what is the difference between purines and pyrimidines

what is the difference between purines and pyrimidines


Purines and pyrimidines are the 2 households of nitrogenous bases that make up nucleic acids – in different phrases, they’re the constructing blocks of DNA and RNA. Whereas they’re comparable in lots of respects, there are a variety of key variations between them that you may be anticipated to know for the AP® examination. Earlier than we get into these, nonetheless, let’s be sure you perceive what purines and pyrimidines are so you may acknowledge questions on them even when the wording is difficult.

What are Purines and Pyrimidines?: The Fundamentals

Every DNA strand has a ‘spine’ that’s made up of a sugar-phosphate chain. Hooked up to every considered one of these sugars is a nitrogenous base that’s composed of carbon and nitrogen rings. The variety of rings this base has determines whether or not the bottom is a purine (two rings) or a pyrimidine (one ring). The purines on one strand of DNA type hydrogen bonds with the corresponding pyrimidines on the alternative strand of DNA, and vice versa, to carry the 2 strands collectively. Inside DNA molecules, that is their most essential operate and is called base pairing. As a result of hydrogen bonds will not be as sturdy as covalent bonds, base pairings can simply be separated, permitting for replication and transcription.

You're reading: what is the difference between purines and pyrimidines

As a result of purines all the time bind with pyrimidines – often known as complementary pairing – the ratio of the 2 will all the time be fixed inside a DNA molecule. In different phrases, one strand of DNA will all the time be an actual complement of the opposite so far as purines and pyrimidines go.This phenomenon is called Chargaff’s Rule, named after Irwin Chargaff, who first seen it. This complementary pairing happens as a result of the respective sizes of the bases and due to the sorts of hydrogen bonds which might be doable between them (they pair extra favorably with bases with which they will have the utmost quantity of hydrogen bonds).

There are two predominant kinds of purine: Adenine and Guanine. Each of those happen in each DNA and RNA. There are three predominant kinds of pyrimidines, nonetheless solely considered one of them exists in each DNA and RNA: Cytosine. The opposite two are Uracil, which is RNA unique, and Thymine, which is DNA unique. One technique that will enable you to bear in mind that is to consider pyrimidines like pyramids which have sharp and pointy tops. So sharp and pointy actually, that they may CUT (Cytosine, Uracil, Thymine) you.

Which purines pair with which pyrimidines is all the time fixed, as is the variety 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 option to bear in mind which bases go collectively is to take a look at the shapes of the letters themselves. The letters made up of solely straight strains (A and T) are paired with one another, whereas the letters which might be made up of curves (G and C) additionally go collectively. Simply be sure you don’t write your A’s in cursive!

These particular pairings additionally issue into Chargaff’s Rule, which we talked about earlier than. The variety of adenines in a DNA molecule will all the time be equal to the variety of thymines. The identical goes for guanines and cytosines. Due to this, if the share of 1 nitrogen base inside a DNA molecule, you may determine the odds of every of the opposite three as properly – its complementary pair may have the identical proportion, and every of the opposite two bases would be the sum of the primary pair subtracted from 100% and divided by two. Count on a query asking you to calculate one thing just like this on the examination.

If what we’ve got lined to date is complicated to you, be sure you return and overview your notes on DNA/RNA construction earlier than shifting on to finding out the variations between purines and pyrimidines.

Purines vs. Pyrimidines

When it comes figuring out the principle variations between purines and pyrimidines, what you’ll wish to bear in mind is the ‘three S’s’: Construction, Measurement, and Supply. The very fundamentals of what you must know are within the desk beneath, however you could find extra particulars about every one additional down.




Double carbon-nitrogen ring with 4 nitrogen atoms Single carbon-nitrogen ring with two nitrogen atoms


Larger Smaller


Adenine and Guanine in each DNA and RNA Cytosine in each DNA and RNA Uracil solely in RNA Thymine solely in DNA

An important distinction that you’ll want to know between purines and pyrimidines is how they differ of their constructions.

The purines (adenine and guanine) have a two-ringed construction consisting of a nine-membered molecule with 4 nitrogen atoms, as you may see within the two figures beneath.

Chemical Construction of Adenine in vector format. Picture Supply: Wikimedia Commons
Construction of guanine. Picture Supply: Wikimedia Commons

Read more: what is a monocle used for

The pyrimidines (cytosine, uracil, and thymine) solely have one single ring, which has simply six members and two nitrogen atoms.

Cytosine chemical structure
Cytosine chemical construction. Picture Supply: Wikimedia Commons
Construction of uracil. Picture Supply: Wikimedia Commons
Thymine skeletal
Skeletal chemical construction of Thymine. Picture Supply: Wikimedia Commons

As a result of purines are basically pyrimidines fused with a second ring, they’re clearly greater than pyrimidines. This measurement distinction is a part of the rationale that complementary pairing happens. If the purines in DNA strands bonded to one another as a substitute of to the pyrimidines, they’d be so vast that the pyrimidines wouldn’t have the ability to attain different pyrimidines or purines on the opposite aspect! The house between them can be so massive that the DNA strand wouldn’t have the ability to be held collectively. Likewise, if the pyrimidines in DNA bonded collectively, there wouldn’t be sufficient house for the purines.

Take a look at Your self!

Listed below are some examples of questions you would possibly discover on the AP® examination in regards to the variations between purines and pyrimidines. In the event you can reply all of those with ease, you need to be in fairly fine condition so far as purines vs. pyrimidines go, however be sure you additionally overview basic DNA construction and nucleotides.

Query 1: Which of those is a pyrimidine used to supply DNA?

A. Adenine

B. Cytosine

C. Uracil

D. Thymine

E. Each B and C

F. Each B and D

Query 2: The diagram beneath reveals examples of which of the next?

sorts of purine. Picture Supply: Wikimedia Commons

A. Sugar-phosphate backbones

B. Amino acids

C. Uracil and Thymine

D. Purines

Query 3: Which of the next choices is true of the variations between purines and pyrimidines in DNA?

You might want to know: what is a deba knife used for

A. The purines, adenine and thymine, are smaller two-ringed bases, whereas the pyrimidines, cytosine and uracil, are bigger and have a single ring.

B. The pyrimidines, cytosine and uracil, are smaller and have a single ring, whereas the purines, adenine and guanine, are bigger and have two rings.

C. The purines, adenine and guanine, are bigger and have two a one-ringed construction, whereas the pyrimidines, thymine and cytosine, have two rings and are smaller.

D. The pyrimidines, cytosine and thymine are smaller constructions with a single ring, whereas the purines, adenine and guanine, are bigger and have a two-ring construction.

E. The purines, adenine and cytosine, are massive with two rings, whereas the pyrimidines, thymine and uracil, are small with one ring.

Solutions and Explanations:

Query 1: The proper alternative is F: each B and D. Cytosine and Thymine are each used to supply DNA. Watch out with questions like these! If the wording had been “which of those is a pyrimidine used solely to supply DNA,”the reply would have been ‘D: Thymine’ as a substitute.

Query 2: The proper alternative is D: Purines. The diagram reveals adenine and guanine, which you’ll be able to determine by their two-ringed construction. Even in the event you didn’t bear in mind this, you might rule out the opposite choices like this: the sugar-phosphate backbones include no nitrogen, amino acids will need to have amine, and uracil and thymine solely have one ring.

Query 3: The proper alternative is D. This was a troublesome one, so in the event you acquired it proper, give your self a pat on the again – you’ve realized the principle variations between purines and pyrimidines! A key level to note on this query is that it asks particularly about purines vs. pyrimidines in DNA. In the event you have been confused about why choice B was incorrect, that is the rationale (uracil is discovered solely in RNA, not DNA). The examination will usually have trick solutions like this early on within the choices, which is why it’s essential that you simply learn ALL the choices earlier than selecting.


Congratulations on making it by means of the entire information! Right here’s a fast recap of the details we’ve lined on this overview:

  • Purines and pyrimidines are the nitrogen bases that maintain DNA strands collectively by means of hydrogen bonds.
  • They pair collectively by means of complementary pairing primarily based on Chargaff’s Rule (A::T and G::C).
  • The purines in DNA are adenine and guanine, the identical as in RNA.
  • The pyrimidines in DNA are cytosine and thymine; in RNA, they’re cytosine and uracil.
  • Purines are bigger than pyrimidines as a result of they’ve a two-ring construction whereas pyrimidines solely have a single ring.

It is best to now really feel assured in your means to determine and differentiate between purines and pyrimidines, in addition to in your information of what function they play in DNA construction. Be sure you don’t simply focus in on the small particulars although – don’t neglect to take a look at the massive image or how this all performs into biology as a complete!

In search of Biology apply?

Take a look at our different articles on Biology.

You too can discover hundreds of apply questions on newurbanhabitat.com. newurbanhabitat.com allows you to customise your studying expertise to focus on apply the place you want probably the most assist. We’ll offer you difficult apply questions that can assist you obtain mastery in Biology.

Begin working towards right here.

Are you a trainer or administrator excited about boosting Biology pupil outcomes?

Study extra about our college licenses right here.

Read: what is used to euthanize dogs