![]() For the two strands of the double helix to fit neatly, a pyrimidine must always be paired with a purine. If these pairings were ever to occur, the structure of DNA would be like a staircase made with stairs of different widths. If two pyrimidines were paired or two purines were paired, the two pyrimidines would take up less space than a purine and a pyrimidine, and the two purines would take up more space, as illustrated in Figure 19.8 "Difference in Widths of Possible Base Pairs". First, a pyrimidine is paired with a purine in each case, so that the long dimensions of both pairs are identical (1.08 nm). Figure 19.7 "Complementary Base Pairing" shows the two sets of base pairs and illustrates two things. 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 Specific base pairings in the DNA double helix., are the steps, or treads, in our staircase analogy ( Figure 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 The secondary structure of DNA.-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. Like proteins, nucleic acids have a primary structure that is defined as the sequence of their nucleotides. Figure 19.5 Structure of a Segment of DNAĪ similar segment of RNA would have OH groups on each C2′, and uracil would replace thymine.
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