The brand new pyrimidines (cytosine, uracil, and you will thymine) simply have one single ring, which has just half a dozen users and two nitrogen atoms

With regards to distinguishing a portion of the differences when considering purines and you will pyrimidines, what you ought to think of is the ‘around three S’s’: Structure, Size, and you may Provider. The maxims from all you have to discover are in the newest desk lower than, but you can look for additional info regarding the each one of these next off.


The most important improvement that you’ll wish to know between purines and you can pyrimidines is how it disagree in their structures.

The brand new purines (adenine and you may guanine) enjoys a two-ringed build composed of a beneficial nine-membered molecule which have four nitrogen atoms, as you can see regarding several figures below.

Since the purines are basically pyrimidines fused with an additional ring, he could be definitely larger than pyrimidines. That it proportions difference is part of how come you to subservient pairing happens. If your purines into the DNA strands fused to one another as an alternative out of into the pyrimidines, they might getting very greater that the pyrimidines would not be capable arrive at almost every other pyrimidines otherwise purines on the reverse side! The space among them is thus high that DNA strand wouldn’t be capable of being held along with her. At the same time, in case the pyrimidines within the DNA bonded along with her, truth be told there would not be room enough to the purines.

Test Yourself!

Here are some samples of inquiries you could find on the AP® exam towards differences between purines and you may pyrimidines. When you can respond to many of these with ease, you should be from inside the pretty good figure as much as purines vs. pyrimidines wade, however, make sure you also opinion standard DNA framework and nucleotides.

Good. The brand new purines, adenine and you will thymine, is faster a few-ringed bases, because pyrimidines, cytosine and uracil, is big as well as have just one band.

B. The pyrimidines, cytosine and you will uracil, is actually quicker and get just one ring, due to the fact purines, adenine and you will guanine, are larger as well as have two bands.

C. This new purines, adenine and you may guanine, is big and have now two a one-ringed build, since the pyrimidines, thymine and you can cytosine, keeps several bands and are also shorter.

D. The fresh pyrimidines, cytosine and you will thymine siti incontri over 50 is actually faster formations which have one band, as the purines, adenine and you will guanine, try large and have a-two-band construction.

E. The newest purines, adenine and cytosine, are high having a couple groups, due to the fact pyrimidines, thymine and you will uracil, was small that have one band.

Matter step 1: The correct choice is F: both B and D. Cytosine and Thymine are both used to produce DNA. Be careful with questions like these! If the wording had been “which of these is a pyrimidine used only to produce DNA,”the answer would have been ‘D: Thymine’ instead.

Concern dos: The correct choice is D: Purines. The diagram shows adenine and guanine, which you can identify by their two-ringed structure. Even if you did not remember this, you could rule out the other options like this: the sugar-phosphate backbones contain no nitrogen, amino acids must have amine, and uracil and thymine only have one ring.

Question step three: The correct choice is D. This was a tough one, so if you got it right, give yourself a pat on the back – you’ve learned the main differences between purines and pyrimidines! A key point to notice in this question is that it asks specifically about purines vs. pyrimidines in DNA . If you were confused about why option B was incorrect, this is the reason (uracil is found only in RNA, not DNA). The exam will often have trick answers like this early on in the options, which is why it is crucial that you read ALL the options before choosing.