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Question 22M.2.HL.TZ1.4

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Date May 2022 Marks available [Maximum mark: 5] Reference code 22M.2.HL.TZ1.4
Level HL Paper 2 Time zone TZ1
Command term Explain, Outline, State Question number 4 Adapted from N/A
4.
[Maximum mark: 5]
22M.2.HL.TZ1.4

Hemoglobin is a conjugated protein consisting of four polypeptide chains with attached heme groups.

[Source: LAGUNA DESIGN/Science Photo Library via Getty Images.]

(a)

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

[1]

Markscheme

quaternary / fourth level;

Examiners report

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

(b)

Explain the shape of the polypeptide chain at X.

[2]

Markscheme
  1. alpha helix / α 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;
Examiners report

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.

(c)

Outline the role of tRNA in hemoglobin synthesis.

[2]

Markscheme
  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);
Examiners report

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.

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