Genetic code start codons

Genetic code Start AUG (also codes for Met) Stop UAG,UGA,UAA Unambiguous (1 codon = 1 amino acid) Redundant (1 amino acid >1 codon) often differ at base 3 ... 

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... 

As the genetic code provides 4 = 64 codons for the 20 amino acids, there are several synonymous codons for most amino acids— the code is degenerate. Three triplets do not code for amino acids, but instead signal the end of translation (stop codons). Another special codon, the start codon, marks the start of translation. The code shown here is almost universally applicable only the mitochondria (see p. 210) and a few microorganisms deviate from it slightly. 

The following table indicates the standard codons that serve as the basis of the genetic code. Note (a) Stop codons have no amino acids assigned to them (b) Met indicates the AUG start codon. 

The existence of 64 - 20 = 44 excess codons allows a valuable redundancy in the genetic code. It also permits the signaling for the start and end of the protein chain. 

C, G or T. This is not sufficient to encode the 20 possible amino acids. In triplets of 3 positions, there are 64 possible combinations. Hence, the system uses triplets, called codons. The code for each protein starts with an ATG (start codon) and ends with a TAA, TAG or a TGA (stop codons). The code is almost universal only mitochondria and ciliated protozoa have a different genetic code. 

Nonoverlapping and commaless The genetic code is nonoverlapping and commaless, that is, the code is read from a fixed starting point as a continuous sequence of bases, taken three at a time. For example, ABCDEFGHIJKL is read as ABC/DEF/GHI/JKL without any "punctuation" between the codons. 

In the next step, translation, the sequence of nucleotides in the newly synthesized mRNA strand is used to determine the sequence of amino acids in the protein to be synthesized. This is done by way of a genetic code, which was fully deciphered by 1966 and is shown in Figure 13.34. According to the genetic code, it takes three mRNA nucleotides—each three-nucleotide unit is called a codon—to code for a single amino acid. The mRNA nucleotide sequence AGU, for example, codes for the amino acid serine, and AAG codes for lysine. (Note from Figure 13.34 that more than one codon can call for the same amino acid.) A few codons, such as AUG and UGA, are the signals for protein synthesis to either start or stop. 

It is immediately obvious that there are three ways of reading the genetic code in mRNA depending upon which nucleotide is used to start each codon. For... 

Table 1.9 The genetic code. The start codon is always AUG. Some codons act as start and stop signals in protein synthesis. Codons are always written left to right, 5 to 3 ...

The genetic code is the rules that specify how the nucleotide sequence of an mRNA is translated into the amino acid sequence of a polypeptide. The nucleotide sequence is read as triplets called codons. The codons UAG, UGA and UAA do not specify amino acids and are called termination codons or Stop codons. AUG codes for methionine and also acts as an initiation (Start) codon. 

The characteristics of the genetic code render it a suitable object for a mathematical discussion. The facts that have influenced the thinking in this connection are the possible specificity of the codons and the existence of a starting point for the transfer of information. 

The genetic code is the relation between the sequence of bases in DNA (or its RNA transcript) and the sequence of amino acids in proteins. Amino acids are encoded by groups of three bases (called codons) starting from a fixed point. 

The genetic code is nonoverlapping (i.e., each nudeotide is used only once), beginning with a start codon (AUG) near the 5 end of the mRNA and ending with a termination (stop) codon (UGA, UAG, or UAA) near the 3 end. 

Part of the triplet genetic code involving mRNA codon triplets that start with U is shown below. 

As noted above, the genetic code used by cells is a triplet code, with every three-nucleotide sequence, or codon, being read from a specified starting point In the mRNA. Of the 64 possible codons in the genetic code, 61 specify individual amino acids and three are stop codons. Table 4-1 shows that most amino acids are encoded by more than one codon. Only two—methionine and tryptophan—have a single... 

The first step in translation is the assembly of the initiation complex. In bacteria, this is done in a three-step process that involves the initiation factors IF-1, IF-2, and IF-3, the 30S and 50S ribosomal subunits, and the hydrolysis of GTP. As shown in Figure 26.10, the first step is the formation of a preinitiation complex consisting of IF-1, IF-3, and the 30S ribosomal subunit. This preinitiation complex binds with high affinity to a translational start site on the mRNA transcript which is close to the 50 end and includes the first codon of the open-reading frame. This codon is called the initiation codon and it is usually AUG, which specifies the amino-acid methionine in the genetic code (Fig. 26.6). 

In the genetic code, methionine is coded for by the codon AUG. This codon is called the start codon because methionine is the first amino acid used to build a protein chain. Methionine forms the so-called amino terminus of a protein. In prokaryotes, a modified form of methionine, formyl-methionine is used as the first (but not subsequent) amino acid in proteins. Formyl-methionine is carried by a modified tRNA from the tRNA that carries unmodified methionine. 

Similarities between protein synthesis in bacteria and protein synthesis in eukaryotes same start and stop codons same genetic code same chemical mechanisms of synthesis interchangeable tRNAs. Major differences in prokaryotes, the Shine-Dalgarno sequence and no introns in eukaryotes, the 5 -cap and 3 -tail on mRNA and introns have been spliced out. 

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