Question

How many codons are possible? All of these codons are used in molecules of mRNA and there are only 20 amino acids normally encoded (plus start codons and stop codons). Is there exactly one codon per amino acid, must some codons select for more than one amino acid, or must there be some amino acids for which there is more than one codon?

Short answer

There are 64 codons; some amino acids are encoded by multiple codons.

Step-by-step solution

Understanding Codons

Codons are sequences of three nucleotide bases. Since mRNA uses four bases—adenine (A), cytosine (C), guanine (G), and uracil (U)—each position in a codon can be filled by one of these four bases, making the total number of possible codons determined by combinations of these bases.

Calculate Total Codon Combinations

The number of possible codons is calculated by raising the number of nucleotide options per position in a codon to the power of the number of positions. Thus, the total number of codons is given by the formula: \[4^3 = 64\] This calculation results in 64 possible combinations of codons.

Determine Codon Usage

There are 64 possible codons but only 20 amino acids, plus start and stop signals. This implies that not every codon corresponds to a unique amino acid due to the surplus number of codons compared to amino acids.

Evaluate Codon Assignment

Each amino acid is encoded by one or more codons. The surplus of 64 codons implies that some amino acids must be encoded by more than one codon. Additionally, some codons are designated as start or stop codons, which also reduces the number of codons available for amino acid encoding.

Conclusion

Because 64 codons encode only 20 amino acids (and are used for start/stop signaling), there are more codons than amino acids, meaning some amino acids are associated with multiple codons. This phenomenon is commonly known as the redundancy or degeneracy of the genetic code.

Key concepts

Codons

Codons are an essential component of genetic coding. They are sequences of three nucleotide bases. In the messenger RNA (mRNA) framework, these bases include adenine (A), cytosine (C), guanine (G), and uracil (U).
This combination allows codons to be highly specific and instructions precise. For instance, by arranging three bases, a unique instruction can be communicated during protein synthesis.

• Each codon is made up of three positions.
• There are four possible nucleotide options for each position: A, C, G, and U.

Given the structure of codons, a total of 64 combinations can be achieved. This results from the mathematical expression \[ 4^3 = 64 \] which stands for the 4 bases raised to the power of 3 positions.

Amino Acids

Amino acids are the building blocks of proteins. There are 20 standard amino acids, each playing a crucial role in forming proteins that influence everything from enzyme function to cell structure.
Interestingly, the genetic code has a surplus of codons as there are 64 possible codon combinations created by three-base sequences versus just 20 amino acids.
This surplus means multiple codons can encode the same amino acid. This phenomenon is referred to as the redundancy or degeneracy of the genetic code. Redundancy ensures that even with minor mutations or errors in the genetic sequence, the correct amino acid can often still be incorporated into the protein, minimizing errors in protein synthesis.

mRNA

Messenger RNA (mRNA) serves as the vital link between the genetic information encoded in DNA and the synthesis of proteins. It is transcribed from the DNA template and carries the genetic instructions to the ribosome, where protein synthesis occurs.
In mRNA, genetic information is organized into codons, which are read in sequence during translation. Each codon specifies a particular amino acid, guiding the ribosome in assembling the corresponding protein according to the genetic script.
The pivotal role of mRNA in gene expression highlights its importance in cellular function and the larger process of genetic coding. Its interactions with codons form the backbone of the translation process, ensuring the accurate conversion of genetic instructions into functional proteins.

Nucleotide Bases

The genetic code is composed of four types of nucleotide bases that form the letters of DNA and RNA: adenine (A), cytosine (C), guanine (G), and thymine (T) in DNA, but uracil (U) replaces thymine in RNA (like mRNA).
These nucleotide bases pair up through hydrogen bonds in specific ways: A with T (or U in RNA) and C with G, creating the uniform structure of the RNA and DNA sequences. In mRNA, these bases order themselves into codons, shaping the precise genetic instructions required for protein synthesis.

• A pairs with T (or U in RNA)
• C pairs with G

The distinct sequence of these bases forms the codons that encode the entire range of biological proteins, adhering to the universal principles of the genetic code.