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What space Purines and also Pyrimidines?: The Basics

Each DNA strand has a ‘backbone’ the is consisted of of a sugar-phosphate chain. Attached to each one of these sugars is a nitrogenous base the is composed of carbon and also nitrogen rings. The number of rings this base has determines even if it is the base is a purine (two rings) or a pyrimidine (one ring). The purines ~ above one strand that DNA form hydrogen bonds through the equivalent pyrimidines on the contrary strand the DNA, and also vice versa, to hold the 2 strands together. Within DNA molecules, this is their most important role and is well-known as base pairing. Because hydrogen bonds room not as solid as covalent bonds, basic pairings can easily be separated, enabling for replication and also transcription.

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Because purines constantly bind through pyrimidines – known as complementary pairing – the proportion of the 2 will always be continuous within a DNA molecule. In various other words, one strand the DNA will always be an accurate complement of the various other as far as purines and pyrimidines go.This phenomenon is known as Chargaff’s Rule, named after ~ Irwin Chargaff, who very first noticed it. This safety pairing occurs because the corresponding sizes of the bases and also because of the type of hydrogen bonds that are feasible between castle (they pair much more favorably v bases through which they have the right to have the maximum quantity of hydrogen bonds).

There space two main varieties of purine: Adenine and also Guanine. Both that these occur in both DNA and also RNA. There space three main species of pyrimidines, however only among them exists in both DNA and RNA: Cytosine. The other two space Uracil, i m sorry is RNA exclusive, and Thymine, i beg your pardon is DNA exclusive. One strategy that may aid you remember this is to think of pyrimidines like pyramids that have actually sharp and pointy tops. For this reason sharp and pointy in fact, that they can CUT (Cytosine, Uracil, Thymine) you.

Which purines pair with which pyrimidines is always constant, together is the variety of hydrogen bonds between them:

ADENINE pairs v THYMINE (A::T) v two hydrogen bondsGUANINE pairs v CYTOSINE (G::C) with 3 hydrogen bonds

One means to remember which bases go with each other is to look at the forms of the letter themselves. The letters made up of only straight present (A and T) room paired with each other, if the letter that are made up of curve (G and also C) also go together. Simply make sure you don’t compose your A’s in cursive!

These certain pairings likewise factor right into Chargaff’s Rule, i beg your pardon we pointed out before. The variety of adenines in a DNA molecule will always be equal to the number of thymines. The same goes because that guanines and cytosines. Since of this, if you understand the percent of one nitrogen base within a DNA molecule, you can number out the percentages of each of the various other three also – its complementary pair will have actually the exact same percentage, and also each of the other two bases will certainly be the sum of the first pair subtracted native 100% and also divided by two. Suppose a question asking friend to calculate something similar to this top top the exam.

If what we have actually covered so far is confusing come you, make certain you go ago and review your note on DNA/RNA structure before moving on to examining the differences between purines and pyrimidines.

Purines vs. Pyrimidines

When it comes identifying the key differences in between purines and also pyrimidines, what you’ll want to remember is the ‘three S’s’: Structure, Size, and also Source. The really basics that what you need to understand are in the table below, however you can find an ext details around each one further down.

StructureDouble carbon-nitrogen ring with 4 nitrogen atomsSingle carbon-nitrogen ring with two nitrogen atoms
SourceAdenine and Guanine in both DNA and RNACytosine in both DNA and also RNAUracil only in RNAThymine only in DNA

The many important difference that girlfriend will have to know in between purines and also pyrimidines is how they differ in your structures.

The purines (adenine and guanine) have a two-ringed structure consisting of a nine-membered molecule with four nitrogen atoms, together you can see in the two figures below.

Chemical framework of adenin in vector format. Image Source: Wikimedia CommonsStructure the guanine. Picture Source: Wikimedia Commons

The pyrimidines (cytosine, uracil, and thymine) only have actually one single ring, which has just 6 members and also two nitrogen atoms.

Cytosine chemistry structure. Image Source: Wikimedia CommonsStructure of uracil. Image Source: Wikimedia CommonsSkeletal chemical structure of Thymine. Image Source: Wikimedia Commons

Because purines are basically pyrimidines fused v a 2nd ring, they room obviously bigger 보다 pyrimidines. This size difference is part of the reason that safety pairing occurs. If the purines in DNA strands external inspection to every other instead of come the pyrimidines, they would certainly be so broad that the pyrimidines would not have the ability to reach various other pyrimidines or purines ~ above the other side! The an are between them would certainly be so big that the DNA strand would not have the ability to be organized together. Likewise, if the pyrimidines in DNA bonded together, there would not it is in enough room for the purines.

Test Yourself!

Question 1: which of these is a pyrimidine provided to develop DNA?

A. Adenine

B. Cytosine

C. Uracil

D. Thymine

E. Both B and also C

F. Both B and also D

Question 2: The diagram below shows instances of which of the following?

kinds of purine. Picture Source: Wikimedia Commons

A. Sugar-phosphate backbones

B. Amino acids

C. Uracil and Thymine

D. Purines

Question 3: i beg your pardon of the following alternatives is true the the differences in between purines and also pyrimidines in DNA?

A. The purines, adenine and thymine, are smaller sized two-ringed bases, if the pyrimidines, cytosine and also uracil, space larger and also have a solitary ring.

B. The pyrimidines, cytosine and also uracil, space smaller and also have a single ring, when the purines, adenine and guanine, are larger and have two rings.

C. The purines, adenine and also guanine, are larger and have two a one-ringed structure, when the pyrimidines, thymine and cytosine, have two rings and are smaller.

D. The pyrimidines, cytosine and thymine are smaller sized structures with a solitary ring, if the purines, adenine and also guanine, are larger and also have a two-ring structure.

E. The purines, adenine and also cytosine, are big with 2 rings, while the pyrimidines, thymine and also uracil, are small with one ring.

Answers and also Explanations:

Question 1: The correct choice is F: both B and also D. Cytosine and also Thymine room both provided to develop DNA. Be careful with questions favor these! If the wording had been “which of this is a pyrimidine used only to produce DNA,”the price would have actually been ‘D: Thymine’ instead.

Question 2: The correct selection is D: Purines. The diagram shows adenine and guanine, i m sorry you can identify by their two-ringed structure. Even if girlfriend did no remember this, you might rule out the other options like this: the sugar-phosphate backbones contain no nitrogen, amino acids must have amine, and uracil and also thymine only have one ring.

Question 3: The correct an option is D. This was a tough one, so if you got it right, offer yourself a beat on the ago – did you do it learned the key differences in between purines and also pyrimidines! A key point to notification in this question is that it asks specifically about purines vs. Pyrimidines in DNA. If you were confused about why choice B to be incorrect, this is the factor (uracil is discovered only in RNA, not DNA). The test will regularly have trick answers like this at an early stage in the options, i beg your pardon is why the is vital that you review ALL the options before choosing.


Congratulations on enduring the whole guide! here’s a fast recap of the main points we’ve extended in this review:

Purines and pyrimidines room the nitrogen bases that hold DNA strands together through hydrogen bonds.They pair together v complementary pairing based upon Chargaff’s dominance (A::T and G::C).The purines in DNA room adenine and also guanine, the very same as in RNA.The pyrimidines in DNA are cytosine and thymine; in RNA, they room cytosine and uracil.Purines are bigger than pyrimidines due to the fact that they have actually a two-ring structure while pyrimidines only have actually a solitary ring.

You must now feel confident in your capability to identify and also differentiate in between purines and pyrimidines, and also in your expertise of what duty they beat in DNA structure. Make sure you don’t just emphasis in top top the little details despite – don’t forget come look at the huge picture or exactly how this all plays right into biology as a whole!

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