A-LevelBiologyNucleic acids and protein synthesis9700/22 · Oct/Nov 2025 · Q313 Marks
The gene LCT codes for the enzyme lactase. In babies, lactase synthesis is necessary for digesting lactose, the sugar found in milk. Another gene, MCM6, has introns that have a regulatory role in the expression of gene LCT. Gene MCM6 codes for a protein that has no involvement in lactase synthesis. As children get older, the MCM6 introns are responsible for a decrease in lactase synthesis. This decrease in lactase synthesis is known as lactase non-persistence.
- (a)LCT and MCM6 are located on the same chromosome in humans. Suggest differences between gene LCT and gene MCM6, other than their locations in different positions on the same chromosome.[3]
- (b)With reference to the process of lactase synthesis, explain the relationship between: • a transcribed strand and a primary transcript • a primary transcript and messenger RNA (mRNA).[2]
- (c)A mutation in a regulatory intron of MCM6 allows lactase synthesis to continue. This is known as lactase persistence. In this mutation, the number of nucleotides in the intron remains the same, but one of the nucleotides is different to the original nucleotide. State the type of mutation that is the cause of lactase persistence.[1]
- (d)Fig. 3.1 summarises the reaction catalysed by lactase. [Figure 3.1] Draw the ring structure of α-glucose in the box provided to complete Fig. 3.1.[1]
- (e)(i)Lactase supplements (tablets) can be taken before milk or milk-based products are consumed to avoid symptoms of lactose intolerance. A student compared the activity of two different concentrations of a lactase supplement using an artificial substrate, ONPG, instead of lactose. A solution of ONPG is colourless, but the hydrolysis of ONPG by lactase releases a coloured product. The student planned to follow the progress of the reaction for each concentration of lactase using a colorimeter. Explain why the student chose to use a colorimeter to follow the progress of the reaction for each concentration of lactase.[2]
- (e)(ii)The lactose in milk can be hydrolysed using immobilised lactase or using lactase free in solution (free lactase). This results in milk and milk products that do not contain lactose and so are suitable for lactose-intolerant people. Scientists carried out an investigation to compare the activity of lactase immobilised in very small magnetic beads (magnetic microspheres) with free lactase, at different temperatures and at different pH values. Fig. 3.2 shows the activity of immobilised lactase and the activity of free lactase at 5 different temperatures. Fig. 3.3 shows the activity of immobilised lactase and the activity of free lactase at 8 different pH values. [Figure 3.2] [Figure 3.3] One advantage of using magnetic microspheres with immobilised lactase is that they can be easily recovered using an electric field and reused. With reference to Fig. 3.2 and Fig. 3.3, explain why the results indicate that there are other advantages in using immobilised lactase instead of free lactase to produce lactose-free products.[4]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.
- [3] any three from: / different sequence of, (DNA) nucleotides / bases ;R RNA nucleotides / code for / result in, different, polypeptides / proteins; A result in different sequences of amino acids / A LCT, codes for / AW, lactase / enzyme, and, MCM6, codes for a different protein / involved in regulation / have different numbers of, introns / exons ; / different, lengths / number of nucleotides / number of bases ; context is DNA / AVP; e.g. one may be continually expressed, the other switched on and off / MCM6 associated with different regulatory sequences
- [2] statement showing relationship between transcribed strand and primary transcript ; e.g. / (transcribed strand is the) DNA (strand) / template (strand), used to form the primary transcript / primary transcript is RNA (molecule first) formed from transcription of transcribed strand / primary transcript is a complementary copy of the transcribed strand (but uracil replaces thymine) / statement showing relationship between primary transcript and mRNA ; e.g. / primary transcript undergoes, gene / RNA, splicing, to form mRNA / primary transcript undergoes modification to produce mRNA / mRNA formed by removal of introns from primary transcript / mRNA formed only from exons of primary transcript / one mark for a response that has correct ideas but does not show a direct relationship
- [1] base substitution / substitution (mutation) ; I missense
- [1] CH2OH / H / H / OH H / OH / H / OH / all correct for one mark – if deciding to leave out the H, then this must be for all C1 to C5 hydrogens – leaving out only the C5 H is zero marks / OH
- [2] any two from: / idea of can, detect increase in / distinguish between, the intensity of colour (as products form); / can obtain quantitative results (for coloured product of ONPG) ; / A can provide numerical, values / readings / results / measurements / R if stated that the results are wavelengths / ref. to not subjective / AW ; idea of not by-eye judgments / difficulty in describing different colour shades / can obtain accurate, results / readings / values; / A idea of greater accuracy in obtaining results / vague references to accuracy should be considered for ideas linked to 'not subjective' / can construct numerical graphs of results to compare curves of each (lactase) concentration; AW / AVP; e.g. more sensitive to small colour changes at the beginning of the experiment / not able to see initial reaction proceeding by eye / ref. to use of a (prepared) calibration, curve / graph (to compare rates / product concentrations)
- [4] any four from: / 1 immobilised (overall) greater, productivity / yield / hydrolysis (of lactose in milk); I activity / 2 (process / immobilised) more tolerant to, temperature / pH, fluctuations/changes; AW / A idea that can be used in processes covering a wide range of, temperature / pH / 3 detail immobilised vs free lactase from Fig. 3.2; e.g. / (at temperatures) above optimum / higher than 38 / 39 °C, (immobilised) has a higher activity AW / above optimum / higher than 38 / 39 °C, (partial / extent of) denaturation (for immobilised) is less at each temperature / AW / A higher than 40 °C if accompanying context is correct / R if optimum stated as 35 °C / 4 data from Fig. 3.2 to support ; any one comparison (units only for temperature required) / 5 detail immobilised vs free lactase from Fig. 3.3 ; e.g. higher activity at all pHs except pH 6.5 and pH 7.0 / higher activity, below pH 6.5 / above pH 7.0 / free not active at pH 5.5 but immobilised active, at all pHs / for a wider / pH range / 6 data From Fig. 3.3 to support ; / 7 AVP ;; e.g. can use a lower temperature / cheaper, for same quantity of product / 8 more, heat stable / thermostable / stable at high temperatures I longer shelf-life R thermostatic / better in alkaline pHs
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