Amino acids nucleotide codon

Microparticles composed of each of four homopolyribonucleotides and the same lysine-rich proteinoid is found to influence the incorporation of individual amino-acyl adenylate 59). The incorporation favors the amino acids whose codons are related to the nucleotide in the particles (Fig. 6), when conditions are appropriately chosen. Other conditions yield other preferences. These results support a stereochemical basis for the genetic code. 

The protein defective in Menkes disease is a membrane-bound protein consisting of 1500 amino acids. Evidence suggests that this protein occurs in the membrane of the endoplasmic reticulum, not in the plasma membrane. The mutations in the gene coding for Menkes protein that are responsible for the disease take a number of forms. The gene, as studied in hundreds of human subjects, may contain insertions (an extra nucleotide), deletions (one less nucleotide), conversions of an amino acid s codon to a stop codon (resulting in a truncated protein), and other types of mutations. 

The answer is b. (Murray, pp 452-467. Scriver, pp 3-45. Sack, pp 1-40. Wilson, pp 101-120.) The genetic code uses three-nucleotide words, or codons, to specify the 20 different amino acids (see the chart below). There are 64 different three-base pair codons (three positions with four possible nucleotides at each). It follows that the genetic code must be degenerate, i.e., different codons can specify the same amino acid. Three codons are reserved as stop signals that result in peptide chain termination. The linear correspondence of codons in DNA and of amino acids in protein domains is interrupted by the presence of introns in DNA. Codons differ from the dinucleotide tandem repeats that provide useful DNA polymorphisms, or the trinucleotide repeats that can be responsible for disease. The genetic code is universal in the sense that codon-amino acid relationships are the same in all organisms. 

The first nucleotide in a codon has less specificity than the others Only one group of nucleotides codes for each single amino acid Every codon (three nucleotide bases) specifies an amino acid Specific nucleotide sequences signal termination of peptide chains... 

The synthesis of proteins is a more complex process (Fig. 124—11). Proteins consist of chains of amino acids in very specific seqnences. As in DNA synthesis, the double helix must unwind. However, in protein synthesis, only the portion of the DNA molecnle that codes for the desired protein is exposed. The enzyme RNA polymerase matches free complementary RNA nncleotides to the exposed DNA nucleotides, and the resultant chain of nncleotides is called mRNA. This process is called transcription. The mRNA travels to ribosomes in the cytoplasm, where protein synthesis occurs. Each three nucleotides of the mRNA chain compose a codon, whose seqnence is specific for a particular amino acid. The codon is recognized by tRNA, which then carries the amino acid to the ribosome, where it is added to the growing peptide chain. This process is known as translation. The... 

Three contiguous nucleotides is the minimum that can serve as a codon. There are four kinds of nucleotides in mRNA. A codon consisting of only two nucleotides (either of which could be any of the four possible nucleotides) allows only 16 possible combinations (4 x 4 = 16). This would not be sufficient to specify all 20 of the amino acids. A codon consisting of three nucleotides, however, allows 64 possible combinations (4x4x4 = 64), more than enough to specify the 20 amino acids. 

Various rate heterogeneity corrections are implemented in several tree-building programs. For nucleotide data, PAUP 4.0 implements both invariants and discrete gamma models for separate or combined use with time-reversible distance and likelihood tree-building methods and invariants in conjunction with the log-det distance method (see below). For nucleotide, amino acid, and codon data, PAML implements continuous, discrete, and autodiscrete models. For nucleotide and amino acid data, PHYLIP implements a discrete gamma model. 

The use of a large variety of random copolynucleotides along these lines resulted in the determination of the nucleotide composition of close to fifty amino acid-specifying codons in the laboratories of Drs. Ochoa and Nirenberg between the fall of 1961 and the summer of 1963. 

Each tRNA anticodon corresponds to a particular codon of the mRNA, so that each specific amino acid attached to its tRNA becomes ordered according to the sequence of codons in the mRNA. The information of the amino acid nucleotide code found in mRNA is translated into polypeptides, the messenger RNA being read in the 5 to 3 direction, with the polypeptides being synthesized from the amino-terminal to the car-boxy-terminal amino acid. Upon completion of synthesis of a polypeptide chain, the chain leaves the ribosome and assumes its particular conformation. It is able to combine with other polypeptide chains, and subsequent covalent modifications of the protein molecule can take place. 

Codon Sequence of three nucleotides that codes for a single amino acid (or a termination signal)... 

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. 

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 genetic code (Table 28.3) is the message caiiied by mRNA. It is made up of triplets of adjacent nucleotide bases called codons. Because mRNA has only four different bases and 20 amino acids must be coded for, codes using either one or two nucleotides per amino acid are inadequate. If nucleotides are read in sets of three, however, the four mRNA bases generate 64 possible words, more than sufficent to code for 20 amino acids. 

Among the 76 nucleotides of tRNA are two sets of three that are especially important. The first is a group of three bases called the anticodon, which is complementary to the inRNA codon for the amino acid being transfened. Table 28.3 lists two inRNA codons for phenylalanine, UUU and UUC (reading in the 5 3 direction). Because base-pairing requires the rnRNA and tRNA to be antiparallel, the two anticodons are read in the 3 5 direction as AAA and AAG. 

Codon (Section 28.11) Set of three successive nucleotides in mRNA that is unique for a particular amino acid. The 64 codons possible from combinations of A, T, G, and C code for the 20 amino acids from which proteins are constructed. 

THE NUCLEOTIDE SEQUENCE OF AN mRNA MOLECULE CONSISTS OF A SERIES OF CODONS THAT SPECIFY THE AMINO ACID SEQUENCE OF THE ENCODED PROTEIN... 

Twenty different amino acids are required for the synthesis of the cellular complement of proteins thus, there must be at least 20 distinct codons that make up the genetic code. Since there are only four different nucleotides in mRNA, each codon must consist of more than a single purine or pyrimidine nucleotide. Codons consisting of two nucleotides each could provide for only 16 (4 ) specific codons, whereas codons of three nucleotides could provide 64 4 ) specific codons. 

The terms first, second, and third nucleotide refer to the individual nucleotides of a triplet codon. U, uridine nucleotide C, cytosine nucleotide A, adenine nucleotide G, guanine nucleotide Term, chain terminator codon. AUG, which codes for Met, serves as the initiator codon in mammalian cells and encodes for internal methionines in a protein. (Abbreviations of amino acids are explained in Chapter 3.)... 

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