AQA AS Level Biology Paper 2, June 2024: Question 4

✅ Correct Answer

• Nucleus

💡 Key Knowledge

In eukaryotes, genetic material is enclosed within the nuclear envelope. Transcription (DNA → pre-mRNA) occurs in the nucleus, whereas translation occurs at ribosomes in the cytoplasm.

🧠 Exam Technique

Keep it concise. Single-word recall questions do not require full sentences. Avoid naming organelles involved in translation (like ribosomes or rough endoplasmic reticulum).

❌ Common Errors

• Confusing transcription with translation and answering cytoplasm or ribosome.
• Writing nucleolus specifically—while rRNA is made there, the mark scheme expects nucleus for general gene transcription.

✅ Correct Answers (Any 3 from 5)

• 1. Loops (of DNA) contain introns (accept: 'non-coding' DNA);
• 2. DNA regions forming the hybrid molecule contain exons;
• 3. Complementary base pairing occurs between DNA and mRNA OR Hydrogen bonding occurs between DNA and mRNA;
• 4. Introns have been removed during splicing / to produce mature mRNA;
• 5. The mRNA is shorter than the original DNA template / pre-mRNA strand due to splicing;

💡 Key Knowledge: R-Loop Hybridisation

Adenovirus pre-mRNA contains both introns and exons. Splicing removes non-coding introns and joins coding exons together. When single-stranded template DNA is incubated with mature mRNA:

• Linear hybridized regions: Exons pair with complementary mRNA bases via hydrogen bonds.
• Unpaired loops: Intron sequences in the DNA find no complementary partner in the processed mRNA and loop outward.

🧠 Exam Technique: "Describe and Explain"

Always split your response into two distinct halves:

• Describe (what is seen): Single-stranded loops of DNA that do not bind to mRNA; mRNA is shorter than the DNA strand.
• Explain (underlying biology): Loops correspond to introns removed during splicing; hybridized regions are exons joined together that form hydrogen bonds by complementary base pairing.

❌ Common Errors

• Mixing up exons and introns (stating loops are exons). Remember: Exons are expressed in mRNA.
• Failing to mention the molecular force binding them (hydrogen bonds / complementary base pairing).
• Describing the loops without explaining why they formed (i.e. because their matching sequences were excised).

✅ Correct Answer (Both required for full marks)

• Description: No loops of DNA would be seen / the entire length would form a hybrid molecule [1 mark];
• Explanation: Because prokaryotic DNA does not contain introns / prokaryotes do not undergo splicing [1 mark];

💡 Key Knowledge

Prokaryotic genes do not contain introns. Their mRNA is produced directly from DNA transcription without the need for post-transcriptional splicing. Therefore, mature prokaryotic mRNA is directly complementary along its entire coding length to the template DNA strand.

🧠 Exam Technique

Look at the mark allocation (2 marks). One mark is strictly for the observation (what you would see visually: no loops) and the second is for the biological reason (prokaryotes lack introns). Providing only the reason loses the first mark.

❌ Common Errors

• Writing "it wouldn't work" or "there would be no hybrid" — the mRNA and DNA still pair fully!
• Stating "prokaryotes have circular DNA" — while true, it does not answer how the hybridization pattern changes in this experiment.

✅ Correct Answer (Any 1)

• A change in the base/nucleotide sequence of exons occurs;
• Deletion or addition of bases/nucleotides occurs (frameshift);
• Deletion of an exon / inclusion of an intron occurs;

💡 Key Knowledge

If splicing enzymes (spliceosomes) cut at an incorrect nucleotide:

• Part of an intron may be retained or part of an exon may be removed.
• This changes the triplet reading frame (frameshift) or changes specific codons, ultimately altering the amino acid sequence (primary structure) and potentially leading to a non-functional tertiary structure.

🧠 Exam Technique

The mark scheme notes: Accept nucleotide/s for base/s. Ignore references to DNA or RNA. Focus directly on the change in the sequence of bases/nucleotides or the loss/addition of coding bases.

❌ Common Errors

• Vague answers like "it causes a mutation" — the question already states that; you must explain at the molecular level (e.g. changes the base sequence).
• Jumping straight to the protein ("alters active site shape") without first mentioning the direct effect on the base sequence.