Genetic code features

Most genetic code tables designate the codons for amino adds as mRNA sequences (Figure 1-4-1). Important features of the genetic code include ... 

FIGURE 1-2 Diverse living organisms share common chemical features. Birds, beasts, plants, and soil microorganisms share with humans the same basic structural units (cells) and the same kinds of macromolecules (DNA, RNA, proteins) made up of the same kinds of monomeric subunits (nucleotides, amino acids). They utilize the same pathways for synthesis of cellular components, share the same genetic code, and derive from the same evolutionary ancestors. Shown here is a detail from "The Garden of Eden," by Jan van Kessel the Younger (1626-1679). 

A striking feature of the genetic code is that an amino acid may be specified by more than one codon, so the code is described as degenerate. This does not suggest that the code is flawed although an amino acid may have two or more codons, each codon specifies only one amino acid. The degeneracy of the code is not uniform. Whereas methionine and tryptophan have single codons, for example, three amino acids (Leu, Ser, Arg) have six codons, five amino acids have four, isoleucine has three, and nine amino acids have two (Table 27-3). 

Importance of the Second Genetic Code Some aminoacyl-tRNA synthetases do not recognize and bind the anticodon of their cognate tRNAs but instead use other structural features of the tRNAs to impart binding specificity. The tRNAs for alanine apparently fall into this category. 

The genetic code is shown in Table S.A2. All the possible 64 combinations are shown and there are codes for either start and stop signals or for amino-acids. There are up to six codes for some amino-acids, e.g. arginine, leucine and serine, while tryptophan and methionine have unique codes. The major features of the genetic code are ... 

Extremely sophisticated biological machinery based on specific non-covalent molecular recognition is involved in biological processes such as DNA replication, the transcription of the genetic code and in RNA mediated protein synthesis at the ribosome. This biochemical molecular machinery has recognisable mechanical features. 

Genes and chromosomes, the biological units of heredity, are also chemical units. The strands of DNA carry the genetic code, the information needed for the transmission of heritable features, in their sequence or arrangement of nucleotides. This is the basis for the transmission of chemical information and biological characteristics from one generation to the next. 

The EDITSEQ application allows you to manually enter DNA or protein sequence information into your computer. This application has several features that make it useful to the research scientist. First, it can identify open reading frames (possible gene sequences) within a DNA sequence. Second, it can provide the percent base composition (A,G,C,T), the percent GC, the percent AT, and the melting temperature of the entire sequence or a small subset of that sequence. Third, EDITSEQ can translate a nucleotide sequence into a protein sequence. Finally, the application is capable of translating or reverse translating a nucleotide sequence of interest using codes other than the standard genetic code. 

The genetic code is the relation between the sequence of bases in DNA (or its RNA transcripts) and the sequence of amino acids in proteins. Experiments by Francis Crick, Sydney Brenner, and others established the following features of the genetic code by 1961 ... 

Transfer RNA molecules (tRNAs), messenger RNA, and many proteins participate in protein synthesis along with ribosomes. The link between amino acids and nucleic acids is first made by enzymes called aminoacyl-tRNA synthetases. By specifically linking a particular amino acid to each tRNA, these enzymes implement the genetic code. This chapter focuses primarily on protein synthesis in prokaryotes because it illustrates many general principles and is relatively well understood. Some distinctive features of protein synthesis in eukaryotes also are presented. 

DNA is the biological blueprint material, which authentically carries all the necessary cellular information and passes it from generation to generation. Thus, it is essential for the cells to preserve the integrity of the DNA and keep it as error free as possible. One is amazed with the versatile DNA molecule, which dictates both the unique and the similar features of the offspring from its parents. What are the processes that take place in a cell to duplicate and interpret this genetic code into functional signals DNA is duplicated in a semiconservative fashion in a process called DNA replication. 

Since nucleic acids generally cannot go in and out of mitochondria, all mitochondria appear to code for their own rRNAs and tRNAs. For the same reason, only the mRNAs that have been transcribed from the mitochondrial genome are translated in the mitochondria. A unique feature of mitochondrial mRNAs is the lack of a m G cap at the 5 end (reviewed by Bag, 1991). There are only 22-25 tRNA species in the mitochondria, indicating that a single tRNA can recognize more than one codon. There are some structural and sequence differences in the mitochondrial tRNAs. Furthermore, deviations from the standard genetic code, for example, utilization of AUA as the initiation codon instead of AUG, and reading UGA as a tryptophan instead of a stop codon, are a unique feature of mitochondria (Lapointe... 

Cytosine is an essentially flat molecule. The three-dimensional structure of cytosine crystals revealed helices. It is involved in the Genetic Code of 17 amino acids and controls essential features of living systems. Cytosine can form under prebiotic conditions. Nonchiral cytosine spontaneously forms highly enantioenriched crystals upon stirring during crystallization. Furthermore, chiral crystals of cytosine act as chiral initiator for asymmetric autocatalysis with amplification of chirality to provide for a virtually enantiopure compound (Fig. 3.5). 

It has been suggested that the triplet genetic code evolved from a two-nucleotide code. Perhaps there were fewer amino acids in the ancient proteins. Examine the genetic code in Figure 24.16. What features of the code support this hypothesis ... 

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