Codons, of messenger RNA

How does a cell read the code This question is dealt with in detail in Chapters 28 and 29. A key step is the positioning of each amino acid on the ribosome in proper sequence. This is accomplished by the pairing of codons of messenger RNA with the anticodons of the appropriate transfer RNA molecules as is indicated at the bottom of Fig. 5-30. Each tRNA carries the appropriate "activated" amino acid at its 3 end ready to be inserted into the growing peptide. 

The codons of messenger RNA recognize the anticodons of trans-fer RNAs rather than the amino acids attached to the tRNAs. A codon on mRNA forms base pairs with the anticodon of the tRNA. Some tRNAs are recognized by more than one codon because pairing of the third base of a codon is less crucial than that of the other two (the wobble mechanism). 

An anticodon is a sequence of three nucleotides in a transfer RNA (tRNA) that is complementary to a codon of messenger RNA (mRNA). The relationship between codons and the amino acids they code for is called the genetic code. The process of converting mRNA sequence information to the amino acid sequence of a protein is called translation. 

Andcodon (Section 25.5C) A sequence of three bases on transfer RNA (tRNA) that associates with a codon of messenger RNA (mRNA). 

Section 28 11 Three RNAs are involved m gene expression In the transcription phase a strand of messenger RNA (mRNA) is synthesized from a DNA tern plate The four bases A G C and U taken three at a time generate 64 possible combinations called codons These 64 codons comprise the genetic code and code for the 20 ammo acids found m proteins plus start and stop signals The mRNA sequence is translated into a prescribed protein sequence at the ribosomes There small polynucleotides called... 

Figure 1.8 Translation of messenger RNA. The attachment of a ribosome to the mRNA involves protein initiation factors and the recognition of a particular base sequence, the start codon. A single mRNA can be simultaneously translated by more than one ribosome, forming a polyribosome. Synthesis occurs in the direction from the 5 end of messenger RNA to the 3 end. For further details of protein synthesis see Chapter 20.

There is one separate tRNA for each amino acid and one separate specific synthetase. The enzyme must bind not only the correct amino acid but also the correct tRNA, so that each synthetase has specific recognition sites for both. Transfer RNAs contain a three-base sequence that is an anticodon, which binds to its complementary codon on messenger RNA. The importance of the synthetase in relation to fidelity of translating the information in messenger RNA is indicated by the fact that, once an amino acid is bound to tRNA, its identity as an amino acid is dictated by the anticodon site on the transfer RNA and not by the amino acid itself. (The enzyme can be considered as a dictionary, since it provides a cross-reference between the nucleic acid and amino acid languages.)... 

The arrangement of amino acids in polypeptide chains is determined by the arrangement of codons in messenger RNA molecules. 

The fidelity of protein synthesis requires the accurate recognition of three-base codons on messenger RNA. Recall that the genetic code relates each amino acid to a three-letter codon (Section 5.5.1). An amino acid cannot itself recognize a codon. Consequently, an amino acid is attached to a specific tRNA molecule that can recognize the codon by Watson-Crick base pairing. Transfer RNA serves as the adapter molecule that binds to a specific codon and brings with it an amino acid for incorporation into the polypeptide chain. 

Fig. 12.4 Outline of the process of protein synthesis (translation of messenger RNA) in bacterial cells. The four stages of synthesis are shown initiation, elongation, translocation and termination with the sites of action of antibiotics. AUG is the start codon on messenger RNA (mRNA) specifying the first amino acid in bacterial proteins, N-formylmethionine. UAG, UAA and UGA are termination codons specifying no amino acid. 30S and 50S are the subunits of the ribosome. Other protein factors involved in protein synthesis are initiation factors (IF-1,2,3), elongation factors (EF-Tu and EF-G) and release factors (RF-1,2,3).

It has been shown that the first step in ribosome-dependent peptide synthesis is activation of amino acids to form amino acid adenylates. The amino acids are then transferred to RNA present in the soluble extract of the cell, the so-called transfer RNA (tRNA) to which the amino acids become fixed by an ester linkage. These two steps are usually referred to as the formation of aminoacyl-tRNA. The next step, the translation step of codons in messenger RNA (mRNA), which is associated with ribosomes, to provide a polypeptide includes three stages (1) chain initiation by mutual coordination with initiation factors, (2) chain elongation in aid of elongation factors, and (3) chain termination in support of release factors. 

The genetic code specifies unique three base sequences (called codons) for each of the 20 amino acid residues. Transfer RNA molecules (tRNAs), which are composed of a nucleic acid and a specific amino acid, provide the link between the nucleic acid sequence of messenger RNA (mRNA) and the amino acid sequence it codes for. 

Today, the evolution of genes, programmed cell death (apoptosis), and the action of messenger RNA (mRNA) are three major targets of research. mRNA contains the blueprint for every protein in the body. It is transcribed from a DNA template, and carries information to ribosomes, the sites of protein synthesis. The sequences of nucleic acid polymers are translated by transfer RNA (tRNA) into amino acid polymers. tRNA recognizes the three-nucleotide sequences that encode each amino acid. Ribosomal RNA directs the ribosome s production of proteins. Codons carry the messages that terminate protein synthesis. 

These procedures have permitted researchers to determine the base composition and the base sequence of the codons for a large number of amino acids and to establish that the code is degenerate, universal, operates in vivo, and is not overlapping. The accepted code for each amino acid is presented in Table 2-4. There are 64 sequence combinations of trinucleotides when four different bases are used to build the triplet. A brief look at Table 2-4 shows that 60 of the 64 combinations are involved in amino acid coding. Two sequences have no known coding properties (CUA and CUG) and are therefore called nonsense codons. The intercalation of such a triplet into the sequence of messenger RNA is responsible for nonsense mutation. UAA and UAG are now known to be involved in punctuation. 

Anticodon The three-base region of transfer RNA which recognizes and binds to the codon on messenger RNA. 

As a general principle, translation is the step in which the nucleotide sequence of messenger RNA is translated into a definite amino-acid sequence, using the adaptor transfer RNAs decoding units. As stated before, the recognition mechanism between messenger RNA and transfer RNAs is of the codon-anti-codon type. 

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. 

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