Codons, DNA

Figure 1 shows the standard code in DNA language (i. e., as a sequence of triplets in the sense strand of DNA, read in the 5 3 direction see p. 84), represented as a circular diagram. The scheme is read from the inside to the outside. For example, the triplet CAT codes for the amino acid histidine. With the exception of the exchange of U for T, the DNA codons are identical to those of mRNA. 

Basis of the Sickle-Cell Mutation Sickle-cell hemoglobin has a Val residue at position 6 of the j3-globin chain, instead of the Glu residue found in normal hemoglobin A. Can you predict what change took place in the DNA codon for glutamate to account for replacement of the Glu residue by Val ... 

Sequence in DNA Codon in mRNA Amino acid in protein... 

The genetic code indicates the amino acids that are coded for by the codons appearing in DNA and mRNA. To acquire a genetic code for the codons in mRNA, chaise each thymine (T) to uracil (U). In actual practice, scientists usually refer to a table that contains the DNA codons, in analyzing genetic information, and rarely use a table that cOTitains the codons appearing in mRNA. 

Hemoglobin Amino Acid Position Change DNA Codon Alterations Remarks ... 

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. 

These steroid—receptor complexes are formed in, or diffuse into, the nucleus where they bind to the chromatin (no steroid is bound in the absence of its protein). In this way, a length of DNA (codon) is de-repressed and produces the required mRNA which, in turn, uses cytoplasmic ribosomes to make the characteristic protein. So rapid is this process that the physiological response characteristic of the hormone can sometimes be seen within an hour. These are very typical examples of the induction of a characteristic protein (for a review of induction, see Cohen, 1966). 

The genetic code can be expressed in mRNA codons (as we have shown in Table 25.2) or in DNA codons. We have chosen to show the mRNA codons because these are the codons that are actually read during the synthesis of polypeptides (the process called translation that we discuss next). However, each mRNA molecule (Section 25.5A) acquires its sequence of nucleotides by transcription from the corresponding gene of DNA. In transcription, RNA polymerase (along with other transcription factors) opens the DNA double helix and begins the process. 

Because the synthesis of mRNA proceeds in the 5 to 3 direction, the codons for the sense strand of DNA (with the exception of thymine replacing uracil) are the same as those for the mRNA. For example, one DNA codon for valine is GTA. The corresponding mRNA codon for valine is GUA. 

Missense mutation A mutation that alters a DNA codon so as to cause one amino acid in a protein to be replaced by a different one. 

Researchers of Praecis Pharmaceuticals, which was later acquired by GlaxoSmithKline (GSK), invented another encoding method. The key encoding technique involved enzymatic ligation of double-stranded DNA codons. Scheme 10.2 shows the DNA ligation and chemical synthesis on a sym-chlorotriazine scaffold, which were cycled in a split-and-pool manner to generate different encoded libraries with up to 4 biUion compoimds. 

Fig. 9. The relationship between codijgen on the.DNA, codon on the mR-NA and anticodon on the tRNA (modified from Hess 1968).

Point mutation change in a single purine or pyrimidine base in DNA codon... 

Unequal crossing over of DNA strands a. Loss of one or more DNA codons... 

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