6.2- Mitosis

Question 1

What is mitosis and its importance?

Answer 1

Mitosis is the entire process of cell devision in eukaryotic cells.

• refers to the division of the nucleus.
• ensures that both daughter cells produced are genetically identical, have an exact copy of the dna present in the parent cell and the same number of chromosomes.
• necessary for growth, replacement and repair of tissues in multicellular organisms as daughter cells have to be identical.
• also necessary for asexual reproduction.

Question 2

What is asexual reproduction?

Answer 2

Production of genetically identical off spring from one parent in multicellular organisms such as plants, fungi and some animals. Also in single celled organisms.
Bacteria don’t have a nucleus so reproduce asexually by binary fission.

Question 3

What are chromatids and the centromere?

Answer 3

When dna in the nucleus is replicated during interphase, each chromosome is converted into 2 identical dna molecules called chromatids.

The two chromatids are joined together at a region called the centromere. It is necessary to keep the chromatids together during mitosis so they can be precisely manoeuvred and segregated equally, one each into 2 new daughter cells.

Question 4

What are the 4 stages of mitosis and how can they be viewed?

Answer 4

Prophase, metaphase, anaphase, telophase.
They flow seamlessly from one to another.
- can be viewed and identified using a light microscope.
- dividing cells easily obtained from growing root tips of plants. They can be treated with a chemical to allow cells to be separated and then squashed.
- stains that bind to dna used to make chromosomes clearly visible.

Question 5

Explain prophase.

Answer 5

1. Chromatin fibres begin to coil and condense to form chromosomes. Nucleolus disappears. Nuclear membrane starts to break down.
2. Protein microtubules form spindle- shaped structures linking the poles of the cell. Fibres forming spindle are necessary to move chromosomes into correct positions before division.
3. Two centrioles migrate to opposite poles of the cell. (In animals/some plants).
4. Spindle fibres attach to specific areas on the centromeres and move chromosomes to the centre.
5. By the end, nuclear envelope has disappeared.

Question 6

Explain metaphase?

Answer 6

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

Question 7

explain anaphase.

Answer 7

• centromeres holding together the pairs of chromatids in each chromosome divide during anaphase.
• chromatids are separated by being pulled to opposite poles by the shortening spindle fibres.
• ‘V’ shape of chromatids is because they are dragged by their centromeres through the liquid cytosol.

Question 8

Explain telophase.

Answer 8

• the chromatids have reached the poles and are now called chromosomes.
• the 2 new sets of chromosomes assemble at each pole.
• nuclear envelope reforms around them.
• chromosomes start to uncoil, nucleolus formed.
• cytokinesis begins.

Question 9

What is cytokinesis?

Answer 9

The actual division of the cell into two separate cells.

Begins during telophase.

Question 10

Explain cytokinesis in animal cells.

Answer 10

• a cleavage furrow forms around the middle of the cell.
• cell surface membrane pulled inwards by the cytoskeleton until it is close enough to fuse around the middle, forming two new cells.

Question 11

Explain cytokinesis in plant cells.

Answer 11

have cell walls so formation of cleavage furrow not possible.

• vesicles from the Golgi apparatus assemble in the same place as the metaphase plate.
• vesicles fuse with each other and the cell surface membrane, dividing the cell in 2.
• new sections of cell wall form along the new membrane sections.
• (if cell wall is formed before the daughter cells separated, they would immediately undergo osmotic lysis from the surrounding water.)