All these abilities depend on the pairing of complementary bases. The structure proposed by Watson and Crick provided clues to the mechanisms by which cells are able to divide into two identical, functioning daughter cells how genetic data are passed to new generations and even how proteins are built to required specifications. (b) This represents a schematic representation of the double helix, showing the complementary bases. (a) This represents a computer-generated model of the DNA double helix. These specific base pairs, referred to as complementary bases, are the steps, or treads, in our staircase analogy ( Figure 19.6 “DNA Double Helix”).įigure 19.6 DNA Double Helix. The purine and pyrimidine bases face the inside of the helix, with guanine always opposite cytosine and adenine always opposite thymine. Moreover, as their model showed, the two chains are twisted to form a double helix-a structure that can be compared to a spiral staircase, with the phosphate and sugar groups (the backbone of the nucleic acid polymer) representing the outside edges of the staircase. Using the information from Chargaff’s experiments (as well as other experiments) and data from the X ray studies of Rosalind Franklin (which involved sophisticated chemistry, physics, and mathematics), Watson and Crick worked with models that were not unlike a child’s construction set and finally concluded that DNA is composed of two nucleic acid chains running antiparallel to one another-that is, side-by-side with the 5′ end of one chain next to the 3′ end of the other. Watson and Francis Crick announced that they had a model for the secondary structure of DNA. Chargaff drew no conclusions from his work, but others soon did.Īt Cambridge University in 1953, James D. Similarly, he showed that the molar amount of guanine (G) was the same as that of cytosine (C). In 1950, Erwin Chargaff of Columbia University showed that the molar amount of adenine (A) in DNA was always equal to that of thymine (T). Initial work revealed that the polymer had a regular repeating structure. The three-dimensional structure of DNA was the subject of an intensive research effort in the late 1940s to early 1950s. The sequence of nucleotides in the DNA segment shown in Figure 19.5 “Structure of a Segment of DNA” would be written 5′-dG-dT-dA-dC-3′, which is often further abbreviated to dGTAC or just GTAC. The final nucleotide has a free OH group on the 3′ carbon atom and is called the 3′ end. For DNA, a lowercase d is often written in front of the sequence to indicate that the monomers are deoxyribonucleotides. In writing nucleotide sequences for nucleic acids, the convention is to write the nucleotides (usually using the one-letter abbreviations for the bases, shown in Figure 19.5 “Structure of a Segment of DNA”) starting with the nucleotide having a free phosphate group, which is known as the 5′ end, and indicate the nucleotides in order. For amino acid sequences in proteins, the convention is to write the amino acids in order starting with the N-terminal amino acid. Unlike proteins, which have 20 different kinds of amino acids, there are only 4 different kinds of nucleotides in nucleic acids. Laboratory of Biophysical Chemistry, University of Groningen, The Netherlands.Like proteins, nucleic acids have a primary structure that is defined as the sequence of their nucleotides. Lawson, C.L., Bergsma, J., Bruinenberg, P.M., De Vries, G., Dijkhuizen, L., Dijkstra, B.W. Maltodextrin-Dependent Crystallization of Cyclomaltodextrin Glucanotransferase from Bacillus Circulans.The maltose molecules bound in the E domain interact with several residues implicated in a raw starch binding motif conserved among a diverse group of starch converting enzymes. The maltose-dependence of CGTase crystallization can be ascribed to the proximity of two of the maltose binding sites to intermolecular crystal contacts. Three maltose binding sites are observed at the protein surface, two in domain E and one in domain C. The structure of the enzyme is nearly identical to the CGTase from B. The X-ray structure of the CGTase was elucidated in a maltodextrin-dependent crystal form and refined against X-ray diffraction data to 2.0 A resolution. It was found to code for a mature protein of 686 amino acid residues, showing 75% identity to the CGTase from B. The cyclodextrin glycosyltransferase (CGTase, EC 2.4.1.19) gene from Bacillus circulans strain 251 was cloned and sequenced. Biologically Interesting Molecule Reference Dictionary (BIRD).
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