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18.3: Nucleic Acid Structure

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     formed by linking together and are found in every cell. Deoxyribonucleic acid (DNA) is the nucleic acid that stores genetic information. If all the DNA in a typical mammalian cell were stretched out end to end, it would extend more than 2 m. () is the nucleic acid responsible for using the genetic information encoded in DNA to produce the thousands of found in living organisms. are joined together through the phosphate group of one nucleotide connecting in an linkage to the OH group on the third carbon atom of the sugar of a second nucleotide. This joins to a third nucleotide, and the process is repeated to produce a long nucleic acid chain (Figure \(\PageIndex{1}\)). The backbone of the chain consists of alternating phosphate and sugar units (2-deoxyribose in DNA and ribose in ). The purine and pyrimidine bases branch off this backbone. , nucleic acids have a that is defined as the sequence of their . Unlike , which have 20 different kinds of amino acids, there are only 4 different kinds of in nucleic acids. For amino acid sequences in , the convention is to write the amino acids in order starting with the N-terminal amino acid. In writing nucleotide sequences for nucleic acids, the convention is to write the (usually using the one-letter abbreviations for the bases, shown in Figure \(\PageIndex{1}\)) starting with the nucleotide having a free phosphate group, which is known as the 5′ end, and indicate the in order. For DNA, a lowercase is often written in front of the sequence to indicate that the monomers are deoxyribonucleotides. The final nucleotide has a free OH group on the 3′ carbon atom and is called the . The sequence of in the DNA segment shown in Figure \(\PageIndex{1}\) would be written 5′-dG-dT-dA-dC-3′, which is often further abbreviated to dGTAC or just GTAC. of DNA. 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. 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. The purine and pyrimidine bases face the inside of the helix, with guanine always opposite cytosine and adenine always opposite thymine. These specific pairs, referred to as , are the steps, or treads, in our staircase analogy (Figure \(\PageIndex{2}\)). are built to required specifications. All these abilities depend on the pairing of . Figure \(\PageIndex{3}\) shows the two sets of pairs and illustrates two things. First, a pyrimidine is paired with a purine in each case, so that the long dimensions of both pairs are identical (1.08 nm). were paired, the two pyrimidines would take up less space than a purine and a pyrimidine, and the two would take up more space, as illustrated in Figure \(\PageIndex{4}\). If these pairings were ever to occur, the structure of DNA would be like a staircase made with stairs of different widths. For the two strands of the double helix to fit neatly, a pyrimidine must always be paired with a purine. The second thing you should notice in Figure \(\PageIndex{3}\) is that the correct pairing enables formation of three instances of between guanine and cytosine and two between adenine and thymine. The additive contribution of this imparts great stability to the DNA double helix.

    18.3: Nucleic Acid Structure is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.

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