Mitosis (c,e)

Question 1

What is mitosis?

Answer 1

The division of the nucleus

Question 2

Why is mitosis important?

Answer 2

• it ensures genetically identical daughter cells
• necessary in growth, replacement and repair of tissues in multicellular organisms
• necessary for asexual reproduction in unicellular and multicellular organisms

Question 3

What happens to chromosomes due to DNA replication + why?

Answer 3

• each chromosome is converted into 2 identical chromatids joined together by the centromere
• it is necessary to keep the chromatids together during mitosis so that they can be precisely manoeuvred + segregated equally, one each into the 2 new daughter cells.

Question 4

How can dividing cells be obtained (practical)?

Answer 4

• from growing root tips of plants
• can be treated with a chemical to allow the cells to separate - then can be squashed to form a single layer of cells on a microscope slide
• stains that bind DNA are used to make the chromosomes clearly visible

Question 5

Describe prophase

Answer 5

• chromosomes condense getting shorter + fatter
• centrioles start moving to opposite ends of the cell, forming a network of protein fibres across it - the spindle
• nuclear envelope breaks down + chromosomes lie free in the cytoplasm
• spindle fibres attach to centromeres

Question 6

Describe metaphase

Answer 6

• chromosomes are moved by spindle fibres to form a plane in the centre of the cell - metaphase plate - then held in position

Question 7

Describe anaphase

Answer 7

• centromeres divide, separating each pair of sister chromatids
• spindle fibres shorten, pulling chromatids to opposite ends of the cell by their centromeres through liquid cytosol

Question 8

Describe telophase

Answer 8

• chromatids have reached the poles and are chromosomes again
• 2 new sets of chromosomes assemble at each pole + nuclear envelope reforms around them
• chromosomes uncoil becoming long and thing again

Question 9

Cytokinesis in animals

Answer 9

• cleavage furrow forms around middle of the cell
• cell-surface membrane is pulled inwards by the cytoskeleton until close enough to fuse around the middle, forming 2 cells

Question 10

Cytokinesis in plants

Answer 10

• vesicles from the Golgi apparatus begin to assemble in the same place the metaphase plate was formed
• the vesicles fuse with each other + the cell-surface membrane, dividing the cell into 2
• new sections of cell wall form along new membrane