Mammary gland cells make casein, the main protein in milk. Fig. 17.1 shows amino acids joined into a chain that folds into a protein. One short casein chain...
Question 1 Report
Mammary gland cells make casein, the main protein in milk. Fig. 17.1 shows amino acids joined into a chain that folds into a protein. One short casein chain has 214 amino acids.
(a) Name the monomers that make up casein [1](b) Calculate the minimum number of DNA bases in the gene that codes for this chain [2]
(c) State the number of mRNA bases needed to code for the same chain [1]
(d) Describe how the base sequence of the casein gene decides the order of amino acids [3]
Answer Details
(a) Name the monomers of casein [1]: amino acids [1]. A protein is a polymer built from many amino acid monomers joined by peptide bonds, as shown by the chain in Fig. 17.1.
(b) Minimum number of DNA bases in the gene [2]: each amino acid is coded by a triplet of three bases, so \(214\times 3\) [1] \(=642\text{ bases}\) [1]. "Minimum" because it counts only the bases that code for the 214 amino acids.
(c) Number of mRNA bases [1]: mRNA is a direct base-for-base copy of the coding sequence, so the same number is needed: 642 [1].
(d) How the base sequence decides the amino acid order [3]: the bases are read in threes (as codons / triplets) [1]; each triplet codes for one particular amino acid [1]; so the order of the bases fixes the order in which the amino acids are joined [1]. Because the amino acid sequence in turn determines how the chain folds, the base sequence ultimately decides the shape and function of the casein.
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