Students should be able to: relate the base sequence of nucleic acids to…

The idea

Questions require translating between DNA, mRNA, tRNA, and amino acid sequences using a provided table. Apply base pairing rules carefully: mRNA is complementary to the DNA template strand, but uses uracil (U) instead of thymine (T). For example, a DNA template triplet 'TAC' transcribes to an mRNA codon 'AUG'. The tRNA anticodon is complementary to the mRNA codon (e.g., 'UAC'). Always verify if the table lists mRNA codons or DNA triplets. Group bases into triplets from the correct reading frame; a frameshift alters the downstream polypeptide. When interpreting experimental data, like radioactive labelling of nucleic acids, identify the independent variable, cite specific data points, and link trends to protein synthesis mechanisms.

What earns the marks

• Convert a DNA base sequence into the correct complementary mRNA sequence by substituting uracil in place of thymine.
• Group the bases into triplets from the correct starting point to establish the proper reading frame.
• Read the supplied table accurately, ensuring you distinguish between mRNA codons and tRNA anticodons, to list the amino acids in the correct order.
• Derive tRNA anticodons by applying complementary base pairing rules to the mRNA codons.
• Describe trends shown by experimental data using specific values before explaining them in terms of nucleic acid function.
• Identify the independent variable in an experiment and use the results to deduce the specific role of a nucleic acid.

Common mistakes and how to put them right

• Pairing adenine with thymine when writing an mRNA sequence.

  Put it right: Ensure you pair adenine with uracil instead.

• Looking up a DNA triplet in a table that lists mRNA codons.

  Put it right: Convert the DNA sequence to mRNA first if the table requires it.

• Starting the triplets from the wrong base, causing a frameshift.

  Put it right: Carefully identify the start codon or the specified starting base to maintain the correct reading frame.

• Drawing a conclusion the data do not support, such as calling a difference significant.

  Put it right: Only state significance if a statistical test is provided.

Exam tips

• Write the mRNA sequence directly underneath the DNA sequence and bracket it into triplets before using the data table.
• Always check the table heading first to confirm whether it provides mRNA codons, tRNA anticodons, or DNA triplets.
• When instructed to use a figure or table, explicitly quote actual values with their units to support your explanation.