It’s a fascinating process that involves the sequence of nucleotides in DNA and the translation of that code into the amino acids that make up proteins. Here are some general features of the genetic code that make it so unique and important:
Triplet Code: The genetic code is a triplet code, meaning that it consists of three nucleotides called a codon. This triplet code is necessary because a doublet code involving two adjacent nucleotides would not be sufficient to code for all the different amino acids that make up proteins.
Unambiguous: Each triplet specifies only one amino acid, making the code unambiguous. This means that there is no confusion as to which amino acid is being coded for.
Start and Stop Signals: The genetic code contains specific triplets that are necessary to initiate and terminate translation. These are known as start and stop signals and are essential for the proper functioning of the code.
Commaless: There is no internal punctuation or breaks between codons in the genetic code. This means that once the translation of mRNA begins, the codons are read one after the other with no interruptions.
Degenerate: A given amino acid can be specified by more than one triplet codon. This is known as degeneracy and is the case for 18 of the 20 amino acids. The different codons for a given amino acid are known as synonyms.
Nonoverlapping: After translation commences, any single ribonucleotide at a specific location within the mRNA is part of only one triplet. This ensures that there is no overlap or confusion between different codons.
Universal Code: The genetic code is commonly described as a universal code, meaning that it is used throughout all life forms. However, there are some exceptions to this, such as context-dependent codons that code for different amino acids in certain organisms.
Codon Bias
Refers to the unequal usage of codons in the genes of a particular organism.
Human genes, for example, use GTG four times more frequently than GTA among the four valine codons.
The biological reason for codon bias is unknown.
All organisms have a bias, which varies between species.
How was the genetic code cracked?
Marshall Nirenberg and Heinrich J. Matthaei elucidated the first codon in 1961 at the National Institutes of Health.
They used a cell-free system that included RNA templates, ribosomes, nucleotides, amino acids, stabilizing agents, and energy.
Homopolymer RNA sequence (UUUUU…) was translated using the cell-free system.
Polynucleotide phosphorylase enzyme, which catalyzes the formation of RNA polymers starting from ADP, UDP, CDP, and GDP, was discovered, resulting in the ability to synthesize long-chain messenger RNAs.
The enzyme produces polymers with a base composition that reflects the relative concentrations of nucleoside 5′-diphosphate precursors in the medium.
Under physiological conditions, the enzyme favors RNA degradation into nucleoside diphosphates.
Synthesis of homopolymer
Nirenberg and Matthaei used the enzyme polynucleotide phosphorylase in E. coli cell-free system to make uridine diphosphates into a poly-U messenger RNA.
They found that phenylalanine residues were incorporated into a polypeptide, and the first code word established was UUU for phenylalanine.
They continued the same approach and found that synthetic poly(C) codes for the formation of a polypeptide containing only proline (polyproline), and polyadenylate or poly(A) codes for polylysine.
The triplet CCC was assigned as a proline codon, and the triplet AAA as a lysine codon.
However, this type of experiment did not reveal what amino acid GGG specifies.
Synthesis of the mixed copolymer
Nirenberg and Matthaei used RNA mixed copolymer to decipher the codes.
Two or more different ribonucleoside diphosphates were added in combination to form the artificial message.
The researchers predicted the frequency of any particular triplet codon occurring in the synthetic mRNA by knowing the relative proportion of each type of ribonucleoside diphosphate.
Polynucleotide phosphorylase was presented with UDP, it makes only poly(U).
If it is presented with a mixture of one part of ADP and five of CDP, it will make a polymer with about one-sixth of the residues as adenylate and five-sixth cytidylate.
For AAA, the frequency is ( 1/6 )3 , or about 0.4 percent. For AAC, ACA, and CAA, the frequencies are identical, which is (1/6)2 (5/6), or about 2.3 percent for each. Together, all three 2A:1C triplets account for 6.9 percent of the total three-letter sequences.
Each of the three 1A:2C triplets accounts for (1/6) (5/6)2, or 11.6 percent (or a total of 34.8 percent). CCC is represented by (5/6), or 57.9 percent of the triplets.
By examining the percentages of any given amino acid incorporated into the protein synthesized under the direction of this message, it is possible to propose probable base composition.
Synthesis of repeating copolymer
Har Gobind Khorana devised biochemical methods to produce well-defined nucleic acids, including long RNA strands with every nucleotide in an exact position.
He chemically synthesized long RNA molecules consisting of short sequences repeated many times.
He created shorter sequences (di-, tri-, and tetranucleotides), which were replicated and joined enzymatically to form long polynucleotides.
He made a strand repeating the two nucleotides UCUCUC, which translated into a strand of amino acids, reading serine-leucine-serine-leucine.
Synthetic RNA was later used to decipher the rest of the genetic code.
Triplet binding assay
Nirenberg and P. Leder developed a triplet binding assay in 1964.
Ribosomes bind to an RNA sequence of three ribonucleotides and form a complex.
Triplets of known sequences were chemically synthesized, and the amino acid to be tested was made radioactive.
A charged tRNA was produced, and the radioactively charged tRNA, RNA triplet of known sequence, and ribosomes were incubated together on a nitrocellulose filter.
If radioactivity is retained on the filter, the charged tRNA has bound to the triplet associated with the ribosome, and a specific codon assignment was made.
Wobble in the anticodon
Frances Crick devised the wobble concept in 1966 to explain the observation that not every codon has a specific tRNA anticodon.
The wobble position is the base at the 5′ end of the anticodon that shows non-standard base pairing with any of several bases located at the 3′ end of a codon.
U at the wobble position can pair with either adenine or guanine, while I can pair with U, C, or A.
Inosine occupying the first (5) position of the anticodon permits recognition of three codons by a single tRNA molecule.
Conclusion
the genetic code is a complex yet fascinating process that is essential for the proper functioning of all living organisms. Its unique features ensure that the correct amino acids are coded for and that there is no confusion or overlap between different codons. So next time you think about DNA and proteins, remember the amazing power of the genetic code!
