Passage of information from parents to offspring — practice questions

(a) During transcription, mRNA is synthesised complementary to the template DNA strand. Remember that Thymine (T) in DNA is replaced by Uracil (U) in mRNA. Template DNA: 3'-T A C G T T A C G A C G-5' Complementary mRNA: 5'-A U G C A A U G C U G C-3'

(b) Using the mRNA sequence 5'-A U G C A A U G C U G C-3' and the provided genetic code: The mRNA is read in codons (groups of three): Codon 1: AUG -> Methionine (Met) Codon 2: CAA -> Glutamine (Gln) Codon 3: UGC -> Cysteine (Cys) Codon 4: UGC -> Cysteine (Cys) Therefore, the amino acid sequence is: Met-Gln-Cys-Cys

(c) The genetic code is described as degenerate because most amino acids can be specified by more than one codon. For instance, while UGC codes for Cysteine, the codon UGU also codes for Cysteine. This redundancy provides a protective mechanism, as a point mutation changing one base (e.g., from UGC to UGU) might still result in the same amino acid being incorporated, thus having no effect on the protein's function.

(a) Step 1: Use complementary base pairing rules. In double-stranded DNA, the amount of guanine (G) equals the amount of cytosine (C), and the amount of adenine (A) equals the amount of thymine (T). Given: %G = 22% Therefore, %C = 22%

Step 2: Calculate the total percentage of G and C. Total % (G + C) = 22% + 22% = 44%

Step 3: Calculate the remaining percentage for A and T. The total percentage of all bases is 100%. Total % (A + T) = 100% - % (G + C) = 100% - 44% = 56%

Step 4: Calculate the percentage of adenine. Since %A = %T, we divide the remaining percentage by 2. %A = 56% / 2 = 28% Answer (a): The percentage of adenine is 28%.

(b) Step 1: Calculate the number of each type of base pair. Total base pairs = 5,000 Number of G-C pairs = % (G + C) of total pairs = 0.44 × 5,000 = 2,200 pairs Number of A-T pairs = % (A + T) of total pairs = 0.56 × 5,000 = 2,800 pairs (Check: 2,200 + 2,800 = 5,000)

Step 2: Calculate the number of hydrogen bonds for each type of pair. Each G-C pair is joined by 3 hydrogen bonds. Each A-T pair is joined by 2 hydrogen bonds.

Step 3: Calculate the total number of hydrogen bonds. Total H-bonds = (Number of G-C pairs × 3) + (Number of A-T pairs × 2) Total H-bonds = (2,200 × 3) + (2,800 × 2) Total H-bonds = 6,600 + 5,600 = 12,200 Answer (b): The total number of hydrogen bonds is 12,200.