Nitrogenous Bases: Hydrogen Bonding, Overview

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  • 0:01 Nitrogenous Bases
  • 0:40 Types of Nitrogenous Bases
  • 1:55 What's in a Nitrogenous Base?
  • 2:40 Pairing Up
  • 3:45 Lesson Summary
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Lesson Transcript
Instructor: Beth Skwarecki
Adenine, cytosine, thymine, guanine, and uracil! They are the 'rungs' in the DNA ladder. Learn how these ring-shaped molecules spell out your genes and pair with each other in this lesson.

Nitrogenous Bases

If you think of DNA as a twisted ladder, nitrogenous bases are the 'rungs.' They are shaped like flat rings stacked on top of each other in the middle of the DNA strand. Their function is to spell out the sequences that we call genes; you can think of the bases as letters and the genes they make as words or sentences.

There are four bases in DNA, and each one is different than the others, making four 'letters.' Amazingly, this is enough variety to spell out all the genes in all living things! The related molecule RNA also has four bases: three are the same as in DNA and one is different. Let's meet the different nitrogenous bases.

Types of Nitrogenous Bases

Adenine, abbreviated 'A,' has a 2-ring structure, so that makes it a purine. When it's in DNA, it pairs up with thymine. When it's in RNA, it pairs up with uracil. Adenine also has other jobs in the cell: it makes up part of the energy molecule ATP and electron carriers FAD and NAD, which are used in cellular respiration.

Thymine, abbreviated 'T,' is a pyrimidine, which means it has a 1-ring structure. It's only present in DNA, where it pairs with adenine. As we just noted, its cousin uracil does the same job in RNA.

Uracil, abbreviated 'U,' is found in RNA. It's a pyrimidine, and it bonds with adenine (think of it as having the same job in RNA that thymine does in DNA).

Guanine, abbreviated 'G,' is part of both DNA and RNA, where it bonds with cytosine. Guanine is a purine (fun fact: it was first discovered in bird feces, known as guano, hence the name).

Cytosine, abbreviated 'C,' is part of DNA and RNA and bonds with guanine. It has one ring, so it's a pyrimidine.

What's in a Nitrogenous Base?

So, what's in a nitrogen base? The first clue is in the name - they contain nitrogen, which you can see as the letter 'N' in the pairing diagrams. While carbon always makes four bonds to other atoms, nitrogen makes three real, covalent bonds and has an extra pair of electrons. This pair makes the nitrogen slightly negative so it can be attracted to hydrogen and form a weak hydrogen bond. We'll talk more about that in the section on pairing up.

As I mentioned earlier, there are two types of nitrogenous bases: purines, which are shaped like a double ring, and pyrimidines, which are shaped like a single ring. Having trouble remembering which ones are purines? Think of the phrase 'Girls Are Pure' for guanine and adenine and imagine a girl wearing two hoop earrings.

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