AQA AS BIOLOGY EXAMS SCRIPT 2025/2026 QUESTIONS AND ANSWERS GRADED A+

AQA AS BIOLOGY EXAMS SCRIPT 2025/2026 QUESTIONS
AND ANSWERS GRADED A+
✔✔A dipeptide consists of two amino acids joined by a peptide bond. Dipeptides may
differ in the type of amino acids they contain.
Describe two other ways in which all dipeptides are similar and one way in which they
might differ.
[3 marks] - ✔✔Similarities
-Amine/NH2 (group at end);
-Carboxyl/COOH (group at end);
-Two R groups;
-All contain C and H and N and O;
Difference
-Variable/different R group(s);

✔✔A solution contained a mixture of three different amino acids. A scientist passed an
electric current through the solution to separate the amino acids.
She placed a drop of the mixture at one end of a piece of filter paper, attached an
electrode to each end of the paper and switched on the current. She switched off the
current after 20 minutes and stained the paper to show spots of the amino acids at
new positions.
Explain what the positions of the spots in Figure 6 show about these amino acids.
[3 marks] - ✔✔-Moved to negative (electrode) because positive(ly charged);
-Spots move) different distances/rates because (amino acids) different charge/mass;
-Two spots (not three) because (amino acids) same charge/massOROne spot has 2
amino acids because (amino acids) same charge/mass;

✔✔Read the following passage.
In laboratory tests, scientists investigated the effects of a new drug called ABZ on
stomach tumour cells. They found ABZ stopped mitosis by preventing the formation of
spindle fibres. They also found that ABZ affected some healthy cells.
Mitosis is a controlled process. Cyclin B is a protein found in a cell's nucleus. 5 It
regulates the timing of mitosis during the cell cycle. Mitosis starts when the
concentration of Cyclin B in the nucleus rises sharply and ends when it falls.The
scientists found that ABZ increased, and maintained, a high concentration
of Cyclin B in stomach tumour cells.
Programmed cell death is called apoptosis. Two nuclear proteins, Bcl-2 and 10 Bax, are
involved in controlling apoptosis. Apoptosis is prevented when theratio of Bcl-2 to Bax is
high and is promoted when this ratio is low. Thescientists found that ABZ decreased the
concentration of Bcl-2 and increased
t - ✔✔1. Chromosomes/centromeres cannot attach (to spindle)
OR
Chromosomes cannot line up (on spindle);
2. (So, no) metaphase;
OR

,Chromatids cannot separate (on spindle);
(So, no) anaphase;

✔✔Suggest and explain why ABZ could be used as a treatment for cancer even though
it
affects some healthy cells (lines 3-4).
[1 mark] - ✔✔1. Cancer cells divide more/uncontrollably/rapidly
OR
Healthy cells divide less/slowly;

✔✔What can you conclude about the effect of ABZ on tumour cells?Use information
about changes in the concentrations of Cyclin B (lines 6-7), and
Bcl-2 and Bax (lines 10-14).
[4 marks] - ✔✔1. (ABZ) increases/maintains Cyclin B;
2. (So) mitosis (starts but) does not end (no
tumour growth);
3. (ABZ) lowers ratio of Bcl-2 to Bax;
4. (So) apoptosis occurs/cells die (no tumour
growth);

✔✔Evaluate the scientists' suggestion that ABZ could be used for the successful
treatment of stomach cancer.
[3 marks] - ✔✔In support of suggestion
1. Stops mitosis (at metaphase/anaphase)/cell division (so no tumour
growth)ORPromotes apoptosis/programmed cell death (so tumour destroyed);
Against suggestion
2. Healthy cells (are) damaged/affected
OR
Causes side effects;
3.Results from laboratory tests/tests on (isolated) cells
OR
No clinical trials/tests on patients/people;
4.Dosage unknown;
5.Effectiveness unknown;

✔✔Describe the role of enzymes in the digestion of proteins in a mammal.
[4 marks] - ✔✔1. (Reference to) hydrolysis of peptide bonds;
2. Endopeptidase act in the middle of
protein/polypeptide
OR
Endopeptidase produces short(er) polypeptides/
increase number of ends;
3. Exopeptidases act at end of protein/polypeptide
OR
Exopeptidase produces dipeptides/amino acids;

, 4. Dipeptidase acts on dipeptide/between two
amino acids
OR
Dipeptidase produces (single) amino acids;

✔✔The food eaten by a rabbit is digested mainly by microorganisms in its caecum. The
caecum is a section of intestine attached between the ileum and the large intestine. The
resulting semi-digested material leaves the anus of a rabbit as soft, caecal droppings.
The rabbit then eats these caecal droppings.
Use this information and Figure 2 to suggest how eating its own caecal droppings
helps a rabbit's digestion and absorption of dietary protein.
[3 marks] - ✔✔More/remaining/undigested (protein) broken down;
(So more) amino acids absorbed;
(Because) protein/food passes again through
stomach/ileum;

✔✔Describe and explain the effect of increasing carbon dioxide concentration on the
dissociation of oxyhaemoglobin.
[2 marks] - ✔✔1. Increases/more oxygen dissociation/unloading
OR
Deceases haemoglobin's affinity for O2;
2. (By) decreasing (blood) pH/increasing acidity;

✔✔Name the two scientists who proposed models of the chemical structure of DNA and
of DNA replication.
[1 mark] - ✔✔Watson and Crick
OR
Crick and Watson;

✔✔A scientist replicated DNA in a test tube. To do this, he mixed an enzyme with
identical single-stranded DNA fragments and a solution containing DNA nucleotides.
Name the enzyme used in this DNA replication.
[1 mark] - ✔✔DNA polymerase;

✔✔Use your knowledge of semi-conservative replication of DNA to suggest:
1. the role of the single-stranded DNA fragments
2. the role of the DNA nucleotides.
[3 marks] - ✔✔Role of single-stranded DNA fragments
1. Template;
2. Determines order of nucleotides/bases;
Role of DNA nucleotides
3. Forms complementary pairs / A - T, G - C
OR
Forms complementary (DNA) strand;