Nucleic acids and protein synthesis

Learning objectives

Draw and label a nucleotide (pentose sugar, phosphate, nitrogenous base) and ATP, and sort the bases into double-ring purines (adenine, guanine) and single-ring pyrimidines (cytosine, thymine, uracil).

Describe DNA as a double helix of antiparallel 5′→3′ and 3′→5′ strands with phosphodiester bonds and complementary base pairs (two hydrogen bonds for A–T, three for C–G), and compare it with a molecule of messenger RNA.

Explain semi-conservative replication in S phase, including why DNA polymerase adding nucleotides only in the 5′→3′ direction gives a continuous leading strand and a lagging strand built in fragments joined by DNA ligase.

Describe the universal triplet code with start and stop codons, and explain transcription of a gene from the template (transcribed) strand by RNA polymerase and the splicing of the primary transcript (introns removed, exons joined) to give mRNA.

Explain translation at ribosomes, including how tRNA anticodons pair with mRNA codons to bring amino acids that are joined by peptide bonds into a polypeptide.

Explain how substitution, insertion or deletion of nucleotides changes the base sequence of a gene and may alter the polypeptide, including frame shifts and silent mutations.