1.6 Cell Division

Question 1

Cell Cycle

Answer 1

is a series of events that take place in a cell leading to its division and duplication
it includes both mitosis and cell growth
the stages are:
G1 - period of cell growth before DNA is replicated
S - period when DNA is duplicated
G2 - period after DNA is duplicated, cell prepares for cell division
(these three stages are all part of interphase)
Prophase, Metaphase, Anaphase, Telophase and Cytokinesis

Question 2

Role of Cyclins

Answer 2

cyclins are chemicals that undergo periods of chemical synthesis and breakdown as they interact with the cell cycle and the environment to signal the beginning and end of different stages

Question 3

Why does Mitosis occur?

Answer 3

development and growth: number of cells in an organism increases due to mitosis
cell replacement: in some parts of body cells are constantly needing to be replaced
Asexual reproduction: some organisms produce genetically similar offspring by mitosis

Question 4

What is mitosis?

Answer 4

it is the division of the nucleus into two genetically identical daughter nuclei

Question 5

Interphase

Answer 5

• DNA is present as uncondensed chromatin (not visible under microscope)
• DNA is contained within a clearly defined nucleus
• Centrosomes and other organelles have been duplicated
• Cell is enlarged in preparation for division

Question 6

Prophase

Answer 6

• DNA supercoils and chromosomes condense (becoming visible under microscope)
• Chromosomes are comprised of genetically identical sister chromatids (joined at a centromere)
• Paired centrosomes move to the opposite poles of the cell and form microtubule spindle fibres
• The nuclear membrane breaks down and the nucleus dissolves

Question 7

Metaphase

Answer 7

• Microtubule spindle fibres from both centrosomes connect to the centromere of each chromosome
• Microtubule depolymerisation causes spindle fibres to shorten in length and contract
• This causes chromosomes to align along the centre of the cell (equatorial plane or metaphase plate)

Question 8

Anaphase

Answer 8

• Continued contraction of the spindle fibres causes genetically identical sister chromatids to separate
• Once the chromatids separate, they are each considered an individual chromosome in their own right
• The genetically identical chromosomes move to the opposite poles of the cell

Question 9

Telophase

Answer 9

• Once the two chromosome sets arrive at the poles, spindle fibres dissolve
• Chromosomes decondense (no longer visible under light microscope)
• Nuclear membranes reform around each chromosome set
• Cytokinesis occurs concurrently, splitting the cell into two

Question 10

Cytokinesis differences in Animal and Plant Cells

Answer 10

animals lack a cell wall, while plant cells have a cell wall
so in plants a cell plate beings to grow which will become the new cell wall
in animals the cell membranes on opposite sides of the cell become pinched, forming a cleavage furrow until the two sides are touching

Question 11

Calculating mitotic index

Answer 11

is the % of cells undergoing mitosis
mitotic index = (number of cells in mitosis)/(total number of cells) x 100

Question 12

When do cells become cancerous?

Answer 12

when they lose their ability to stop dividing, to attach to other cells, to stay where they belong and to die at the proper time

Question 13

benign tumours

Answer 13

non cancerous

Question 14

malignant tumours

Answer 14

cancerous

Question 15

Metastasize

Answer 15

ability for cells to spread to other parts of the body

Question 16

Tumour Suppressor Genes

Answer 16

are genes that act as the brakes on uncontrolled cell growth
cancer occurs when both copies of the tumour suppressor genes fail to work properly