Introduction

Purines and pyrimidines are the essential building blocks of all life. They make up DNA and RNA, molecules that carry the genetic code in all living organisms. They are two different families of nitrogenous bases that pair together in the DNA ladder and are vital to the structure of RNA. Without them, we wouldn’t be here! The unique properties and uses of purines and pyrimidines are SUPER important for DNA replication, transcription, and translation to be able to happen. In this article, you’ll learn exactly what purines and pyrimidines are, as well as their similarities and differences. Let’s dive right in! 

What is a Purine and Pyrimidine?

Purines and pyrimidines are two different types of nitrogenous bases that are a HUGE part of the structure of nucleic acids, which is what carries all of the important genetic information that makes you, YOU! Here’s how it works: each DNA strand has a backbone made of a sugar-phosphate chain, and along with each sugar is a nitrogenous base (either a purine or a pyrimidine!) made up of carbon and nitrogen rings. The number of rings in this nitrogenous base determines whether it is a purine or a pyrimidine. Purines distinctly have two rings, while pyrimidines only have one (something important to remember!). The purines lie on one side of the DNA, forming hydrogen bonds with the pyrimidines on the other side. This is known as base pairing. The hydrogen bonds holding the purines and pyrimidines together aren’t nearly as strong as covalent bonds; this is so the strands can easily unzip during DNA replication and transcription, the genetic processes where all of this genetic material is copied down to make new proteins. But besides being a super important part of genetic replication, purines and pyrimidines also serve as a form of energy for cells and regulate enzymes. They’re a triple threat! 

Purines and pyrimidines each have specific bases that pair with each other to form the structure of DNA and RNA. 

Purine vs Pyrimidine Bases

Purine Bases

  • Adenine (A): Adenine is a purine base found in both DNA and RNA. In DNA, adenine pairs with thymine (A-T) and in RNA, and pairs with uracil. 

  • Guanine (G): Guanine is a purine base found in both DNA and RNA. Both times, guanine pairs with cytosine. 

Pyrimidine Bases

  • Cytosine (C): Cytosine is a pyrimidine base found in both DNA and RNA. Both times, cytosine pairs with guanine.

  • Thymine (T): Thymine is a pyrimidine base that is ONLY found in DNA. Thymine pairs with adenine. It cannot be found in a strand of RNA.

  • Uracil (U) : Uracil is a pyrimidine base that is ONLY found in RNA, where it replaces thymine. In RNA, uracil pairs with adenine. It is involved in processes unique to RNA like protein synthesis. It cannot be found in a strand of DNA.

Purine and Pyrimidine Flashcards & Resources

While studying purines and pyrimidines, take a second to review your understanding of what you learned! These resources and exam tips will provide pretty much everything you need to know to ace the protein synthesis section of the AP Biology Exam!

  • If you’re in AP Biology, this is usually covered in the Protein Synthesis section. This study guide for AP biology has a section on protein synthesis that goes over what you should know about each of the purines, pyrimidines, and base parings - all written by a former AP Biology student! 

  • AP Bio Flashcards - this is an ultimate collection of terms to know for the AP exam, from the cell structures to the the steps of protein synthesis 

Chargaff’s Rule

In each DNA strand, the proportion of purine and pyrimidines will always be constant, a phenomenon called Chargaff’s rule. Erwin Chargaff, an Austro-Hungarian-born American biochemist, discovered it and came up with the following rule. It states that in a DNA molecule, the amount of adenine will always be equal to the amount of thymine, the pyrimidine that adenine bonds with, and the amount of cytosine will always be equal to the amount of guanine, the purine that cytosine bonds with. This means that if you have 20% of a DNA strand is adenine, it will also have to be 20% of thymine and vice versa (there will be a question about the percentage of purines and pyrimidines on the AP Biology exam, so make sure you know this)!! All of this happens because of the complementary pairing in the DNA helix. Since each purine and pyrimidine has a very specific partner and will not pair with anyone else, there is not a random mix of adenines, thymines, cytosines, and guanines, allowing for a perfect proportion each time. This allows for the stability and uniformity of the DNA helix. Not only is the proportion of purines and pyrimidines constant, but also is the number of hydrogen bonds between them.

Adenine bonds with thymine and will ALWAYS have two hydrogen bonds

Guanine bonds with cytosine and will ALWAYS have three hydrogen bonds

The consistent pairing makes sure that the double helix has a uniform shape, which is SUPER important for it to be able to interact and work properly with other molecules. 

Pyrimidines vs. Purines

You’ve learned about how purines and pyrimidines work together and their similarities, but they still have some key differences that separate them into the two nitrogenous base groups they are in. See what they are below!

Difference Between Purine and Pyrimidine

Purine

Pyrimidine

Ring Structure

The structure of purines is a double carbon-nitrogen ring that has four nitrogen atoms

The structure of pyrimidines is a single carbon-nitrogen ring that has two nitrogen atoms 

Size

Purines are bigger

Pyrimidines are smaller

Bases

Purines have adenine and guanine for its nucleobases

Pyrimidines have cytosine, thymine, and uracil as nucleobases

Functions

Besides helping create genetic code in RNA and DNA, purines are also a part of energy transfer (ATP, GTP), cell signaling, and as coenzymes 

Primarily function as parts of DNA and RNA, but also involved in metabolic processes. 

Melting point

214º Celsius

20-22º Celsius

Source

Adenine and guanine are in both DNA and RNA

Cytosine is in both DNA and RNA

Uracil is ONLY in RNA

Thymine is ONLY in DNA

The most important difference between them is definitely between their structures. 

The purines, adenine and guanine, are two-ringed nine-membered molecules and made up of four nitrogen atoms.

However, pyrimidines have just one ring and are made up of six members and two nitrogen atoms.

Since purines are SO much bigger than pyrimidines, it is important for them to bond with their opposite, otherwise, there wouldn’t be any room!

How to Remember Pyrimidine vs Purine 

A good mnemonic for purines is “Pure As Gold.” “Pure” is for purines and “as gold” stands for the two purine bases, adenine and guanine.

To remember the pyrimidine bases, use the mnemonic “CUT the Pie.” “CUT” is for each of the pyrimidine bases, cytosine, uracil, and thymine, and the P in “pie” stands for pyrimidine! 

To remember which bases pair with each other, viewing the letters visually is super helpful. Adenine and thymine pair with each other (A-T) and both letters are very straight and rigid. On the other hand, cytosine and guanine pair with each other (C-G) and they both are very round in shape. A good mnemic to remember these are also “apples in the tree, cars in the garage.” The A and T in the first phrase present the base pairs adenine and thymine, and the C and G represent the base pairs cytosine and guanine. 

For even more practice, make sure to check out our purine and pyrimidine flashcards to really let it all sit in! 

Conclusion

Way to go on learning so much about purines and pyrimidines! Here, you learned all about the differences between purine and pyrimidines, the building blocks of DNA and thus life! You learned the different types of these nitrogenous bases, such as adenine and guanine for the purines and thymine, cytosine and uracil for the pyrimidines and what they bond with. You also read about Chargaff’s rule and the structural differences between purines and pyrimidines. If you understand all of these concepts, you will be more than ready to ace this section on the AP exam! 


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