DNA (“deoxyribonucleic acid”) is the carrier of genetic information in all living things except for retroviruses (which use RNA instead). DNA is a polymer that is made by stringing together various combinations of four different monomers called “nucleotides.” Each nucleotide is made by reacting 1 three compounds: 2
The pyrimidines are the single-ring bases thymine (T) and cytosine (C):
Figure App-1 (Wikipedia, “Gene”) shows two DNA strands (the “sense” strand that is read and the complementary “anti-sense” strand), each strand in the figure having four nucleotides. Each nucleotide is made up of three groups – a phosphate, a deoxyribose, and a base; all four pairs of the possible base pairs, A-T, C-G, T-A, and G-C, are shown. Analogizing to a ladder, the rails (“backbones”) of the ladder are formed by alternating phosphate and deoxyribose groups and the rungs of the ladder are formed by a pair of bases. Adenine (“A”) always bonds to thymine (“T”), and cytosine (“C”) always bonds to guanine (“G”), so all the base pairs are either A-T or C-G. 3 Since A and G each have a single ring and C and T each have two rings, there are always 3 rings separating the rails and not 2 rings or 4 rings. The paired bases weakly bond the two rails together, so they can be easily separated when the strands are read. 4 The two rails are reversed, one going from the 5’ end to the 3’ end and the other from the 3’ end to the 5’ end. (The “5” and “3” come from counting clockwise around the ribose ring, starting with the oxygen atom, “O.”) Note that there are two weak bonds (dotted lines) between adenine (A) and thymine (T), but there are three weak bonds between cytosine (C) and guanine (G); this gives the structure a slight twist, forming a double helix.
RNA (“ribonucleic acid”) is the same as DNA, but ribose replaces deoxyribose and uracil replaces thymine:
Since each nucleotide can be formed with one of four bases (A, C, G, or T), every group of three adjacent nucleotides in the DNA sense strand will have one of 64 possible combinations (4x4x4) of the four T-G-C-A bases. The three base pairs in those three nucleotides (a “codon”) correspond to one of the 20 amino acids that are linked together to form proteins. For example, the base sequence TGC codes for the amino acid cysteine, so that when that codon (T-G-C) is read the amino acid cysteine will be added to the polypeptide that is being formed. Since there are 64 different codons and only 20 different amino acids, different codons may code for the same amino acid, i.e., those codons are “synonymous.” A “gene” is a portion of the DNA sense chain that codes for a product, usually a polypeptide; various polypeptides are then assembled to form different proteins.
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1. In the first reaction, a hydrogen atom (H) on one of the four bases, A, C, G, or T, combines with a hydroxyl group (OH) on deoxyribose, bonding the base to the deoxyribose and forming water (H-OH). In the second reaction, hydrogen atoms (H) from phosphoric acid combine with the remaining two hydroxyl radicals (OH) on the deoxyribose, splitting out more water (H-OH) and forming long strings of alternating phosphoric acid and deoxyribose groups (with bases attached).
2. In the chemical formulas, the letters represent atoms of various elements. “H” is hydrogen, “O” is oxygen, “C” is carbon, “N” is nitrogen, and “P” is phosphorus; a carbon atom is at every vertex in the rings that is not occupied by an “N” or an “O.” Back
3. For that reason, a sense strand can be the same as its anti-sense strand read backwards, e.g., ACCTAGGT and TGGATCCA, a palindrome. Many of the sequences on the Y chromosome are palindromes, which is useful in making repairs. Back
4. Typically, the sense strand is read, but now scientists are finding that the anti-sense strand can also be read. (Stark, 2008). Back