The structure of DNA

DNA is a double helix of two antiparallel strands (Watson, Crick, Franklin & Wilkins, 1953). Each strand is a chain of nucleotides, and each nucleotide has three parts: a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases — adenine (A), thymine (T), cytosine (C), guanine (G). The sugar-phosphate backbones run outside; the bases pair inside by hydrogen bonds following complementary base pairing:

Because A always pairs with T and C with G, one strand specifies the other — the basis of both replication and information storage.

DNA replication — semi-conservative

Before division, DNA copies itself. The helix is unwound by helicase; each old strand serves as a template; DNA polymerase adds complementary nucleotides. Each new molecule has one old and one new strand — hence semi-conservative replication (proven by Meselson & Stahl, 1958).

The central dogma: DNA → RNA → protein

• Transcription (in the nucleus): RNA polymerase reads a gene and builds a complementary messenger RNA (mRNA) copy. RNA differs from DNA: single-stranded, ribose sugar, and uracil (U) replaces thymine.
• Translation (at a ribosome): the mRNA is read in three-base units called codons. Each codon specifies one amino acid via the genetic code. Transfer RNA (tRNA) molecules bring the matching amino acids, pairing their anticodon to the codon, and the ribosome links the amino acids into a polypeptide.

The genetic code is a triplet code: 4³ = 64 codons encode 20 amino acids plus start (AUG) and stop signals. It is degenerate (most amino acids have several codons) and nearly universal across all life.

Information, copied and expressed

Think of DNA as a reference library that must never leave the building (the nucleus). To use a recipe, the cell makes a disposable photocopy — mRNA — of just the needed gene, and carries that copy out to the workshop (the ribosome). There the code is read three letters at a time and translated into a chain of amino acids that folds into a working protein. Copying (replication) preserves the archive; transcription and translation express it.

Why triplets? With only 4 bases, singlets could code just 4 amino acids and doublets 16 — too few for 20. Triplets give 64 combinations, comfortably enough. The surplus makes the code degenerate, which is protective: many single-base changes are "silent" because a different codon still codes the same amino acid. A mutation — a change in the base sequence — may be silent, may swap one amino acid (missense), may create a premature stop (nonsense), or, worst, may shift the reading frame (insertion/deletion) and garble everything downstream.

Worked example: transcribe and translate a gene

Given the template (antisense) DNA strand 3'-TAC GGA TTT ACC ATT-5', work out the protein it encodes.

1. Transcribe to mRNA. RNA polymerase builds the complementary RNA copy (A→U, T→A, G→C, C→G). Reading the template gives mRNA: 5'-AUG CCU AAA UGG UAA-3'.
2. Split into codons. AUG · CCU · AAA · UGG · UAA.
3. Translate using the genetic code. AUG = Methionine (start); CCU = Proline; AAA = Lysine; UGG = Tryptophan; UAA = STOP.
4. Write the polypeptide. Met – Pro – Lys – Trp (translation halts at the stop codon, which codes no amino acid).
5. Test a mutation. Suppose the template's third codon changes so the mRNA codon AAA becomes UAA. That is a nonsense mutation — a premature stop — truncating the protein after Proline. A tiny change, a broken product.
6. Reflect. Notice AUG doubles as start signal and Methionine, and that the reading frame (where you start grouping threes) is everything — begin one base off and the entire message is misread.

Test yourself

Q1 Explain why DNA replication is described as "semi-conservative" and how Meselson and Stahl showed it.

Semi-conservative means each new DNA molecule keeps one original (parental) strand and one newly made strand — half is conserved. Meselson & Stahl grew bacteria in heavy nitrogen (¹⁵N) so all DNA was heavy, then switched them to light ¹⁴N. After one replication, all DNA was of intermediate density (one heavy + one light strand) — ruling out conservative replication (which would give separate heavy and light molecules). After two replications, they saw intermediate and light DNA, exactly as the semi-conservative model predicts.

Q2 Give three differences between DNA and mRNA.

(1) DNA is a double helix; mRNA is single-stranded. (2) DNA's sugar is deoxyribose; RNA's is ribose. (3) DNA uses the base thymine (T); RNA uses uracil (U) instead. (Also: DNA is a stable long-term archive kept in the nucleus, while mRNA is a short-lived working copy that carries the message to the ribosome.)

Q3 The genetic code is "degenerate." What does this mean and why is it advantageous?

Degenerate means most amino acids are encoded by more than one codon (there are 64 codons for 20 amino acids plus stop). This is advantageous because many base substitutions are "silent" — a mutation changes the codon but it still codes the same amino acid, so the protein is unaffected. Degeneracy thus buffers organisms against the harmful effects of point mutations, adding robustness to the genetic system.

Q4 Compare the likely effects of a base substitution versus a base deletion in a coding sequence.

A substitution changes a single codon, affecting at most one amino acid — it may be silent (same amino acid), missense (one amino acid changed), or nonsense (premature stop). A deletion (or insertion) of a base causes a frameshift: every codon after the deletion is regrouped and misread, usually producing a completely wrong amino-acid sequence and often an early stop codon. Frameshifts are generally far more damaging because they corrupt the entire downstream message, not just one codon.

Q5 The genetic code is nearly universal — the same codons mean the same amino acids in bacteria, plants and humans. Why is this a powerful piece of evidence, and how is it exploited in biotechnology?

A shared code is strong evidence for the common ancestry of all life: the code was fixed so early that it has been inherited by every lineage. It is exploited in genetic engineering: because a human gene inserted into a bacterium is read the same way, bacteria can be programmed to manufacture human proteins such as insulin. Universality is what makes transgenic technology possible — a gene means the same thing in any organism.