and are no longer visible under a light microscope).
• Chromosomes arrive at the poles.
• Microtubule spindle fibers disappear
• New nuclear membranes reform around each set of chromosomes

Question 9

What is cytokinesis in comparison to mitosis?

Answer 9

Mitosis is the division of the nucleus whereas

cytokinesis is the division of the cytoplasm

Question 10

Describe cytokinesis in animal cells.

Answer 10

• Ring of contractile protein immediately inside the plasma membrane at the equator pulls the plasma membrane inward
• Inward pull produces cleavage furrow
• When cleavage furrow reaaches the centre of the cells it is pinched apart to form two daughter cells

Question 11

Describe cytokinesis in plant cells.

Answer 11

• In telophase membrane enclosed vesicles derived from the golgi apparatus migrate to the centre of the cell
• Vesicles fuse to form tubular structures
• The tubular structures merge to form two layers of plasma membrane
• Cell plate continues to develop, connecting with cell’s plasma membrane
• Completes division of the cytoplasm

Question 12

Why do chromosomes supercoil?

Answer 12

Chromosomes need to be stored compactly to fit within the nuclei of cells.

Question 13

How do cells supercoil?

Answer 13

• Strain is placed on a DNA helix by overwinding or underwinding of the helix
• This causes the DNA molecule to coil back on itself becoming shorter and wider

Question 14

What are cyclins?

Answer 14

A family of proteins that control the progression of cells through the cell cycle.

Question 15

How are cyclins involved in the control of the cell cyle?

Answer 15

1. Cells cannot progress to the next stage of the cell cycle unless the specific cyclin reaches it threshold.
2. Cyclins bind to enzymes called cyclin-dependent kinases
3. These kinases then become active and attach phosphate groups to other proteins in the cell.
4. The attachment of phosphate triggers the other proteins to become active and carry out tasks (specific to one of the phases of the cell cycle).

Question 16

Explain ecah specific cyclin.

Answer 16

Cyclin A: activates DNA replication inside the nucleus in S phase.
Cyclin B: promotes the assembly of the mitotic spindle and other tasks in the cytoplasm to prepare for mitosis.
Cyclin D: Triggers cells to move from G0 to G1 and from G1 into S phase.
Cyclin E: prepares the cell for DNA replication in S phase.

Question 17

What is a tumour?

Answer 17

Abnormal growth of tissue that develop at any stage of life in any part of the body.

Question 18

What is cancer?

Answer 18

A malignant tumour and is named after the part of the body where the cancer (primary tumour) first develops.

Question 19

Outline the development of primary tumours.

Answer 19

1. A primary tumor is a malignant tumor growing at the site where the abnormal growth first occurred.
2. Cancerous cells can detach from the primary tumour.
3. Some cancerous cells gain the ability to penetrate the walls of lymph or blood vessels and hence circulate around the body
4. The circulating cancerous cells invade tissues at a different locations and develop, by uncontrolled cell division, into a secondary tumours.
5. Metastasis is the movement of cells from a primary tumour to set up secondary tumours in other parts of the body.

Question 20

What is a mutation?

Answer 20

A change in an organisms genetic code. A mutation/change in the base sequence of a certain genes can result in cancer.

Question 21

What is a mutagen?

Answer 21

Agents that cause gene mutations. Not all mutations result in cancers, but anything that causes a mutation has the potential to cause a cancer.

Question 22

Give examples of what mutagens can be.

Answer 22

• Chemicals that cause mutations are referred to as carcinogens
• High energy radiation such as X-rays
• Short-wave ultraviolet light
• Some viruses

Question 23

What is an oncogene?

Answer 23

In normal cells oncogenes control of the cell cycle and cell division.

Question 24

Outline how oncogenes can become cancerous.

Answer 24

1. Mutation in a oncogene
2. Malfunction in the control of the cell cycle
3. Uncontrolled cell division
4. Tumour formation

Question 25

What factors other than mutagen exposure increase the probabilit of tumour development?

Answer 25

• The vast number of cells in a human body – the greater the number of cells the greater the chance of a mutation.
• The longer a life span the greater the chance of a mutation.