Complementary, antiparallel DNA strands
DNA codes information by ordering the sequence of the A, T, G and C nucleotides in a long polymer (or chain). Just as the order of the letters forms the words you are reading, the order of these A, T, G and C nucleotides codes the biological information stored in a gene. This order must be retained. Thousands to millions of nucleotides form a polymer of DNA in a chromosome that are connected by strong, sugar-phosphate covalent bonds. When you look at the sugars in Figure 3, you will see that they are parallel. The DNA molecule has a constant thickness. But the sugars of each strand are oriented in opposite directions. This is because the phosphate bond connects carbon #3 (the 3 prime or 3′ carbon) of one sugar to the 5′ carbon of the next. This gives a DNA strand directionality. Antiparallel is the term which describes the constant thickness, opposite directionality in the double stranded DNA molecule.
While strong, covalent bonds connect the nucleotides in each DNA strand, weaker hydrogen bonds connect the two strands as they form a double stranded molecule (Fig. 3). The nitrogenous bases in the nucleotides form the hydrogen bond connections between the strands. This is called “base pairing” and it is very specific. As shown in Figure 2 and 3, adenine (A) only pairs with thymine (T) and cytosine (C) only pairs with guanine (G). Thus, the sequences on the two strands of DNA are “complementary”; whenever there is an “A” on one strand, there is a “T” in the corresponding position of the complementary strand.
Figure 3. DNA is a double stranded molecule. Base pairs A and T pair together and G and C pair together. A, T, G, and C are nucleic acids and those nucleic acids attached to a sugar-phosphate create a nucleotide. The sugar phosphates connect together to form a sugar-phosphate backbone. Base pair adv, NIH-NHGRI, n.d. Public Domain.
In addition to containing complementary nucleotide sequences, the strands are in opposite orientations. The 5′ end of one strand is oriented toward the 3′ end of the other. For this reason, the strands are referred to as “antiparallel”.
This antiparallel and opposite orientation influences the replication of DNA and allows the DNA to be a consistent width from one sugar phosphate strand to the other.