What are nucleic acids? Describe their composition and the structure of DNA. Give two biological functions of nucleic acids and three differences between DNA and RNA.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define. Nucleic acids are polynucleotides: long chains of nucleotides found in the nucleus.
2 · Composition. Hydrolysis gives a pentose sugar, phosphoric acid and nitrogenous bases. Base + sugar = nucleoside; + phosphate = nucleotide; nucleotides join by phosphodiester links (C5′ to C3′).
3 · Structure of DNA. Watson and Crick double helix: two chains wound round each other, bases inside, joined by hydrogen bonds; A pairs with T, G with C.
4 · Draw the double helix. Two twisting ribbons (sugar-phosphate backbones) with rungs labelled A-T and G-C.
5 · DNA vs RNA.
Sugar: deoxyribose vs ribose
Bases: thymine vs uracil
Strands: double helix vs usually single
6 · Functions. DNA is the chemical basis of heredity and copies itself in cell division; RNA carries out protein synthesis.
7 · Link to real life. DNA fingerprinting identifies people because each person's base sequence is unique.
8 · Close in one line. So the order of four bases along a sugar-phosphate chain stores the instructions for life.
Scoring tip: A neat DNA vs RNA table of three rows is quicker to write and easier to mark than a paragraph.
