Continuous genetic code

The sequencing of the human genome has been widely touted as a critical scientific milestone that will revolutionize the process of drug discovery. The continuing analysis of the human genetic code will provide the scientific framework on which... 

Nonoverlapping and commaless The genetic code is nonoverlapping and commaless, that is, the code is read from a fixed starting point as a continuous sequence of bases, taken three at a time. For example, ABCDEFGHIJKL is read as ABC/DEF/GHI/JKL without any "punctuation" between the codons. 

Interestingly, the genetic code you have today is the same as the one you had in your yesteryears, but you are now made of a different set of molecules—not unlike two identical twins. Aside from memories, perhaps you are as different from your past self as two identical twins are from each other. Perhaps an individual s identity is continually reestablished each and every moment. 

Fig. 10.1 (Continued), (b) Distribution of local optima in solution space for a basic amino acid dope (30% Arg, 30% Lys, 40% His). Due to die structure of die genetic code, a perfect soludon of fracdons of nucleotides for die given example does not exist. Instead, several islands of different suboptimal solutions are found. Seven local optima, marked by different colors (see die color version on die CD diat accompanies diis book), can be identified. Note diat die SOM uses toroidal boundaries, i.e., die left-most and right-most points lie close togedier, as do die top and bottom points.

The nature of the interaction between nucleic acid bases has been a continual source of fascination ever since the nature of the genetic code was unraveled. However, the numbers of atoms and electrons in these systems erected a high barrier to the application of accurate quantum mechanical methods for many years. For example, the guanine-cytosine (GC) pair contains 29 atoms and 136 electrons. 

The genetic code refers to specific sequences of RNA bases (or DNA bases) that encode specific amino acids. Kach of these sequences is composed of a triplet of bases (three bases in a row). Hence, any mRNA molecule can be considered a continuous polymer of successive triplets. For example, the triplet UUU codes for phenylalanine, CAU encodes histidine, GAG encodes glutamate, AAA encodes lysine, and AUG codes for methionine. DNA is used for information storage, while mRNA is used for information transfer. 

The genetic code has been broken, but research continues, aimed at tracking down the lines of communication. DNA serves as a template on which molecules of RNA are formed. It has been suggested that the double helix of DNA partially undbils, and about the individual strands are formed chains of RNA the process thus resembles self-duplication of DNA, except that these new chains contain... 

This flow of information depends on the genetic code, which defines the relation between the sequence of bases in DNA (or its mRNA transcript) and the sequence of amino acids in a protein. The code is nearly the same in all organisms a sequence of three bases, called a codon, specifies an amino acid. There is another step, between transcription and translation, in the expression of most eukaryotic genes, which are mosaics of nucleic acid sequences called introns and exons. Both are transcribed, but introns are cut out of newly synthesized RNA molecules, leaving mature RNA molecules with continuous exons. The existence of introns and exons has crucial Implications for the evolution of proteins. 

The genetic code is degenerate (several codons have the same meaning), specific (each codon specifies only one amino acid), and universal (with a few exceptions each codon always specifies the same amino acid). In addition, the genetic code is nonoverlapping and without punctuation (i.e., mRNA is read as a continuous coding sequence). 

Three different experimental approaches were taken in the early 1960s to prove that the genetic code is based on a continuous string of triplet ribonucleotides in the mRNA. The general idea for each experiment is illustrated in Figure 26.4. 

The nucleus is separated from the rest of the cell (the cytoplasm) by the nuclear envelope, which consists of two membranes joined at nuclear pores. The outer nuclear membrane is continuous with the rough endoplasmic reticulum. To convert the genetic code of the DNA into the primary sequence of a protein, DNA is transcribed into RNA, which is modified and edited into mRNA. The mRNA travels through the nuclear pores into the cytoplasm, where it is translated into the primary sequence of a protein on ribosomes (Fig. 10.21). Ribosomes, which are generated in the nucleolus, also must travel through nuclear pores to the cytoplasm. Conversely, proteins required for replication, transcription, and other processes pass into the nucleus through these pores. Thus, transport through the pore is specific for the molecule and the direction of transport. 

FIGURE 12.2 Theoretically possible genetic codes, (a) An overlapping versus a nonoverlapping code, (b) A continuous versus a punctuated code. 

The human body is made of about ten trillion cells (Nesse and Williams, 1994), (see also Human Ecology System, Section 5.5.3). Among all the actively dividing cells in the body, there are bound to be mistakes made in the genetic codes of some of them, and so there must be mechanisms to detect genetic mistakes and either (1) correct errors, or (2) destroy the cell with defective genes (apoptosis). If these cells are allowed to continue with defective genomes, they can become tumorous or cancerous. 

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