State the level of protein structure at which the polypeptide chains of hemoglobin are combined.

Explain the shape of the polypeptide chain at X.

Outline the role of tRNA in hemoglobin synthesis.

quaternary / fourth level;

1. alpha helix / $\alpha$ helix
2. type of secondary structure / second level of protein structure;
3. maintained by hydrogen bonds;
4. between C=O group and an N–H group;

1. decoding/translation (of the genetic code/RNA base sequence);
2. carries/brings one amino acid/a specific amino acid/the amino acid (corresponding to codon/anticodon);
3. tRNA has an anticodon which pairs with mRNA/is complementary to a codon (on mRNA);

Many candidates knew that association of polypeptide chains is quaternary structure. Any spelling of this term was accepted — only a minority got it correct.

This was one of the most successfully answered of questions, with better prepared candidates recognizing the alpha helix and explaining it in terms of hydrogen bonding.

There were varied answers to this question. The best made it clear that a tRNA molecule carries one specific amino acid corresponding to its anticodon and to the codon on mRNA to which it binds during translation.