Base sequence in DNA

The relationship between the base sequence in DNA and the amino acid sequence in the protein is known as the genetic code. With four bases (A, C, G and T) 64 three-base combinations are possible to provide the code for the amino acids (e.g. GTA, CCG). All but three of these are used to code for the polymerisation of the 20 different amino acids (in fact, 21, see Chapter 8) to form a polypeptide chain that can then form a protein. Most amino acids are, therefore, coded for by more than one three-base combination (Appendix 20.2). The link between the three-base... 

Change in Base Sequence in DNA Causing Mutation A change in single base pair in DNA can cause substitution of Bio-organic Chemistry... 

Southern blotting—used to identify a specific base sequence in DNA. Northern blotting—used to identify specific base sequences in RNA. 

The sequence of amino acids in a protein is determined by the base sequence in DNA. What determines the sequence of sugar residues in oligosaccharides and polysaccharides ... 

The formation of intrastrand crosslinks on GG and AG base sequences in DNA could be deduced from the presence of the adducts ds-Pt(NH3)2(d(pGG)) and cis-Pt(NH3)2(d(pAG)) in digests of platinated DNA. However, no indications for the presence of GA adducts could be found, and therefore crosslinks with a GA sequence are unlikely. 

The genetic code is the relationship between the base sequence in DNA, or its RNA transcript, and the amino acid sequence in a protein. A three-base sequence, called a codon, corresponds to one amino acid. Because there are 4 bases in RNA (A, G, C, and U), there are 4 X 4 X 4 = 64 possible codons. However, there are only 20 common amino acids in proteins. Each codon corresponds to only one amino acid, but the code is degenerate that is, several different codons may correspond to the same amino acid. Of the 64 codons, 3 are codes for stop (UAA, UAG, and UGA) they signal that the particular protein synthesis is complete. One codon, AUG, serves double duty. It is the initiator codon, but if it appears again after a chain has been initiated, it codes for the amino acid methionine. The entire code is summarized in Table 18.2. 

The genetic code is the relationship between the base sequence in DNA and the amino acid sequence in a protein. A three-base sequence located on mRNA, called a codon, corresponds to one amino acid. 

More complicated, and less obvious, is the mechanism for the precise recognition of base sequences in DNA by a given protein. Such recognition is an impressive feat kineti-cally, for an often unique nucleotide sequence (usually only a few base pairs long) must be located among tens or hundreds of thousands of similar sequences in DNA or RNA within a "reasonable" time (milliseconds ). 

Leng, M., Felsenfeld, G. The preferential interaction of polylysine and polyarginine with specific base sequences in DNA. Proc. nat. Acad. Sci. (Wash.) 56, 1325—1332 (1966). 

The RNA polymerases evince more sequence specificity than the repair enzymes since they transcribe the information in DNA into specific classes of RNA. In prokaryotes, transcription can be regulated by repressor and related proteins, which are very sensitive to the base sequence in DNA. Similar regulatory requirements undoubtedly exist for the eukaroytic genome but are not as simple. These are discussed subsequently. Regulating proteins combine with certain DNA molecules through recognition of a particular nucleotide base sequence. 

However, the majority of base sequences in DNA serve functions other than coding for structural genes. As mentioned above, one important function is the binding of certain enzymes—for example, of endonucleases that cut the double strand of DNA. Other specific sequences are recognized by enzymes that bind and either initiate or terminate transcription. Still other sequences bind proteins that inhibit transcription. Translation into a polypeptide begins when an initiator codon in a strand of mRNA becomes bound to a ribosome, and continues consecutively, at the behest of the amino acid code, until a termination codon is reached. Translation from nucleotide to amino acid occurs at the level of the tRNA molecule, since on the one hand it pairs with mRNA by codon-anticodon pairing, while on the other it binds a specific amino acid. 

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