urines and pyrimidines are the two families of nitrogenous bases that make up nucleic acids – in other words, they are the building blocks of DNA and RNA. While they are similar in many respects, there are a number of key differences between them that you will be expected to know for the AP® exam. Before we get into those, however, let’s make sure you understand what purines and pyrimidines are so you can recognize questions about them even if the wording is tricky.
What are Purines and Pyrimidines?: The Basics
Each DNA strand has a ‘backbone’ that is made up of a sugar-phosphate chain. Attached to each one of these sugars is a nitrogenous base that is composed of carbon and nitrogen rings. The number of rings this base has determines whether the base is a purine (two rings) or a pyrimidine (one ring). The purines on one strand of DNA form hydrogen bonds with the corresponding pyrimidines on the opposite strand of DNA, and vice versa, to hold the two strands together. Within DNA molecules, this is their most important function and is known as base pairing. Because hydrogen bonds are not as strong as covalent bonds, base pairings can easily be separated, allowing for replication and transcription.
Because purines always bind with pyrimidines – known as complementary pairing – the ratio of the two will always be constant within a DNA molecule. In other words, one strand of DNA will always be an exact complement of the other as far as purines and pyrimidines go.This phenomenon is known as Chargaff’s Rule, named after Irwin Chargaff, who first noticed it. This complementary pairing occurs because the respective sizes of the bases and because of the kinds of hydrogen bonds that are possible between them (they pair more favorably with bases with which they can have the maximum amount of hydrogen bonds).
There are two main types of purine: Adenine and Guanine. Both of these occur in both DNA and RNA. There are three main types of pyrimidines, however only one of them exists in both DNA and RNA: Cytosine. The other two are Uracil, which is RNA exclusive, and Thymine, which is DNA exclusive. One strategy that may help you remember this is to think of pyrimidines like pyramids that have sharp and pointy tops. So sharp and pointy in fact, that they might CUT (Cytosine, Uracil, Thymine) you.
Which purines pair with which pyrimidines is always constant, as is the number of hydrogen bonds between them:
ADENINE pairs with THYMINE (A::T) with two hydrogen bonds
GUANINE pairs with CYTOSINE (G::C) with three hydrogen bonds
One way to remember which bases go together is to look at the shapes of the letters themselves. The letters made up of only straight lines (A and T) are paired with each other, while the letters that are made up of curves (G and C) also go together. Just make sure you don’t write your A’s in cursive!
These specific pairings also factor into Chargaff’s Rule, which we mentioned before. The number of adenines in a DNA molecule will always be equal to the number of thymines. The same goes for guanines and cytosines. Because of this, if you know the percentage of one nitrogen base within a DNA molecule, you can figure out the percentages of each of the other three as well – its complementary pair will have the same percentage, and each of the other two bases will be the sum of the first pair subtracted from 100% and divided by two. Expect a question asking you to calculate something similar to this on the exam.
If what we have covered so far is confusing to you, make sure you go back and review your notes on DNA/RNA structure before moving on to studying the differences between purines and pyrimidines.
Purines vs. Pyrimidines
When it comes identifying the main differences between purines and pyrimidines, what you’ll want to remember is the ‘three S’s’: Structure, Size, and Source. The very basics of what you need to know are in the table below, but you can find more details about each one further down.
