1.6 Cell Division

Question 1

The process of mitosis simplified

Answer 1

• Mitosis is the division of the nucleus into two genetically identical daughter nuclei
• Before mitosis occurs all DNA is replicated during interphase
• Each DNA molecule is converted into two identical DNA molecules called chromatids
• During mitosis one of the chromatids pass to each daughter nucleus

Question 2

What happens to some cells in interphase (don’t progress)

Answer 2

• Some cells do not progress beyond G1 but enter G0 temporarily or permanently

Question 3

Supercoiling of chromosomes

Answer 3

• DNA molecules are very long (50,000 uM) therefore they must be packaged efficiently to become shorter structures, this is known as condensation of chromosomes and occurs during the first stage of mitosis
• Condensation occurs by repeatedly coiling the DNA molecule to make the chromosome shorter and wider, this process is called supercoiling
• Proteins called histones help with the supercoiling and enzymes are also involved

Question 4

the process of cytokinesis

Answer 4

• In animal cells: the plasma membrane is pulled inwards around the equator of the cell to form a cleavage furrow, this process uses contractile proteins; actin, myosin
• When the cleavage furrow reaches the centre the cell is pinched apart into two daughter cells
• In plant cells: vesicles move to the equator where they fuse to form tubular structures across the equator
• The fusion of many vesicles makes the tubular structures create two layers of membrane across the whole of the equator which develop into the plasma membranes of the two daughter cells and are connected to the existing plasma membranes at the sides of the cell, this completes the division of the cytoplasm
• Pectins and other substances are brought in by vesicles and exocytosis between the two new membranes, this forms the lamella. Both daughter cells bring cellulose to the equator and deposit it by exocytosis adjacent to the middle lamella, as a result each cell wall builds its own cell wall adjacent to the equator

Question 5

What are cyclins and what do they do

Answer 5

• Important phases within the cell cycle are controlled by a group of proteins called cyclins
• Cyclins insure that tasks are performed at the correct time and the cell only moves onto the next stage when appropriate
• Cyclins bind to enzymes called cyclin-dependant kinases, these kinases then become active and attach phosphate groups to other proteins, the phosphate attachment triggers other proteins in the cell to become active and carry out tasks specific to one of the phases of the cell cycle

Question 6

What causes tumours

Answer 6

• Mutagens, oncogenes, and metastasis are involved in the development of primary and secondary tumours

Question 7

What are the four phases of mitosis

Answer 7

Prophase, Metaphase, Anaphase, Telophase

Question 8

The role of mitosis

Answer 8

• Mitosis is the division of the nucleus into two genetically identical daughter nuclei
• Mitosis is used in cell replication: embryonic development, growth, tissue repair and asexual reproduction

Question 9

What is the cell cycle + two main phases

Answer 9

• The cell cycle is the sequence of events between one cell division and the next
• Two main phases: interphase, cell division

Question 10

what happens during interphase

Answer 10

• During interphase many metabolic reactions occur, as well as during cell division
• DNA replication in the nucleus and protein synthesis in the cytoplasm only occur during interphase
• During interphase the number of mitochondria in the cytoplasm increase due to growth and division of pre-existing mitochondria
• In plant cells algae and chloroplasts increase in the same way
• They also synthesize cellulose and add it to their cell wall

Question 11

What are the three phases of interphase

Answer 11

G1 phase, S phase and G2 phase

Question 12

What happens in S phase

Answer 12

• In S phase the cell replicates all genetic material in its nucleus

Question 13

when does cytokinesis occur

Answer 13

• Cytokinesis occurs after mitosis and is different in plant and animal cells
• Cytokinesis happens just before the end of mitosis

Question 14

what is cytokinesis

Answer 14

Cytokinesis is the division of a cell with two identical nuclei into two daughter cells with identical nuclei

Question 15

what are the four main types of cyclins in the human cell

Answer 15

Cyclin D, Cyclin E, Cyclin A, Cyclin B

Question 16

What does cyclin D do?

Answer 16

• Cyclin D triggers cells to move from G0 to G1 and from G1 to S phase

Question 17

What does Cyclin E do?

Answer 17

• Cyclin E prepares the cell for DNA replication in S phase

Question 18

What does cyclin A do?

Answer 18

• Cyclin A activates DNA replication inside the nucleus in S phase

Question 19

What does cyclin B do?

Answer 19

• Cyclin B promotes the assembly of the mitotic spindle and other tasks in the cytoplasm to prepare for mitosis

Question 20

What are tumours

Answer 20

• Tumours are abnormal groups of cells in the body, when they do not invade nearby tissues or move to other parts of the body they are called benign and are relatively harmless
• Other tumours that become detached and move to other parts of the body are malignant, create secondary tumours and are often life-threatening
• Diseases due to malignant tumours are known as cancer

Question 21

What causes cancer + what are mutagens

Answer 21

• Carcinogens cause cancer
• All mutagens are carcinogenic
• Mutagens are agents that cause gene mutations and mutations cause cancer
• Mutations are random changes to the base sequence of genes
• The few genes that become cancerous after mutating are called oncogenes
• Oncogenes are involved in control of the cell cycle and cell division this is why mutation can lead to uncontrolled cell division
• Several mutations must occur in the same cell for it to become tumorous this is very unlikely but given the number of cells in the human body it does happen

Question 22

what is metastisis

Answer 22

Metastasis is the movement of cells from a primary tumour to set up a secondary tumour in other parts of the body

Question 23

Example: smoking and cancer

- the correlation between smoking and incidence of cancers

Answer 23

• there is a positive correlation between smoking and cancer
• studies show that the more cigarettes smoked per day the higher death rate due to cancer
• studies show a higher death rate among people that smoked but had stopped
• results show large increases in death rates due to cancer of the; mouth, pharynx, larynx and lung
• smokers are more likely to die from cancer than non-smokers
• 20 substances in cigarette smoke have been shown to cause tumours in the lungs
• 40 other substances in cigarette smoke are carcinogenic