Question

The Genetic Code. Suppose that, as evidence suggests, very early life on Earth used a genetic code that consisted of only two-base "words" rather than three- base "words." Could such life have made use of the same set of 20 amino acids that life uses today? Explain, using quantitative arguments.

Short answer

No, a two-base code with 16 combinations cannot represent all 20 amino acids.

Step-by-step solution

Understand the Current Genetic Code

In modern biology, the genetic code is made up of three-base 'words' called codons. Each codon consists of three nucleotides, which might be one of four types: Adenine (A), Cytosine (C), Guanine (G), and Thymine (T). This results in several combinations.

Calculate the Number of Possible Codons

In the current three-base system, each position in the codon can be one of four nucleotides. Therefore, the number of possible codons is \[ 4 \times 4 \times 4 = 64 \] and these 64 codons encode for 20 amino acids plus start and stop signals.

Consider a Two-Base System

In the hypothetical two-base 'word' system, each 'word' would have only two positions to fill with four nucleotides. Calculate the number of possible combinations: \[ 4 \times 4 = 16 \]

Analyze the Coverage of 20 Amino Acids

With only 16 possible combinations from a two-base code, it would not be sufficient to cover all 20 amino acids. This would result in a shortage because 16 codons are insufficient to represent 20 different entities.

Explain the Conclusion

Given that the hypothetical two-base system provides only 16 possible codons, it cannot encode for all 20 amino acids needed. Therefore, such early life could not have used the same full set of amino acids as today.

Key concepts

Two-Base Genetic Code

Imagine if early life on Earth used a simpler genetic coding system. Instead of three-nucleotide codons, there might have been codons made up of only two bases. This is like having shorter "words" in the language of DNA. In such a system, each codon consists of only two positions, which can each be occupied by one of the four unique nucleotides: Adenine (A), Cytosine (C), Guanine (G), and Thymine (T).
Here's a twist: With these two-base "words," the total number of possible combinations is limited. You can calculate these combinations by multiplying the possibilities for each position. Thus, a two-base system produces:

• 4 choices for the first position
• 4 choices for the second position

Which adds up to a total of \[ 4 \times 4 = 16 \] combinations. Compare this with the more robust three-base system, and you'll see why it limits the variety of what can be encoded.

Codons

Codons are like the syllables of the genetic language. In both the ancient and modern genetic systems, these sequences dictate how the blueprint for proteins is executed. In our current understanding, codons are sequences of three nucleotides, each complete with hundreds of possible combinations. However, in the hypothetical early two-base genetic system, the codons would be shorter, containing only two nucleotides.
This change significantly alters the number of possible codons. With two bases per codon, only 16 different codons would exist compared to the 64 possible in a system of three bases. These codons instruct the ribosome on which amino acid to add next in a growing protein chain, which can greatly impact the diversity and flexibility of life.
In essence, codons are critical because they are responsible for the translation of genetic information into proteins, but the choices they offer are directly linked to their length.

Amino Acids

Amino acids play a crucial role in building the proteins necessary for every cellular function. Proteins are like the workers in a cell, carrying out various important jobs. The genetic code's main output is these amino acids. In all known life forms, a total of 20 standard amino acids are used in various combinations to form proteins.
Each amino acid is represented by one or more codons. The three-base codon system provides 64 different codons, which comfortably cover all 20 amino acids, with some redundancy (multiple codons can code for the same amino acid). But in a two-base system, there are only 16 possible codons, which falls short of representing all 20 amino acids needed.
To imagine a scenario where early life still functioned efficiently with fewer codons, it's possible that fewer amino acids were used, or there was a simpler form of proteins being created. But the sophisticated life we know today couldn't function like that.

Nucleotides

Nucleotides form the basic building blocks of genetic material. In DNA, they pair up to form the structure of the double helix. There are four types in DNA: Adenine (A), Cytosine (C), Guanine (G), and Thymine (T). Their sequences determine the genetic instructions in organisms.
In both the current and early proposed systems, these nucleotides combine to form codons, the basic units of genetic coding. Each nucleotide is akin to a letter in the genetic alphabet. Their arrangement defines the genetic "words" that the cell machinery reads to synthesize proteins and execute other essential processes.
In the two-base system, the sequence possibilities decrease, limiting the range of instructions that can be coded. However, even in this simplified form, nucleotides are essential since they provide the fundamental blueprint from which all biological processes are derived.