AQA AS Level Biology Paper 1, November 2021: Question 4

✅ Correct Answer

Any valid pair from the following marking points:

• Point 1 & 2: Chromosomes are becoming visible/distinct because they are still condensing (or shortening/thickening).
• Point 3 & 4: Chromosomes are arranged at random / not lined up because there is no spindle activity / they are not attached to spindle fibres.

💡 Key Knowledge

During prophase, chromatin condenses into visible chromosomes. Because spindle fibers have not yet formed or attached to centromeres, chromosomes are scattered randomly throughout the nuclear region rather than aligning at the equator.

🧠 Exam Technique

This is a paired mark question ("Describe and explain"). You must couple what you see (description) with the cellular cause (explanation). Simply stating "they are condensing" or "they are random" will only gain 1 mark unless linked to visibility or the lack of spindle fibres.

❌ Common Errors

Students often confuse prophase with metaphase and claim chromosomes are "lined up on the equator." Always inspect micrographs carefully for alignment before jumping to conclusions.

✅ Correct Answer

Box A should be ticked.

💡 Key Knowledge

A diploid number of 4 means 2n = 4 . During metaphase of mitosis, replicated chromosomes (each consisting of two sister chromatids joined at a centromere, making an 'X' shape) line up individually along the cell's equator. Therefore, you need to see 4 distinct X-shaped structures aligned in single file.

🧠 Exam Technique

Break down the visual clues systematically:
1. Check the ploidy number: 4 chromosomes needed.
2. Check the stage: Metaphase requires single-file alignment at the equator (unlike meiosis I homologous pairs).
3. Check structure: Replicated mitotic chromosomes look like an 'X'.

❌ Common Errors

Selecting B (only 2 chromosomes shown) or C/D (unreplicated single lines representing anaphase or telophase chromatids, or incorrect numbers).

✅ Correct Answer

Locus (accept plural: loci).

💡 Key Knowledge

A locus is the specific, physical location of a gene or other DNA sequence on a chromosome.

🧠 Exam Technique

Definitive recall question. Ensure spelling is precise; do not confuse with "allele" or "genome".

❌ Common Errors

Writing "allele" (which refers to an alternative form of a gene, not its position).

✅ Correct Answer

1. It has a specific base / nucleotide sequence;
2. Consisting of triplets;
3. Which determines the order / sequence of amino acids (forming the primary structure) in the polypeptide.

💡 Key Knowledge

A gene is a sequence of DNA nucleotides. Genetic information is stored as non-overlapping triplets of bases (codons). The sequence of these triplets dictates the precise linear sequence of amino acids assembled during protein synthesis, establishing the protein's primary structure.

🧠 Exam Technique

Read the negative constraint carefully: "Do not include information about transcription or translation." Mentioning mRNA, ribosomes, or tRNA will instantly forfeit marks even if your biology is correct.

❌ Common Errors

Failing to read the restriction and describing transcription/translation processes. Another common error is referring to "amino acids" without specifying their sequence or link to primary structure.