Question
explain why scientists rejected a 1- or 2-base code when trying to work out the genetic code
Answers
GPT-4o mini
When scientists were trying to decipher the genetic code, they initially considered various possibilities for how amino acids could be encoded by nucleotide sequences in DNA and RNA. One of these possibilities was a code based on either 1-base or 2-base combinations. However, they ultimately rejected these simpler coding schemes for several reasons:
1. **Limited Combinations**: A 1-base code, where a single nucleotide (A, T, C, or G) represents an amino acid, could encode only four unique signals. Since there are 20 standard amino acids used in protein synthesis, a single nucleotide could not provide enough unique codes. A 2-base code, using pairs of nucleotides (like AA, AT, AC, etc.), would provide only 16 unique combinations (since 4 nucleotides can form 4^2 = 16 combinations), which still falls short of the 20 amino acids.
2. **Need for Specificity**: The genetic code needs to be highly specific. Each amino acid needs a unique sequence to minimize the risk of incorporating the wrong amino acid into a protein, which could lead to nonfunctional or harmful proteins. A 1- or 2-base code would not provide the specificity required to distinguish among more than a few amino acids.
3. **The Codon Hypothesis**: Through experiments, scientists found that three nucleotides—known as a codon—seemed to correlate well with amino acids. The reason a 3-base code makes sense is that it allows for 64 combinations (4^3), which comfortably accommodates the 20 amino acids, while leaving room for stop signals and any potential additional amino acids. This redundancy also helps to mitigate the effects of mutations, as multiple codons can specify the same amino acid.
4. **Experimental Evidence**: Evidence from experiments such as triplet binding assays and the work of Har Gobind Khorana, Marshall Nirenberg, and others showed that triplet codons were functional in directing the incorporation of specific amino acids. They demonstrated that certain combinations of three nucleotides corresponded to distinct amino acids, reinforcing the triplet coding structure.
5. **Historical Context and Progress**: Early work on viruses and simpler organisms such as bacteria revealed patterns consistent with a triplet code. The cumulative experimental data over time made it clear to scientists that a 3-base codon system was not just adequate but necessary to support the diverse and complex nature of proteins.
In conclusion, the rejection of a 1- or 2-base genetic code scheme was based on the limitation of combinations to specify the range of amino acids needed for protein synthesis, the requirement for specificity and redundancy, and the robust experimental evidence supporting a 3-base codon structure.
1. **Limited Combinations**: A 1-base code, where a single nucleotide (A, T, C, or G) represents an amino acid, could encode only four unique signals. Since there are 20 standard amino acids used in protein synthesis, a single nucleotide could not provide enough unique codes. A 2-base code, using pairs of nucleotides (like AA, AT, AC, etc.), would provide only 16 unique combinations (since 4 nucleotides can form 4^2 = 16 combinations), which still falls short of the 20 amino acids.
2. **Need for Specificity**: The genetic code needs to be highly specific. Each amino acid needs a unique sequence to minimize the risk of incorporating the wrong amino acid into a protein, which could lead to nonfunctional or harmful proteins. A 1- or 2-base code would not provide the specificity required to distinguish among more than a few amino acids.
3. **The Codon Hypothesis**: Through experiments, scientists found that three nucleotides—known as a codon—seemed to correlate well with amino acids. The reason a 3-base code makes sense is that it allows for 64 combinations (4^3), which comfortably accommodates the 20 amino acids, while leaving room for stop signals and any potential additional amino acids. This redundancy also helps to mitigate the effects of mutations, as multiple codons can specify the same amino acid.
4. **Experimental Evidence**: Evidence from experiments such as triplet binding assays and the work of Har Gobind Khorana, Marshall Nirenberg, and others showed that triplet codons were functional in directing the incorporation of specific amino acids. They demonstrated that certain combinations of three nucleotides corresponded to distinct amino acids, reinforcing the triplet coding structure.
5. **Historical Context and Progress**: Early work on viruses and simpler organisms such as bacteria revealed patterns consistent with a triplet code. The cumulative experimental data over time made it clear to scientists that a 3-base codon system was not just adequate but necessary to support the diverse and complex nature of proteins.
In conclusion, the rejection of a 1- or 2-base genetic code scheme was based on the limitation of combinations to specify the range of amino acids needed for protein synthesis, the requirement for specificity and redundancy, and the robust experimental evidence supporting a 3-base codon structure.