Termination and Genetic Code of Protein Synthesis

Protein synthesis requires orderly interactions between mRNA and other ribonucleic acids (transfer RNA [tRNA] and ribosomal RNA [rRNA], the ribosome, and over 100 enzymes. The genetic instructions are carried by mRNA, which is produced in the nucleus during transcription and transported to the ribosomes via the nuclear membrane. The process of using the information encoded in mRNA to drive the sequencing of amino acids and, eventually, the production of a protein is known as translation.

Protein Synthesis

Before an amino acid can be integrated into a polypeptide chain, it must be linked to its own tRNA. This critical step requires the aminoacyl-tRNA synthetase enzyme. Each amino acid has its own tRNA synthetase. This high level of specificity is essential for incorporating the correct amino acid into a protein. When an amino acid molecule is attached to its tRNA carrier, protein synthesis can begin.

Genetic Code

The genetic code connects the 20 amino acids to all 64 possible combinations of the four nucleotide bases found in DNA (A, T, G, and C) (A, U, G, and C).

The genetic information in the nucleotide sequence of DNA is transcribed into the precise nucleotide sequence of an RNA molecule in the nucleus. The nucleotide sequence of the RNA transcript is complementary to the nucleotide sequence of the template strand of its gene, according to base-pairing regulations. Several types of RNA collaborate to direct protein synthesis.

In prokaryotes, the gene, the messenger RNA (mRNA) produced by the gene, and the polypeptide product all have a linear relationship. In higher eukaryotic cells, the initial transcript is significantly larger than the mature mRNA, complicating matters. Large mRNA precursors contain both coding segments (exons) that will become mature mRNA and extensive intervening sequences (introns) that separate the exons. As the mRNA is processed within the nucleus, the introns, which make up significantly more of the RNA than the exons, are deleted. Exon splicing produces mature mRNA, which is then transported to the cytoplasm and translated into protein.

The cell must have the machinery to precisely and efficiently translate information from the nucleotide sequence of an mRNA to the amino acid sequence of the corresponding specific protein. The decipherment of the genetic code required clarification of our understanding of the translation process. Because mRNA molecules have no affinity for amino acids, an intermediary adapter molecule must be used to translate the information in the mRNA nucleotide sequence into the amino acid sequence of a protein. This adaptor molecule must detect a specific nucleotide sequence on the one hand, and an amino acid on the other. Using such an adapter molecule, the cell can route a specific amino acid into the proper sequential position of a protein during its production, as dictated by the nucleotide sequence of the specific mRNA. In reality, the functional groups of the amino acids do not make direct contact with the mRNA template.

Because the cellular complement of proteins necessitates the production of twenty different amino acids, the genetic code must contain at least 20 distinct codons. Because mRNA only has four different nucleotides, each codon must contain more than one purine or pyrimidine nucleotide. Codons with two nucleotides can have a maximum of 16(42) distinct codons, whereas codons with three nucleotides can have a maximum of 64(43) distinct codons.

Each codon is now known to be composed of a three-nucleotide sequence, indicating that it is a triplet code. In vitro production of nucleotide polymers, particularly triplets in repeating sequences, was critical in early genetic code decoding. In the test tube, these synthetic triplet ribonucleotides were used as mRNAs to programme protein synthesis, allowing researchers to deduce the genetic code.

The three termination codons used at the end of a protein-coding sequence in mRNA are UAA, UAG, and UGA. TRNAs do not recognise these codons. As a result, one of many proteins known as release factors binds to these tRNAs, allowing mRNA to be released from the ribosome and ribosomal dissociation to occur.

The sequence of nucleotides in deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) determines the amino acid sequence of proteins (RNA). Despite the fact that the linear sequence of nucleotides in DNA provides information for protein sequences, proteins are not created directly from DNA. Instead, a messenger RNA (mRNA) molecule is created from DNA and directs the production of the protein. The four nucleotides that make up RNA are adenine (A), guanine (G), cytosine (C), and uracil (U) (U). The codon is made up of three contiguous nucleotides that code for an amino acid. The codon AUG, for example, specifies the amino acid methionine. Three codons represent the end of a protein but do not code for amino acids. The remaining 61 codons specify the 20 amino acids that make up proteins. The AUG codon is found at the beginning of every mRNA and indicates the beginning of a protein as well as coding for methionine. Methionine and tryptophan are the only two amino acids that have only one codon (AUG and UGG, respectively). The remaining 18 amino acids are coded by two to six codons. The code is referred to as degenerate because the majority of the 20 amino acids are coded by multiple codons.

Conclusion

The genetic code, long assumed to be the same in all forms of life, has been found to differ slightly in some organisms and mitochondria of some eukaryotes. Nonetheless, these changes are uncommon, and the genetic code of nearly every species is identical, with the same codons indicating the same amino acids. In the early 1960s, three American biochemists, Marshall W. Nirenberg, Robert W. Holley, and Har Gobind Khorana, deciphered the genetic code.