A-LevelBiologyGenetic technology9700/51 · May/June 2019 · Q29 Marks

Artificial insemination (AI) is one method used in assisted reproduction programmes for large mammals. For many years, horse breeders have collected semen samples from male horses and used these to inseminate female horses. Success rates have been good. However, due to the sample containing live sperm cells, the process needed to be carried out quickly. New technologies exist to allow horse semen to be frozen in small plastic straws. Semen samples can now be stored for many years and transported all over the world. • Each 0.5 cm³ straw can hold 7.5 × 10⁷ sperm cells. • A typical sample of horse semen contains 7.5 × 10⁹ sperm cells. The semen from a horse is analysed by technicians to determine its quality before the horse is accepted onto the assisted reproduction programme. Technicians use microscopy to look at the appearance and motility of the sperm cells and to estimate the sperm count. Fig. 2.1 shows the surface view of a haemocytometer used to estimate the number of sperm cells in a sample. Each number represents a sperm cell which was counted in the sample. Each star (*) represents a sperm cell not counted in the sample. The depth of the haemocytometer is 0.1 mm. [Figure 2.1]

(a)(i)Calculate the volume of a typical horse semen sample.[1]
(a)(ii)To inseminate one female horse, 5.0 × 10⁸ sperm cells are needed. Calculate the minimum number of straws needed to carry out this process.[1]
(b)Suggest how this apparatus could be used to estimate the number of sperm cells per cm³ of semen and describe how the technician decided which sperm cells to include in the count.[4]
(c)To ensure that the sperm cells remain viable after they have been frozen, a solution is added to the sample before freezing the semen in the straws. The solution contains: • a sugar • a buffer to maintain pH • antibiotics. Explain why it is necessary for the solution to contain these three substances.[3]

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Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[1] 50 (cm³);
[1] 7 (straws);
[4] any 4 of: estimate of numbers: 1. calculate the volume of the sample or volume of sample is, (for mm³) 0.25 × 0.25 × 0.1 / 0.00625 / 6.25 × 10−3 / (for cm³) 0.025 × 0.025 × 0.01 / 0.00000625 / 6.25 × 10-6; 2. divide the number of sperm by the volume and method of converting to cm³; deciding which sperm to include: 3. idea of: count those where sperm (fully) inside the square / 0.0625 (mm²) / all (small) squares / haemocytometer / quadrant/grid ; 4. idea of: include those touching the (line) top and left / up and right or exclude those touching bottom and right / down and left; detail of method: 5. AVP; e.g. ref to dilution of sample / stirring / even spreading / immobilising sperm / use of coverslip / screenshot for counting;
[3] sugar: idea that (it) provides or gives energy / is an energy source / (is used) in respiration / (is used) to produce ATP ; buffer: (change in pH may), denature enzymes / proteins; antibiotics: idea of, killing / stopping reproduction of, bacteria / AW ;

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