Cysteine is an amino acid containing sulfur. Fig. 2.1 shows the structure of the molecule formed by joining two cysteine molecules together. SH H-O O H C-C-N-C-C-N H H CH₂ SH Fig. 2.1

1. (a)Cysteine is an amino acid containing sulfur. Fig. 2.1 shows the structure of the molecule formed by joining two cysteine molecules together. SH H-O O H C-C-N-C-C-N H H CH₂ SH Fig. 2.1 Draw a circle around an R-group in the molecule shown in Fig. 2.1.[1]
2. (b)(i)Goblet cells in the human gas exchange system produce proteins called mucins. The ends of mucin molecules contain many cysteine residues. Mucin strands are formed by joining the ends of mucin molecules together through covalent bonds between R-groups. State the name of these covalent bonds.[1]
3. (b)(ii)Mucin strands are transported out of the goblet cells and then absorb water to form mucus. Suggest and explain how mucin strands are transported out of the goblet cells.[3]
4. (c)Cystic fibrosis is a genetic disease caused by a mutation in the human CFTR gene. This results in mucus that is thicker than normal. Suggest how thicker mucus interferes with the maintenance of healthy gas exchange surfaces in the lungs.[2]
5. (d)(i)Row 1 and row 2 of Table 2.1 show the DNA base sequences of part of the normal CFTR allele and the same part of a mutated CFTR allele. The base sequences shown are for the DNA strands used in the synthesis of RNA. When Table 2.1 is completed, row 3 will show the base sequence of the RNA synthesised from the same part of the mutated CFTR allele. Table 2.1 1 | DNA base sequence of part of the normal CFTR allele | T | A | G | T | A | G | A | A | A | C | C | A 2 | DNA base sequence of part of the mutated CFTR allele | T | A | G | T | A | A | C | C | A | C | A | A 3 | RNA base sequence synthesised from the mutated CFTR allele | | | | | | | | | | | | | The difference between the DNA base sequence in row 1 and the DNA base sequence in row 2 of Table 2.1 is caused by a single gene mutation. State the name of this type of gene mutation.[1]
6. (d)(ii)Row 1 and row 2 in Table 2.1 show the DNA strands used in the synthesis of RNA. State the term used to describe the DNA strand that is used in the synthesis of RNA.[1]
7. (d)(iii)Complete Table 2.1 to show the missing bases in row 3.[1]
8. (d)(iv)The normal CFTR allele is approximately 189000 base pairs in length. The CFTR polypeptide consists of only 1480 amino acids. Explain the reasons for this difference between the number of base pairs and the number of amino acids.[3]

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