Figure 1. Repeating nucleotides of DNA. DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) [not pictured] are polymers of nucleotides. Each nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphate group.

Figure 2.  Purines and Pyrimidines. There are two classes of nitrogenous bases found in DNA and RNA.  Adenine and guanine are purines.  Cytosine, thymine and uracil are pyrimidines. Thymine is found only in DNA; uracil is found only in RNA.  One base attached to a pentose sugar and a phosphate group creates a nucleotide, the primary building block of nucleic acids. The bases are often represented by their first letters only as A, G, C, T or U.

Figure 3.  Sugars in RNA and DNA. This slide shows both deoxyribose, the pentose sugar in DNA, and ribose, the pentose sugar in RNA. Note that the difference between the sugars is the presence of an OH group on carbon #2 in ribose; hence the name deoxyribose for the sugar found in DNA.

Figure 4. Single DNA Strand
In DNA and RNA, the purine or pyrimidine is always attached via a covalent bond to the C-1' position of the pentose sugar. The phosphate group is at the C-5' position of the pentose sugar. Alternating sugar-phosphate groups with attached nitrogenous bases create a polynucleotide chain. The chain is created through the phosphodiester linkage of a phosphate at the C-3' position of one sugar with the C-5' of the adjacent sugar.

Note that the two ends of the chain are not the same. On the end referred to as the 3' end, there is a free OH group off of the C-3' of the pentose sugar. At the opposite end of the chain, the 5' end, there is a free phosphate group off of the C-5' of the pentose sugar. Thus, the nucleotides in the chain or "backbone" are in the same relative orientation: if the C-5' is above the sugar ring and the C-3' below, they are in the same position in all subsequent nucleotides. This asymmetrical characteristic of the polynucleotide chain is described as polarity or directionality. The forward direction is defined as 5' to 3'.  The ends of a strand are usually labeled 5' or 3' as guides to the direction along the chain. For example, the sequence shown is 5'-ACTG-3'.

Figure 5.  Double Stranded DNA
This slide illustrates DNA, composed of two nucleotide chains, with the bases from the sugar-phosphate backbone on one strand forming hydrogen bonds at the center of the molecule with bases from the opposing strand. Note the opposite orientation, or polarity, of the two strands composing the molecule. In this opposing orientation, the two strands are said to be antiparallel to each other.

Figure 6.  Complementary Base Pairing
This slide illustrates pairing between nitrogenous bases in DNA by hydrogen bonding.   Two hydrogen bonds form between the adenine and thymine, and three hydrogen bonds form between the cytosine and guanine. The joined bases are called base pairs.   Thymine pairs only with adenine and cytosine pairs only with guanine. This restricted base pairing imparts complementarity to the strands. Therefore, the nucleotide sequence on one strand can be used to determine the sequence on the other strand.