Genetics nucleotide

In addition to their role in genetics, nucleotides play other important roles in biochemistry. Key enzymes and coenzymes such as nicotinamide adenine dinucleotide (NAD), flavin adenine dinucleotide (FAD), and vitamin B12 also include nucleotides as part of their structures. Also, the major component of viruses is DNA. 

Very few somatic cell genes are actively expressed. There appears to be something in the cell s cytoplasm that directs genetic expression (Saltus, 2006). Some of this may have to do with methyla-tion, wherein a methyl hydrocarbon group (CH3) replaces a hydrogen atom on a genetic nucleotide base. A gene that is methylated lies dormant. 

As important as nucleotides of adenosine are to bioenergetics that is not the only indispensable part they play m biology The remainder of this chapter describes how these and related nucleotides are the key compounds m storing and expressing genetic information... 

The genetic code (Table 28 3) is the message earned by mRNA It is made up of triplets of adjacent nucleotide bases called codons Because mRNA has only four dif ferent bases and 20 ammo acids must be coded for codes using either one or two nucleotides per ammo acid are inadequate If nucleotides are read m sets of three how ever the four mRNA bases generate 64 possible words more than sufficent to code for 20 ammo acids... 

Once the broad outlines of DNA replication and protein biosynthesis were established scien tists speculated about how these outlines af fected various origins of life scenarios A key question concerned the fact that proteins are re quired for the synthesis of DNA yet the synthesis of these proteins is coded for by DNA Which came first DNA or proteins How could DNA store genetic infor mation if there were no enzymes to catalyze the polymerization of its nucleotide components How could there be proteins if there were no DNA to code for them ... 

New Chapter 28 Nucleosides Nucleotides and Nucleic Acids is new Its presence testifies to the importance of these topics and the explosive growth of our knowledge of the molecular basis of genetics... 

In the human cell there are 23 pairs of chromosomes containing approximately 3000 million base pairs of DNA. Short sequences of DNA, perhaps with as few as 20 nucleotide units and sometimes radiolabeled, can be obtained either by chemical synthesis (gene machine) or from cloning. These short sequences can be used to probe for a complementary sequence by looking for the position to which they bind to any DNA sample under investigation, from blood for example. Such probes can detect as little as 100 fg of DNA and are the basis of forensic genetic fingerprinting tests. 

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. 

The DNA molecules in each cell of an organism contain all the genetic information necessary to ensure the normal development and function of that organism. This genetic information is encoded in the precise linear sequence of the nucleotide bases from which the DNA is built. DNA is a linear molecule while its diameter is only about 20 A, if stretched out its length can reach many millimeters. This means that concentrated solutions of DNA can be pulled into fibers in which the long thin DNA molecules are oriented with their long axes parallel. 

New chapter on Nucleosides, Nucleotides, and Nucleic Acids testifies to the explosive growth of the molecular- basis of genetics. 

Proteins are a diverse and abundant class of biomolecules, constituting more than 50% of the dry weight of cells. This diversity and abundance reflect the central role of proteins in virtually all aspects of cell structure and function. An extraordinary diversity of cellular activity is possible only because of the versatility inherent in proteins, each of which is specifically tailored to its biological role. The pattern by which each is tailored resides within the genetic information of cells, encoded in a specific sequence of nucleotide bases in DNA. 

The unique characteristic of each protein is the distinctive sequence of amino acid residues in its polypeptide chain(s). Indeed, it is the amino acid sequence of proteins that is encoded by the nucleotide sequence of DNA. This amino acid sequence, then, is a form of genetic information. By convention, the amino acid sequence is read from the N-terminal end of the polypeptide chain through to the C-terminal end. As an example, every molecule of ribonucle-... 

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