Answers to questions on nucleic acids

1. (a) Hydrogen bonding

(b) Show two bonds between thymine and adenine and three bonds between cytosine and
guanine in the correct positions.

(c) A sugar (deoxyribose) and a phosphate group.

(d) The phosphate of one nucleotide covalently bonds to the –OH of the deoxyribose on the
next nucleotide by a condensation reaction to form a backbone of -sugar-phosphate-sugar-
phosphate-.

2. (a) The triplet code is the sequence of the nitrogen-containing bases on the nucleotides carried by messenger RNA to synthesise particular amino acids to form protein chains. UAU and UAC mean that the sequence of bases are uracil-adenine-uracil and uracil- adenine-cytosine respectively. The four different bases can make up 64 different combinations (permutations) known as codons and there are only about 20 amino acid needed so some of the amino acids have more than one codon. In this case tyrosine has two.

(b) The DNA sequence ACA, AAA, CTT, GTG and ACT will transcribe to the RNA codons UGU, UUU, GAA, CAC and UGA so that the sequence of amino acids will be -Cys-Phe-Glu-His followed by a terminator which means that the chain of the polypeptide will end with these four amino acid residues.

3. Potential benefits: any two from
Plants can be made more resistant herbicides and insect attack.
Anti-cancer substances and increased amounts of vitamins can be incorporated and exposure to less healthy fats reduced.
GM foods can enable animals to give higher yields of milk and eggs etc. and also increase their resistance to disease.
GM foods can lead to soil, water and energy conservation and improve natural waste management making them more environmentally friendly.

Potential concerns: any two from
They may cause an increase in disease as antibiotic-resistant genes might be passed to microorganisms with unknown effects.
They may alter the balance of ecosystems as food chains become disrupted.
They may cause an increase in allergic reactions in humans.

Answers to Nucleic acids questions