Nucleic Acids and Protein Synthesis

• Nucleotide structure, ATP; purines and pyrimidines.
• DNA: antiparallel strands, complementary base pairing, phosphodiester bonds.
• Semi-conservative replication: DNA polymerase, ligase, leading and lagging strands.
• Structure of mRNA.
• Genes and the universal, triplet genetic code.
• Transcription, splicing of introns and exons, and translation.
• Gene mutations: substitution, insertion, deletion.

The building blocks

A nucleotide has three parts: a pentose sugar, a phosphate group and a nitrogen-containing base.

• DNA nucleotides contain deoxyribose and one of the bases A, T, C, G.
• RNA nucleotides contain ribose and one of the bases A, U, C, G.
• ATP (adenosine triphosphate) is a phosphorylated nucleotide: the base adenine, the sugar ribose and three phosphate groups. Hydrolysis of the last phosphate releases energy for cell processes.

The double helix

• Two polynucleotide strands coiled into a double helix.
• Nucleotides in each strand are linked by phosphodiester bonds between the phosphate of one nucleotide and the sugar (3′ carbon) of the next, formed by condensation, giving a sugar–phosphate backbone with the bases pointing inwards.
• The strands are antiparallel: one runs 5′ to 3′, the other 3′ to 5′.
• Complementary base pairing: A pairs with T by two hydrogen bonds; C pairs with G by three hydrogen bonds. A purine always pairs with a pyrimidine, so the helix has a constant width.
• Complementary pairing means each strand can act as a template, so DNA can be copied accurately; the many hydrogen bonds make the molecule stable.

Semi-conservative replication in S phase

1. The double helix unwinds and the hydrogen bonds between bases break, separating the two strands. Both strands act as templates.
2. Free DNA nucleotides (activated, carrying extra phosphates) pair with the exposed bases by complementary base pairing.
3. DNA polymerase joins adjacent nucleotides by forming phosphodiester bonds. It can only add nucleotides to the 3′ end of a growing strand, so new DNA is always made in the 5′ to 3′ direction.
4. The leading strand is made continuously, in the same direction as the unwinding.
5. The lagging strand is made discontinuously, in short fragments in the opposite direction; DNA ligase joins the fragments with phosphodiester bonds.
6. Each new DNA molecule contains one original (parental) strand and one new strand: semi-conservative replication.

Messenger RNA