The genetic code is degenerate, meaning that multiple codons can encode a single amino acid, because the base pairing between the third position in a mRNA codon and a tRNA anticodon is flexible, a phenomenon know as wobble pairing.
There are 64 possible codons in the genetic code, and 61 of these encode 20 different amino acids. This makes the genetic code degenerate, meaning that multiple codons encode the same amino acid. Take a look at the chart below and you’ll see that many amino acids are encoded by two or more codons. For example, the codons UCU, UCC, UCA, and UCG all code for the amino acid serine.
When mRNA is translated into a polypeptide chain during translation, the codons of mRNA base pair with complementary RNA sequences in tRNA molecules (which carry amino acids to the growing polypeptide) at the ribosome. You can see this depicted below:
There are many different tRNA molecules in a cell, each with its own anticodon sequence and associated amino acid. During the pairing between mRNA codons and tRNA anticodons, some tRNAs can form base pairs with more than one codon. This is because atypical base pairing, between nucleotides other than the typical A-U and G-C, can occur at the third position of the codon. This is known as wobble pairing. As shown in the figure below, wobble pairing allows a tRNA molecule carrying a single amino acid to recognize more than one codon. Wobble pairing follows its own set of rules: for example, while G in an anticodon can pair with C or U at the third position of a codon, it cannot pair with A or G. This ensures codons are read correctly despite wobble pairing.
Wobble pairing allows fewer tRNAs to cover all the codons of the genetic code, while still making sure that the code is read accurately.
MCAT Official Prep (AAMC)
Practice Exam 4 C/P Section Passage 9 Question 51
• The genetic code is degenerate, meaning that multiple codons can encode a single amino acid because of wobble pairing.
• Wobble pairing describes how tRNA molecules, each carrying an associated amino acid, can bind through their anticodon to multiple different mRNA codons during translation at the ribosome.
• Wobble pairing occurs because base pairing between the third position of a mRNA codon and tRNA anticodon does not follow typical base pairing rules.
Degenerate code: Multiple codons in the genetic sequence can encode a single amino acid.
Base pairing: Describes how specific pairs of nucleotides interact with one another through hydrogen bonding.
Messenger RNA (mRNA): An RNA molecule, copied from a DNA sequence, that encodes a polypeptide sequence (protein); three consecutive nucleotides constitute a codon.
Codon: The triplet of nucleotides in mRNA that encodes an amino acid.
Transfer RNA (tRNA): An RNA molecule, copied from a DNA sequence, that carries amino acids to the ribosome during translation.
Anticodon: The triplet of nucleotides in tRNA that is complementary to a mRNA codon and allows a tRNA to recognize specific codons during translation.
Wobble pairing: Atypical base pairing between the third position in a codon and anticodon that allows tRNAs to recognize more than one codon.
Translation: The process by which an mRNA molecule is used to create a polypeptide (protein).
Complementary: Describes the pairing between specific nucleotides in DNA and RNA.
Ribosome: A macromolecule made up of RNA and proteins that acts as the site of protein synthesis during translation.