Redundancy, genetic code

A potentially general method of identifying a probe is, first, to purify a protein of interest by chromatography (qv) or electrophoresis. Then a partial amino acid sequence of the protein is deterrnined chemically (see Amino acids). The amino acid sequence is used to predict likely short DNA sequences which direct the synthesis of the protein sequence. Because the genetic code uses redundant codons to direct the synthesis of some amino acids, the predicted probe is unlikely to be unique. The least redundant sequence of 25—30 nucleotides is synthesized chemically as a mixture. The mixed probe is used to screen the Hbrary and the identified clones further screened, either with another probe reverse-translated from the known amino acid sequence or by directly sequencing the clones. Whereas not all recombinant clones encode the protein of interest, reiterative screening allows identification of the correct DNA recombinant. 

A base substitution can also result in the formation of a new inappropriate terminator (or non-sense) codon, and are thus known as non-sense mutations. The polypeptide formed from such mutated genes will be shorter than normal and is most likely to be inactive. Owing to the redundancy of the genetic code, about a quarter of all possible base substitutions will not result in an amino acid replacement and will be silent mutations. 

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 ... 

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 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. 

Characteristics of the genetic code include specificity, universality, and redundancy, and it is nonoverlapping and commaless. 

The 3 terminal redundancy of the genetic code and its mechanistic basis were first appreciated by Francis Crick in 1966. He proposed that codons and anticodons interact in an antiparallel manner on the ribosome in such a way as to require strict Watson-Crick pairing (that is, A-U and G-C) in the first two positions of the codon but to allow other pairings in its 3 terminal position. Nonstandard base pairing between the 3 terminal position of the codon and the 5 terminal position of the anticodon alters the geometry between the paired bases Crick s proposal, labeled the wobble hypothesis, is now viewed as correctly describing the codon-anticodon interactions that underlie the translation of the genetic code. 

The sequence of bases (A, G, T, and C) in a strand of DNA specifies the order in which amino acids are assembled to form proteins. The genetic code is the collection of base sequences that correspond to each amino acid, codon. Since there are only four bases in DNA and twenty amino acids in protein, each codon must contain at least three bases.5 Two bases cannot serve as codons because there are only 42 possible pairs of four bases, but three bases can serve because there are 43 = 64 possible triplets. Since the number of possible triplets are more than enough, several codons designate the same ammo acid. In other words, the genetic code is highly redundant as shown in Table 7.1. For example, UCU, UCC, UCA, UCG, AGU, and AGC are all codes for serine. 

Codon positions Selective weighting of first, second, and third codon positions in translated genes, because of redundancy of genetic code. A general rule is that third-codon positions are under less selective constraint than first and second and, as such, are more... 

According to the genetic code [674], each nucleotide triplet in DNA corresponds to one amino acid in protein. Since there are 43 = 64 possible triplets but only 20 amino acids, there must be a redundancy in the code. This was first postulated by Crick [675] and later verified experimentally [676, 677]. 

The redundancy in the genetic code is settled on the tRNA anticodon side. For each codon on the mRNA, the first two nucleotides (counting from the... 

The genetic code is degenerate (redundant). There is at least 1 codon for each of the 20 common amino adds many amino adds have numerous codons. 

Natural variation in the genes that encode adrenergic receptors (ARs) have been identified. The variations of major interest for common diseases are those that occur with allele frequencies >1% and are termed polymorphisms. Within the coding region, polymorphic variation can result in either a change in the encoded amino acid (nonsynonymous) or, because of the redundancy of the genetic code, have no effect on the encoded residue (synonymous). The most common variants are single nucleotide polymorphisms (SNPs), but insertions and deletions are also found. AR polymorphisms have been considered as poten-... 

According to the central dogma, when one strand of DNA is transcribed into RNA and translated to make proteins, three consecutive nucleotides form a codon. Each codon specifies an amino acid or amino acid chain termination. For example, the nucleotide sequence, or codon, GGA specifies the amino acid glycine. The genetic code has substantial redundancy, in that two or more codons code for the same amino acid. For example, both GGA and GGC code for glycine. Amino acids are the basic constituents of proteins, which mediate all cellular functions. Only 20 different amino acids, in various arrangements, form the basic units of all the proteins in the human body. 

Nucleic acids are polymers of four different nucleotides, whereas proteins are polymers of 20 different amino acids. Thus, each nucleotide cannot stand for one amino acid. Neither can two nucleotides code for one amino acid, because there would be only 16 combinations of two nucleotides (4 = 16). In reality, three nucleotides code for one amino acid in a protein (4 = 64, so there is room for redundancy in the genetic code). The grouping of three nucleotides that code for one amino acid is called a codon. 

Proteins are linear polymers of amino acids. The sequence of a protein s constituent amino acids determines its biochemical function. The mRNA sequence is read in groups of three, called codons. Because there are four bases in DNA or RNA, there are 64 (4 ) codons. Only 20 amino acids are specified by translation, so there is more than one codon per amino acid. In other words, the genetic code is redundant. The code also contains punctuation marks. Three codons, UAG, UAA, and UGA, specify stop signals (like the periods in a sentence). One amino acid, methionine, coded by AUG, is used to initiate each protein (like a capital letter at the beginning of a sentence). Just as a letter that starts a sentence can also appear in an uncapitalized form inside the sentence, so methionine also appears internally in proteins. See Table 4-1. 

Genes will be expressed only if the environmental conditions are favorable. Thus, the presence of a gene, or set of genes, will not completely determine the appearance or behavior of the BU. If the environment is favorable, then the particular behavior or appearance coded by that gene will be manifested. Without the gene, however, the same environment will not likely lead to the same kind of behavior or appearance. The exception would be due to redundant genetic control. 

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