A base pair is two chemical bases bonded to one another forming a "rung of the DNA ladder." The DNA molecule consists of two strands that wind around each other like a twisted ladder. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases--adenine (A), cytosine (C), guanine (G), or thymine (T). The two strands are held together by hydrogen bonds between the bases, with adenine forming a base pair with thymine, and cytosine forming a base pair with guanine.
Base pair describes the relationship between the building blocks on the strands of DNA. So each DNA molecule is made up of two strands, and there are four nucleotides present in DNA: A, C, T, and G. And each of the nucleotides on one side of the strand pairs with a specific nucleotide on the other side of the strand, and this makes up the double helix. So for example, if there's a G on one side of the strand, there will always be a C on the other. If there's a T on one side of the strand, there will always be an A on the other. And those nucleotides always pair. We also count DNA and the amount of DNA, or the length of DNA by using units of base pairs, so if we're discussing a gene and we want to describe how big is a gene, we might say that the gene is a thousand base pairs long. If it's a really big gene, it may be 10,000 base pairs, or essentially 10 kilobases long. So we use base pair as a unit of measurement of DNA and RNA as well as a term to describe the pairing relationship.- Lawrence C. Brody, Ph.D.