OCR A 2.1.3 · 20 written questions · 45 marks · about 55 minutes · marked as you go
This free quiz has 20 written exam questions on nucleotides, DNA and protein synthesis, all taken from OCR A Level Biology A papers (AS and A level, 2016 to 2025). Short answers and calculations mark themselves. For longer answers you write your answer, then tick off the OCR mark-scheme points you made, so you see exactly where the marks are. This quiz covers the whole of OCR A 2.1.3: the structure of nucleotides, ATP and DNA, purifying DNA, semi-conservative replication, the genetic code, and how transcription and translation make a polypeptide.
What the quiz covers (OCR A spec 2.1.3)
Nucleotides and polynucleotides (2.1.3(a)(b)): the parts of a DNA nucleotide, purines and pyrimidines, and how phosphodiester bonds form
ATP and ADP (2.1.3(c)): comparing ATP with a DNA nucleotide and with ADP
DNA structure (2.1.3(d)(i)): the double helix, complementary base pairing and a base-percentage calculation
Purifying DNA (2.1.3(d)(ii)): the job of each step in a DNA extraction, and evaluating a student's method
DNA replication (2.1.3(e)): helicase and DNA polymerase, Meselson and Stahl's evidence, and how mutations arise
The genetic code (2.1.3(f)): triplets, reading a genetic code wheel and calculating the length of a gene
Transcription and translation (2.1.3(g)): RNA polymerase, tRNA anticodons and the steps from gene to polypeptide
How to use it
Write every answer in full, as you would in the exam: the mark for each question is shown, so you know how much to write. When you self-mark, only tick a point if your answer clearly makes it, then read the examiner's tip under each question. Your progress is saved on your device, so you can do it in two sittings, and at the end you can retry just the questions where you dropped marks.
Prefer multiple choice? Try the 15-question DNA and Protein Synthesis multiple choice quiz first.
Open the Nucleotides, DNA and Protein Synthesis quiz in a new tab (easier on a phone)
Where students lose marks (from OCR examiners' reports and mark schemes)
Each point below is followed by wording that gained marks.
Deoxyribose, not pentose or ribose: Asked to name the sugar in a DNA nucleotide, many students wrote 'pentose sugar' or 'ribose', which got no credit. OCR also rejected the misspelling 'dioxyribose'.
✔ Gains credit: Deoxyribose.
Nucleotides are the monomers: When describing how a polynucleotide forms, many students just repeated 'monomers' from the question, or wrote about bases or amino acids being joined.
✔ Gains credit: Nucleotides are joined by phosphodiester bonds in condensation reactions.
Phosphodiester bonds in the backbone: Hydrogen, peptide, glycosidic, ester and 'covalent' bonds were all given for the bond between nucleotides in one strand.
✔ Gains credit: A phosphodiester bond, between the phosphate of one nucleotide and the sugar of the next.
Helicase separates the two strands: Students wrote 'helicase unwinds DNA' without saying that the two strands are separated, and often missed the hydrogen bonds.
✔ Gains credit: Helicase unzips the double helix by breaking the hydrogen bonds between complementary bases, separating the two strands.
Ethanol, not salt, precipitates DNA: Many said salt precipitates the DNA, and few knew what the salt is actually for.
✔ Gains credit: Ice-cold ethanol precipitates the DNA; salt makes DNA less soluble in water by breaking hydrogen bonds between DNA and water.
Convert template DNA to mRNA first: Students read a DNA template triplet straight off an mRNA genetic code table, giving the wrong amino acid.
✔ Gains credit: Write the complementary mRNA codon first, then look it up in the genetic code.
Mutation changes the base sequence: Many said the wrong bases pair up during replication but didn't link this to a change in the DNA base sequence, and some wrote about transcription or uracil.
✔ Gains credit: A mutation is a random change in the DNA base sequence, e.g. a wrong nucleotide is inserted during replication.
Compare with 'more' or 'fewer': When asked for a difference between ATP and ADP or a DNA nucleotide, students described only one molecule, e.g. 'ATP has three phosphates'.
✔ Gains credit: ATP has one more phosphate group than ADP; ATP contains ribose whereas a DNA nucleotide contains deoxyribose.
Keep practising
Try the DNA and Protein Synthesis multiple choice quiz, the Cellular Control and Manipulating Genomes quiz, the Proteins, Ions, Food Tests and Chromatography written quiz or the Enzymes written quiz, or browse all our OCR A Level Biology quizzes. Our OCR A masterclasses (Year 12 on Wednesdays at 5.15pm, Year 13 on Tuesdays at 6.30pm) work through real exam questions on one topic each week.
Questions, mark schemes and examiners' report extracts © OCR.