Dna rna and protein synthesis

DNA and RNA are polynucleotides built from mononucleotides, each containing a pentose sugar, a phosphate group, and a nitrogenous base.

DNA contains the pentose deoxyribose and the bases adenine (A), thymine (T), cytosine (C) and guanine (G).

RNA contains the pentose ribose and uses uracil (U) in place of thymine.

Mononucleotides join through phosphodiester bonds formed in condensation reactions, producing a sugar–phosphate backbone.

DNA exists as a double helix of two antiparallel strands held together by hydrogen bonds between complementary base pairs.

• Adenine pairs with thymine through two hydrogen bonds.
• Cytosine pairs with guanine through three hydrogen bonds.

DNA replicates semi-conservatively, so each daughter molecule contains one parental (template) strand and one newly synthesised strand.

DNA polymerase catalyses the formation of phosphodiester bonds between adjacent nucleotides aligned along a template strand.

Meselson and Stahl confirmed semi-conservative replication using the nitrogen isotopes ¹⁵N (heavy) and ¹⁴N (light).

• A single intermediate-density band after one round of replication ruled out the conservative model.
• Two bands (one intermediate-density and one low-density) after two rounds of replication ruled out the dispersive model.

The genetic code is triplet, non-overlapping and degenerate.

Sequences of three bases (triplets) specify amino acids; there are 64 possible triplets coding for the 20 amino acids, with some triplets acting as stop codons that do not code for an amino acid.

Key Definition A gene is a length of DNA whose base sequence codes for a polypeptide.

Protein synthesis occurs in two stages: transcription and translation.

Transcription

Transcription takes place in the nucleus, where RNA polymerase uses the antisense (template) DNA strand as a template to build a complementary mRNA molecule.

Translation

Translation takes place at ribosomes in the cytoplasm, after the mRNA has left the nucleus.

Codons on the mRNA pair with complementary anticodons on tRNA molecules that carry specific amino acids.

The ribosome catalyses the formation of peptide bonds between adjacent amino acids until a stop codon ends the chain.

The result is a polypeptide whose amino acid sequence is determined by the base sequence of the original gene.

This pathway from gene to protein underpins how genetic information directs cell structure and function.