Question

Prior to the elucidation of the genetic code, prominent scientists, including Francis Crick, had predicted that each mRNA codon, coding for one of the 20 amino acids, needed to be at least three nucleotides long. Why is it not possible for codons to be any shorter?

Short answer

Codons need to be at least three nucleotides long because with only four different RNA nucleotides, one or two nucleotide long codons would not provide enough different combinations to uniquely specify all 20 amino acids.

Step-by-step solution

Understanding Codon Necessity

To understand why codons cannot be shorter than three nucleotides long, we must consider the number of unique amino acids and the available nucleotides. There are 20 standard amino acids that are used to form proteins in biological systems. There are four different RNA nucleotides: adenine (A), uracil (U), cytosine (C), and guanine (G).

Calculating Combinations with One-Nucleotide Codons

If codons were only one nucleotide long, each nucleotide could only specify one amino acid. Since there are four different nucleotides, this would allow for only four unique codons, which is insufficient to code for all 20 amino acids.

Calculating Combinations with Two-Nucleotide Codons

If codons were two nucleotides long, the number of possible codons would be the number of nucleotides squared (4^2). This gives us 16 possible combinations (AA, AU, AC, AG, UA, UU, UC, UG, CA, CU, CC, CG, GA, GU, GC, GG). With only 16 possible codons, this would still be insufficient to uniquely specify all 20 amino acids.

Calculating Combinations with Three-Nucleotide Codons

If codons are three nucleotides long, the number of possible codons is the number of nucleotides cubed (4^3). This results in 64 possible combinations. With 64 combinations, there are enough codons not only to encode for all 20 amino acids but also to have some redundancy (more than one codon per amino acid), which is observed in the genetic code.

Conclusion

Codons cannot be shorter than three nucleotides because there would not be enough different codons to specify all 20 amino acids. With three-nucleotide codons, the genetic code can effectively translate the 20 amino acids necessary for synthesizing proteins.

Key concepts

mRNA Codons

The genetic code is akin to a biological dictionary, where 'words'—known as mRNA codons—comprise three nucleotides that correspond to specific amino acids, the building blocks of proteins. Imagine trying to compose meaningful text with an alphabet containing only two letters; the limitation would severely hamper the variety of words you can create. Similarly, for mRNA codons, a length of three nucleotides is essential as it provides a combination that is vast enough to represent the 20 standard amino acids utilized in life's molecular repertoire.

Each codon acts as a unique signal within the messenger RNA (mRNA) sequence and fits neatly into the ribosome during protein synthesis. As each tRNA molecule brings the appropriate amino acid matching the codon on mRNA, a specific protein chain is built. This specificity is crucial for precise protein function, which underlines most biological processes.

Nucleotide Combinations

The crux of understanding why a codon is three nucleotides long lies within the math of nucleotide combinations. Nucleotides are the letters of genetic code—adenine (A), uracil (U), cytosine (C), and guanine (G). Just as you combine letters to form words, nucleotides are combined to form codons. If we were to use only one nucleotide to make a codon, it's akin to spelling all the words in a language with a single letter—an impossible task given there are 20 amino acids to account for.

The Power of Threes

With two-nucleotide codons, you'd get limited combinations—only 16, far less than needed. It is only when you step up to three nucleotides that the magic happens: the number of possible combinations jumps to 64 (4 cubed). This expansion affords not only the ability to code for all amino acids but also provides redundancy, an important feature that acts as a buffer against certain types of genetic mutations.

Protein Synthesis

Protein synthesis is the molecular process by which cells manufacture proteins, the workhorses that carry out and regulate nearly all cellular functions. This intricate process begins with transcription, where a segment of DNA is copied into mRNA. Then, during translation, ribosomes read the mRNA three nucleotides at a time—one codon—and match them to the corresponding amino acid conveyed by transfer RNA (tRNA) molecules.

From Language to Life

One can think of ribosomes as the 'factories' and tRNAs as the 'delivery trucks'. Together, they decode the mRNA's series of codons to synthesize a polypeptide chain that will fold into a functional protein. This procedural fidelity from genetic code to functional protein is vital. Errors in the process can lead to malfunctions and diseases, demonstrating the genetic code's critical role in maintaining the harmony of life at a cellular level.