20 April 2015

#80 Question 3

(a) The diagrams show a cell in various stages ofthe mitotic cell cycle.

Name the stage represented by each diagram, and arrange them in the correct sequence.

(b) Describe the role of spindle microtubules in mitosis. (3 marks)

(c) The graph below shows the changes in the mass of DNA per cell and total cell mass during two cell cycles. Different vertical scales are used for the two lines.

(i) On the graph, write the letter D to indicate a time at which DNA replication is taking place. 
(1 mark)
(ii) On the graph, write the letter C to indicate a time at which cytokinesis is taking place.
(1 mark)

(d) Describe the roles of mitosis In living organisms.
(3 marks)

Total: 11 marks
Candidate A

(a) A metaphase, ü B prophase, üC telophase, ü D anaphase ü

*  The candidate has named each stage correctly, but has not arranged them in the correct order. 2/3

(b) The spindle microtubules pull the chromatids to opposite ends of the cell. ü

* This is correct, but there is not enough here for three marks. 1/3


* Cytokinesis is identified correctly, but DNA replication is not. The candidate has written D before the DNA has replicated. 1/2

(d) Mitosis is used in growth and repair .ü

* This is correct, but not a good enough answer for more than one mark at AS. 1/3

Candidate B

(a) B prophaseü A metaphase ü D anaphase ü C telophase üü

* All identified correctly, and in the right order. 3/3

(b) Spindle microtubules are made by the centrioles. They latch on to the centromeres üof the chromosomes and help them line up on the equator .ü Then they pull ü on the centromeres so they come apart and they pull the chromatids ü to opposite ends of the cell in anaphase.

* A good answer. 3/3


* Both correct. 2/2

(d) Mitosis produces two daughter cells that are genetically identical ü to the parent cell. Mitosis is used for growth, or for repairing cells. û It is also used in asexual reproduction . ü

* The point about producing genetically Identical cells is a good one, and it is also correct that mitosis is involved in asexual reproduction. However, the candidate's second sentence contains an important error. Mitosis cannot repair cells. Mitosis can produce new cells, which can help to repair tissues. 2/3