DNA, genes and protein synthesis

DNA is the chemical that carries the instructions for making a living thing. It is a long molecule made of two strands twisted into a double helix. A gene is a short section of DNA that holds the code for one protein. Cells read the code in two stages, transcription and translation, to build proteins from amino acids.

Key facts

  • DNA is a polymer made of repeating units called nucleotides. Each nucleotide has a phosphate, a sugar (deoxyribose) and one of four bases: A, T, C or G.
  • The two strands are held together by pairs of bases: A always pairs with T, and C always pairs with G. These are complementary base pairs.
  • A gene is a small section of DNA on a chromosome. Its sequence of bases codes for a particular sequence of amino acids, which makes a specific protein. The genome is all the genetic material of an organism.
  • Three bases (a codon) code for one amino acid. The order of the codons decides the order of the amino acids in the protein.
  • Transcription happens in the nucleus: the DNA of a gene unzips and a complementary copy is made as messenger RNA (mRNA). In RNA the base U (uracil) takes the place of T.
  • Translation happens at a ribosome: the ribosome reads the mRNA three bases at a time, and tRNA molecules bring the matching amino acids, which are joined into a chain. The chain folds into the protein’s shape.
  • A mutation is a change in the base sequence. Most have no effect on the protein; some change its shape, so an enzyme’s active site or a structural protein may no longer work.

Worked examples

Example 1

One strand of DNA reads ATTGCAC. What is the sequence of the complementary strand?

  1. A pairs with T, and C pairs with G.
  2. Go along the strand one base at a time: A→T, T→A, T→A, G→C, C→G, A→T, C→G.
  3. So the complementary strand is TAACGTG.

Answer: TAACGTG

Example 2

In a sample of DNA, 22% of the bases are guanine (G). What percentage are adenine (A)?

  1. G pairs with C, so there is as much C as G. C = G = 22%
  2. So C and G together make up 44%. 22 + 22 = 44
  3. The rest is A and T, in equal amounts. 100 − 44 = 56
  4. Half of that is A. 56 ÷ 2 = 28%

Answer: 28%

Example 3

A protein is 150 amino acids long. How many bases in its gene code for it (not counting the stop codon)?

  1. Each amino acid is coded for by a codon of three bases.
  2. Multiply the number of amino acids by 3. 150 × 3 = 450

Answer: 450 bases

Example 4

The template strand of a gene reads TACGGT. What mRNA is made from it, and which amino acids does it code for? (AUG = Met, CCA = Pro)

  1. In transcription, mRNA pairs with the template: A→U, T→A, C→G, G→C.
  2. So TACGGT gives the mRNA AUGCCA.
  3. In translation, the ribosome reads the codons AUG and CCA.
  4. AUG codes for Met and CCA codes for Pro, so the chain starts Met–Pro.

Answer: AUGCCA, coding for Met then Pro