The structure of DNA will ultimately be revealed as the history section ends. Chargaff’s rules for base pairing and the x-ray diffraction photograph reinforces understanding of this molecule’s structure. Constructing a DNA molecule will demonstrate its basic structure as well as provide a model for future discussion of DNA function.
It is worthy to note that there are several forms of DNA: B-DNA and Z-DNA are the two most important. Z-DNA is far less common than B-DNA. B-DNA is the double helix structure proposed by Watson and Crick and is the form DNA usually has in solution. This unit will focus only on the B form of DNA.
DNA, the hereditary material passed on from cell to cell, is a nucleic acid. A nucleic acid is a macromolecule that is composed of repeating nucleotides. There are two kinds of nucleic acids, DNA is one kind, RNA is the other.
A DNA molecule consists of a very long chain of repeating units. The repeating units are called nucleotides. A nucleotide has three components: a 5-carbon sugar (deoxyribose for DNA or ribose for RNA), a phosphate group (PO4) and a nitrogenous base (either a purine or a pyrimidine, which refer to the structure of the nitrogenous base- either two rings or one respectively). In a DNA molecule the four nitrogenous bases are adenine, thymine, guanine, and cytosine with the purines being adenine and guanine and the pyrimidines being thymine and cytosine. Adenine (purine) always bonds to thymine (pyrimidine) and guanine (purine) always bonds to cytosine (pyrimidine). [NOTE: to remember which is a purine and which is a pyrimidine- just recall that pyrimidine has a “Y” in its name and so do the two pyrimidine bases, thYmine and cYtosine] In an RNA molecule, thymine is not found, but the nitrogenous base uracil (a pyrimidine) is found and bonds with adenine during transcription.
The shape of a DNA molecule is like a ladder, twisted or coiled into a double helix. The rungs of our ladder would be the nitrogenous bases bonded to each other. The base pair adenine/thymine are held together as a rung of this ladder by two hydrogen bonds; the base pair guanine/cytosine are held together by three hydrogen bonds. The nitrogenous base pairs are bonded to a sugar phosphate backbone- a chain of alternating sugar and phosphate groups. A nitrogen from the nitrogenous base forms a covalent bond with the first carbon in a sugar molecule. The fifth carbon in the sugar molecule bonds with an oxygen from a phosphate ion. This same phosphate ion uses another oxygen to bond to the third carbon in another sugar molecule, and this repeated chain forms the backbone of a DNA strand. (See diagram #1 below.) But since DNA is double stranded, there are two sugar phosphate backbones. It is estimated that there are at least 3 billion base pairs on a human DNA molecule.
Diagram #1. A diagram of the sugar phosphate backbone of a DNA molecule. This diagram shows the linkage between a phosphate group and the deoxyribose sugar.