Turn over 11 4 Insulin is a protein that is composed of two polypeptide chains, chain A and chain B. The diagram shows the amino acid sequence of these two polypeptide chains and some of the bonds involved in the folding of this protein. Thr Thr Thr Lys Pro Tyr Tyr Tyr Phe Phe Gly Arg Glu Glu Glu Glu Gly Gly Gly Cys Cys Cys Cys Cys Cys Val Val Val Val Leu Leu Leu Leu Leu Leu Tyr Ala His His Ser Ser Ser Gln Gln Gln Asn Phe Asn Ile Ile Asn 1 2 3 4 5 Chain B Chain A Bond C (a) Cysteine (Cys) is one type of amino acid found in insulin. The presence of cysteine in insulin is important in determining the structure of this protein. The R group of cysteine is: H C H S H (i) Draw a diagram to show the structure of the amino acid cysteine. (3) 12 (ii) Name the covalent bond labelled C in the diagram. (1) .................................................................................................................................................................................................................................................................................... (b) The table shows the DNA genetic codes on the anti-sense DNA strand (template strand) for amino acids. Genetic code Amino acid Genetic code Amino acid Genetic code Amino acid Genetic code Amino acid AAA AAG Lys CAA CAG Gln GAA GAG Glu TAC TAT Tyr AAC AAT Asn CAT CAC His GAC GAT Asn TCA TCC TCG TCT Ser ACA ACC ACG ACT Thr CCA CCC CCG CCT Pro GCA GCC GCG GCT Ala TGG Try AGA AGG Arg CGA CGC CGG CGT Arg GGA GGC GGG GGT Gly TGC TGT Cys AGC AGT Ser CTA CTC CTG CTT Leu GTA GTC GTG GTT Val TTA TTG Leu ATA ATC ATT Ile TTC TTT Phe ATG Met The genetic codes TAA, TAG and TGA are stop codons. (i) Complete the diagram to show one base sequence in the antisense DNA strand (template strand) coding for the five amino acids numbered in the diagram of insulin. (1) Amino acid number 1 2 3 4 5 Amino acid abbreviation Asn Cys Tyr Asn Glu Base sequence on the antisense DNA strand TGT Turn over 13 *(ii) Analyse the information to deduce how gene mutations in the DNA base sequence for Cys (amino acid number 2) could affect the structure of insulin. 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(Total for Question 4 = 11 marks)