routine revision

Question 1

Describe and explain how cell fractionation and Ultracentrifugation can be used to isolate mitochondria from a suspension of animal cells.

Answer 1

1. Cell homogenisation to break open cells;
   Accept suitable method of breaking open cells e.g. blend/grind/chop
2. Filter to remove (large) debris / whole cells;
3. Use isotonic/same water potential solution to prevent mitochondria / organelles bursting/shrinking
   OR
   Use Isotonic/same water potential solution to prevent osmosis
   Reject if reference made to cells
4. Keep cold to prevent / reduce damage by enzymes
   OR
   Keep cold to prevent enzyme activity
5. use buffered solution to prevent protein / enzyme denaturation;
6. Centrifuge (at lower speed / 1000 g) to separate nuclei / cell fragments / heavy
   orzanelles.
7. Re-spin (supernatant / after nuclei / pellet removed) at higher speed to get mitochondria in pellet / at bottom.

Question 2

Describe the appearance and behaviour of chromosomes during mitosis.

Answer 2

(During prophase)
1. Chromosomes
coil / condense / shorten / thicken / become visible;

2. (Chromosomes) appear as (two sister) chromatids joined at the centromere;

(during metaphase)
3. Chromosomes line up on the equator / centre of the cell;
4. (Chromosomes) attached to spindle fibres:
5. by their centromere:

(During anapnase)
6. The centromere splits / divides;
7. (Sister) chromatids / chromosomes are pulled to opposite poles / ends of the cell/ separate

(During telophase)
8. Chromatids / chromosomes
uncoil/ unwind / become longer / thinner.

No marks for naming the stages

reject references to homologous
chromosomes / pairing or chromosomes

Ignore references to spindle formation during prophase

Question 3

Describe how phagocytosis of a virus leads to presentation of its antigens and how this leads to the secretion of an antibody against this virus.

Answer 3

1. Phagosome / vesicle fuses with lysosome;
   Accept vacuole fuses with lysosome
   Reject virus fuses with lysosome
2. (Virus) destroyed by lysozymes / hydrolytic enzymes;
3. Peptides / antigen (from virus) are displayed on the cell membrane.
4. Helper T cell / TH cell binds to the antigen (on the antigen-presenting cell / phagocyte);
5. This helper T / TH cell stimulates a specific B cell;
   ‘Helper’ is required once only.
6. B cell clones
   OR
   B cell divides by mitosis;

‘T cell stimulates B cell to undergo clonal selection. This statement achieves mp5 and mp6.

7. (Forms) plasma cells that release antibodies;

Question 4

Describe how the human immunodeficiency virus (HIV) is replicated once inside helper T cells.

Answer 4

1. RNA converted into DNA using reverse transcriptase;
   Reject ‘messenger’ or ‘m’ before RNA
2. DNA incorporated/inserted into (helper T cell) DNA/chromosome/genome/nucleus;
3. DNA transcribed into (HIV m)RNA;
   Accept descriptions of transcription
4. (HIV MRNA) translated into (new) HIV/viral proteins (for assembly into viral particles);

Accept descriptions of translation

Accept named viral protein, eg capsid
Reject viral cells