Lesson 3 — Protein Synthesis


mRNA Codons

The genetic code in the DNA molecules are packaged into blocks of three nucleotides called DNA codons. When the DNA molecule is transcribed into mRNA, the resulting blocks of three RNA nucleotides are called mRNA codons. The three mRNA nucleotides code for a specific amino acid.  Several amino acids bonded together makes up a protein molecule.

With four nucleotides in the mRNA (A, U, C, and G), how many combinations of three nucleotides can be arranged? Using three nucleotides out of four produces 43 = 64 combinations. Because only 20 amino acids are available, many of these codons code for the same amino acids as shown in the mRNA codon table below.  The genetic code is described as redundant because a single amino acid may be coded for by more than one codon.

What might be beneficial about having multiple codons code for the same amino acid? The redundancy of the codons is another method to prevent transcription and translation mistakes. If one nucleotide is transcribed incorrectly from DNA to mRNA, it can still result in the correct amino acid. For example, if DNA nucleotides GGG was incorrectly transcribed to mRNA codon CCA instead of CCC, it would still result in the same amino acid, proline.

mRNA Codons and Their Corresponding Amino Acids. Alberta Education.


Reading the Codons

A codon is made of three nucleotides. For example, CCU codes for proline. The first letter, C, is in the second row on the first column on the left (1st base). Find the second letter, C, from the top row (2nd base). This will narrow the search to 4 codons. Then, identify the last letter as shown on the last column on the right (3rd base).


mRNA codons are three nucleotide sequences that corresponds to specific amino acids. Corresponding amino acids can be determined by using the mRNA codon table shown above.
 NHGRI. Public Domain. 

In the figure above, the mRNA codons AUG, ACG, GAU, CAG, and CCG correspond to methionine (start), threonine, asparagine, glycine and proline amino acids. These amino acids will be strung together by the transfer RNA molecules until it reads the stop codon (UAA).

Remember that mRNA codons are redundant. In other words, many codons code for the same amino acids. For example, four codons specify proline, and they differ only in the third nucleotide - CCU, CCC, CCA, and CCG.

Example

Find the amino acids that corresponds to each of the following codons.
  1. CCA
  2. AUG
  3. GCA

  1. Proline
  2. Methionine (Start codon)
  3. Alanine