Triplets of nucleotides

Cellular protein biosynthesis involves the following steps. One strand of double-stranded DNA serves as a template strand for the synthesis of a complementary single-stranded messenger ribonucleic acid (mRNA) in a process called transcription. This mRNA in turn serves as a template to direct the synthesis of the protein in a process called translation. The codons of the mRNA are read sequentially by transfer RNA (tRNA) molecules, which bind specifically to the mRNA via triplets of nucleotides that are complementary to the particular codon, called an anticodon. Protein synthesis occurs on a ribosome, a complex consisting of more than 50 different proteins and several stmctural RNA molecules, which moves along the mRNA and mediates the binding of the tRNA molecules and the formation of the nascent peptide chain. The tRNA molecule carries an activated form of the specific amino acid to the ribosome where it is added to the end of the growing peptide chain. There is at least one tRNA for each amino acid. 

The sequence of bases in the polynucleotide chain is also important because it determines the exact sequence of amino acids used in the synthesis of a protein. Twenty amino acids are commonly found in proteins, while only four bases are used in the DNA molecule. Thus, more than one base must specify each amino acid. The genetic code is in fact read as triplets and there are 64 possible triplet combinations using 4 nucleotides. Each triplet of nucleotides is termed a codon, and given the redundancy, some amino acids are specified by more than one codon. 

The triplets of nucleotide units in DNA determine the amino acids in a protein through the intermediary mRNA. One of the DNA strands serves as a template for synthesis of mRNA, which has nucleotide triplets (codons) complementary to those of the DNA. In some bacterial and many eukaryotic genes, coding sequences are interrupted at intervals by regions of noncoding sequences (called introns). 

Several key properties of the genetic code were established in early genetic studies (Figs 27-3, 27-4). A codon is a triplet of nucleotides that codes for a specific amino acid. Translation occurs in such a way that these nucleotide triplets are read in a successive, nonoverlapping fashion. A specific first codon in the... 

In accordance with coding requirements each site is actually a triplet of nucleotide residues, but this feature need not concern us here. 

Before the triplet nature of codons had been established, Crick and associates used frame-shift mutations in a clever way to demonstrate that the genetic code did consist of triplets of nucleotides.7 55/55a Consider what will happen if two strains of bacteria, each containing a frame-shift mutation (e.g., a -1 deletion), are mated. Genetic recombination can occur to yield mutants containing both of the frame-shift mutations. 

Such as is seen in so-called triplet repeat disorders (TRDs) where for unknown specific reasons, a triplet of nucleotide base pairs—such as CGG in Fragile-X syndrome—expands between generations. There might be 5 to 50 repeats in the general population, but when this expands to 45 to 200 repeats this can become a genetic catalyst—a premutation —and if over 230, and up to several thousand repeats are in the male offspring, the boy will have mental retardation that becomes apparent in late infancy. 

As shown in Figure 6-16, the activated amino acid is attached to the end of its specific /RNA molecule. The function of this aminoacyl /RNA molecule is to place the amino acid that it is carrying in the proper sequence position on the template. Triplets of nucleotides— called anticodons—in the /RNA molecule (Fig. 6-16) are attracted to the complementary codon of mRNA). The amino acid carried by its specific /RNA is thus brought to the correct position on the mRNA codon. Since the anticodon on the /RNA is identical to the codon on DNA (except thymine replaces uracil), the DNA directs the amino acids in the protein-forming chain. The process of protein biosynthesis can now be considered. 

Table 3.3. The genetic code relating the possible sequence of a triplet of nucleotides to individual amino acids.

Messenger RNA (mRNA) contains the information (formerly residing in DNA) that is decoded in a way that enables the manufacture of a protein, and migrates from the nucleus to ribosomes in the cytoplasm (where proteins are assembled). A triplet of nucleotides within an RNA molecule (called a codon) specifies the amino acid to be incorporated into a specific site in the protein being assembled. A cell s population of mRNA molecules is very diverse, as these molecules are responsible for the synthesis of the many different proteins found in the cell. However, mRNA makes up only 5 percent of total cellular RNA. 

The final reaction, namely protein synthesis or translation of mRNA information into amino acid sequences, then occurs via a specific anticodon of specific RNASs (Fig. 8.4.4). A triplet of nucleotides codes for a single amino acid, e.g., UUC or UUU for Phe. 

Genetic code The relationship between triplets of nucleotide bases in messenger RNA and the amino acids incorporated into a protein in DNA-directed protein biosynthesis. 

At the loop of still another arm is a specific sequence of bases, called the anticodon. The anticodon is highly important because it allows the tRNA to bind with a specific site—called the codon—of mRNA. The order in which amino acids are brought by their tRNA units to the mRNA strand is determined by the sequence of codons. This sequence, therefore, constitutes a genetic message. Individual units of that message (the individual words, each corresponding to an amino acid) are triplets of nucleotides. 

The triplets of nucleotides (the codons) on mRNA are the genetic code (see Table 25.2). The code must be in the form of three bases, not one or two, because there are 20 different amino acids used in protein synthesis but there are only four different bases in mRNA. If only two bases were used, there would be only 4, or 16, possible combinations, a number too small to accommodate all of the possible amino acids. However, with a three-base code, 4, or 64, different sequences are possible. This is far more than... 

An obvious answer is that there is not one base, but rather a combination of bases coding for each amino acid. If the code consists of nucleotide pairs, there are 4 = 16 combinations this is a more extensive code, but it is still not extensive enough to code for 20 amino acids. If the code consists of nucleotides in groups of three, there are 4 = 64 combinations more than enough to code for the primary structure of a protein. This appears to be a very simple solution to the problem for a system that must have taken eons of evolutionary trial and error to develop. Yet proof now exists, from comparisons of gene (nucleic acid) and protein (amino acid) sequences, that nature does indeed use a simple three-letter or triplet code to store genetic information. A triplet of nucleotides is called a codon. 

Codon A triplet of nucleotides on mRNA that directs the incorporation of a specific amino acid into a polypeptide sequence. 

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