In the DNA structure, a purine molecule always binds with a pyrimidine molecule. How would you expect the structure to differ if Adenine forms base pairing with Guanine and Cytosine forms base pairing with Thymidine? Instead of A-T, G-C; can it be A-G, C-T?
Answers
Answer:
The pyrimidines, cytosine and thymine are smaller structures with a single ring, while the purines, adenine and guanine, are larger and have a two-ring structure.
Explanation:
Attention: This post was written a few years ago and may not reflect the latest changes in the AP® program. We are gradually updating these posts and will remove this disclaimer when this post is updated. Thank you for your patience!
Introduction
Purines 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.
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).
plz follow me and add this as brainly list.