Linear genetic codes

The general nature of the genetic code was suggested by the structure of DNA. Both DNA and proteins are linear polymers. Thus, it was logical to suppose that the sequence of the bases in DNA codes for the sequence of amino acids. There are only four bases in DNA but 20 different amino acids in proteins at the time of their synthesis. It is obvious that each amino acid must be specified by some combination of more than one base. While 16 pairs of bases are possible, this is still too few to specify 20 different amino acids. Therefore, it appeared that at least a triplet group of three nucleotides would be required to code for one amino acid.371 Sixty-four (43) such triplet codons exist, as is indicated in Tables 5-5 and 5-6. 

These four functional groups have similar properties—though alkyl iodides are the most reactive and alkyl fluorides the least. PVC (polyvinyl chloride) is one of the most widely used polymers—it has a chloro group on every other carbon atom along a linear hydrocarbon framework. Methyl iodide (Mel), on the other hand, is a dangerous carcinogen, since it reacts with DNA and can cause mutations in the genetic code. 

The linear deoxyribonucleotide sequence information encoded in genes is copied into a linear sequence of ribonncleotides in messenger RNA (mRNA) by the process of transcription. Ribosomes bind one at a time near to the 5 end of each mRNA molecule and translate the ribonucleotide base sequence into an aminoacyl sequence according to the genetic code. Each mRNA is bound to more than one ribosome at a time, forming a polyribosome, often abbreviated to polysome. Typically the rate of... 

A major goal of the human genome project is to determine the order of occurrence of the four bases, adenine (A), cytosine (C), guanine (G), and thymine (T), in DNA molecules. The sequence defines an individual s genetic code. The need for sequencing DNA has spawned the development of several new analytical instruments. Among the most attractive of these approaches is capillary array electrophoresis. In this technique, as many as 96 capillaries are operated in parallel. The capillaries are filled with a separation matrix, normally a linear polyacrylamide gel. The capillaries have inner diameters of 35 to 75 p,m and are 30 to 60 cm in length. 

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. 

Figure 14.2. The biochemistry of protein. Protein is another linear polymer in which each building block is an amino acid. Amino acids have a central ( alpha ) carbon to which an amino group, a carboxyl group, and a variable side chain are joined. Twenty amino acids are encoded within the standard genetic code.

Figure 14.3. (A) Both nucleic acid and protein sequences, as linear polymers, can be represented as strings of English letters. This is, indeed, exactly how they are stored in global, centralized databases of biological data. (B) The genetic code is the system of rules that maps nucleic acid sequences into proteins. Nucleotides are read, ree at a time (as codons ), and converted into a single amino acid by means of tRNAs, specialized adaptor molecules.

Proteins have many functions in the body. They serve as transporters of hydrophobic compounds in the blood, as cell adhesion molecules that attach cells to each other and to the extracellular matrix, as hormones that carry signals from one group of cells to another, as ion channels through lipid membranes, and as enzymes that increase the rate of biochemical reactions. The unique characteristics of a protein are dictated by its linear sequence of amino acids, termed its primary structure. The primary structure of a protein determines how it can fold and how it interacts with other molecules in the cell to perform its function. The primary structures of all of the diverse human proteins are synthesized from 20 amino acids arranged in a linear sequence determined by the genetic code. 

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