Nitrogenous Bases - Online Tutor, Practice Problems & Exam Prep

Nitrogenous Bases Concept 1

Hey, everyone. So when it comes to nitrogenous bases, we're going to say there are 5 different nitrogenous bases that are grouped into 2 categories. We're going to have our pyrimidines versus our purines. Now, pyrimidines, these are our single-ringed molecules, and our purines are our double-ringed molecules. Now, here we have memory tools to help us remember which is which. When it comes to our Pyrimidines, we're going to have our Cytosine, our Thymine, and our Uracil. When it comes to Thymine, this is only found within DNA and Uracil is only found within RNA. Later on, we'll talk about their given structures. Right now, we're only caring about grouping them. So these 3 nitrogen spaces all are single, molecules, single-ringed molecules. And our memory tool here is that creepy tombs under pyramids. Pyramids, remidines, creepy for cytosine, tombs for thymine, and under for uracil. Here, their one-letter abbreviations would be c, t, u. On the other side, we have our purines, which are our adenine and guanine, so a and g. So it's 2 rings fused together when it comes to these structures, and our memory tool here is pure as gold. Pure for purines, s for adenine, and then g for gold guanine. Alright. So just remember, we have our creepy tombs under pyramids and pure as gold to help us group our 5 nitrogenous bases into their 2 categories, being either single-ringed molecules or double-ringed molecules.

Nitrogenous Bases Example 1

So in this example, it says label each nitrogen space as a pyrimidine (p y) or a purine (p u). So remember when it comes to our pyrimidines, we're going to say here "creepy tombs under pyramids". So here, if we say these represent our pyrimidines, "creepy" stands for cytosine. So this will be p y. "Tombs" is for thymine. So p y. "Under" is for uracil. So p y. And then for purines, we're going to say purines are "pure as gold". So our purines, "pure" is for adenine or adenine. So here this would be p u. And then, "gold" is for guanine. So this is how we label each of the following nitrogenous bases based on these two memory tools. We can group them into pyrimidines or purines.

The four nitrogenous bases commonly found in DNA are:

• A. Uracil, cytosine, guanine, thymine.
• B. Adenine, thymine, cytosine, uracil.
• C. Uracil, adenine, cytosine, guanine.
• D. Adenine, thymine, cytosine, guanine.
• E. None are correct.

Nitrogenous Bases Concept 2

In this beetle, we'll learn some tricks to help us remember the structures of different pyrimidines. So here, first of all, when we say pyrimidine, this is the general structure of pyrimidine. The 3 pyrimidines that we have are just modifications to this original structure, and it all begins with Uracil. Uracil here has its 2 nitrogens just like pyrimidine does, but it also has 2 carbonyl groups. So we have a double bond O here and a double bond O here. Now, here, we had a double bond, but we can no longer have a double bond here because then this carbon would be making 5 bonds. So that's not allowed. But nitrogen ideally wants to make 3 bonds. To do that, it would have to be connected to a hydrogen. So, it'd have a connection to the 2 carbons and then the 3rd bond will be to the hydrogen. We went into the same issue here with this bottom nitrogen. We can't have a double bond like we have here because then this carbonyl carbon will be making 5 bonds. Carbon can only go up to 4. So for nitrogen to reach its 3 bonds, its ideal number of bonds, it'd be connected to a hydrogen. So this represents the structure of uracil. So just remember, we have our 2 carbonyls here and then each of the nitrogens to make their 3rd bond connects to a hydrogen.

Now, here uracil, the other two pyrimidines are just modifications of this, thymine, and cytosine. Thymine and uracil are very similar in structure, the only difference is that there's a methyl group involved. The methyl group would be right here, and then we'd still have our 2 carbonyls here and here, and our nitrogens would still have an H each. So this is thymine.

Cytosine is a little bit trickier. But just remember, we're going to say, cytosine is cytosine amine group. So when we see amine group, kind of have a moment like, oh gosh. Here it goes. Si amine group. So what does this mean? Well, we're going to still have our carbonyl here. We're going to still have an H on this nitrogen here. But now, we're still going to possess this double bond just like this nitrogen here possesses a double bond. It's making its 3 bonds, so it doesn't need an H on it. And then, cytosine amine. Amin. Right? It's pronounced the same way as an amine, the functional group amine, which we know is an NH₂ group. So instead of having 2 carbonyls, we have 1 carbonyl and 1 NH₂ group right here. So this represents our cytosine. So just remember, this is the starting structure of pyrimidine. The 3 pyrimidines are just modifications of it. It all starts with uracil and from there, from uracil, you can adapt it to give us thymine or to give us cytosine. This is the key to remembering the structures of these different types of nitrogenous bases.

Nitrogenous Bases Example 2

Here in this example question, it says, "Complete a structure of thymine base." So remember, to draw thymine, we need to remember the structure for uracil. If we can remember the structure for uracil, we just adapt it to give us thymine. Now, first of all, remember that our pyrimidines have this basic structure involved. We have our nitrogens here. This will make a double bond, double bond here, and double bond here. This is the general structure of a pyrimidine. For uracil, all we do now is adapt this structure; we'd have 2 carbonyl groups here and here. We'd still have our double bond here. The nitrogens still need to make 3 bonds, and they do that by adding an H to each one. This would be uracil. To get thymine, just remember methyl because it's connected to the thymine. So here with that, just realize we have this similar structure. The difference now is we're going to add a methyl group. So we'd still have a double bond here. We'd have our 2 carbonyls still. Each nitrogen would still have an H. Methyl for thymine. The methyl will come off of this carbon here. So this will represent the structure for thymine. Remember, we were able to do it by first remembering what a pyrimidine looks like in terms of this structure, then modifying this structure to uracil. And then just remember, if you know uracil, thymine is almost the same, except we have a methyl involved. So, add the methyl group to the appropriate carbonyl. And there you have it, thymine represents what we have within the box.

Nitrogenous Bases Concept 3

In this video, we'll talk about some tricks that we can remember in order to draw purine structures. Now, a purine, the base form, is this. We have two rings fused together, and we see that we have four nitrogens embedded within those rings. Now for adenine, remember the structure of adenine, just remember adenine, ad amine, and amine is an NH₂ group. Here we're just going to add an NH₂ group to this structure. So here we'd still have a double bond on this nitrogen, and we would add our amine to this carbon right here, our NH₂ group. And this will represent adenine.

Now, guanine, if you remember the structure of guanine, we're going to say go first. This red oxygen indicates that we have a carbonyl. And there goes our red oxygen. Now because we have that carbonyl carbon there, it cannot make a double bond. Otherwise, it'd be making five bonds. Right? So a double bond cannot go here. That means this nitrogen is only making two bonds. Ideally, it wants to make three. To get that third bond, it has to connect to a hydrogen. So guanine, we have the go part. And then the second part is amine. So again, we have amine involved. An amine is an NH₂ group. That NH₂ group would go right here. So this represents our structures of adenine and guanine. But again, it all originates from the base form of purine, which is our two fused rings with four nitrogen atoms embedded within them.

Nitrogenous Bases Example 3

Here it says, complete a structure of the guanine base. So remember, guanine, which is a purine, and its base form of purine would be these two nitrogens having double bonds. But we're going to adapt this base form of purine to give us guanine at the end. Now, to remember the structure of guanine, we just have to remember, "Go Amine." So the "go," that red oxygen indicates that we have a carbonyl group. We have to get rid of that double bond there because if we didn't, that carbonyl carbon would be making five bonds. Carbon can only make up to four bonds. Now this gives us an issue though. This nitrogen now isn't making three bonds as it ideally wants to. It's only making two. So in order to make that third bond, you'd have to gain an H. Next, we have "Amine". Remember, an amine is an NH₂ group. That means we'd have to add an NH₂ group somewhere to the structure which would be right here. So here, this will represent the structure of our guanine nitrogenous base. Remember, we've just adapted the base form of our purine molecule in order to get this particular nitrogenous base.

Select a correct structure for U.

• A.
• B.
• C.
• D.

Draw a structure for cytosine.

• A.
• B.
• C.
• D.

Draw the other 2 tautomeric forms of cytosine. 

Hey everyone. So here it says, draw the other 2 tautomeric forms of cytosine. Alright. So here, we can do as our first tautomer, and we're going to say, remember, this is our keto form which is more stable. So we'd have a reversible arrow more towards our keto form. What we could do here initially is we could remember that this nitrogen here could deprotonate our acid catalyst in the form of H3O+. So it's gonna deprotonate this H3O+, and what we're going to form initially is this structure. And we're going to say, that nitrogen now is making 4 bonds, so it's gonna be positively charged. The H2O that lost its H+ can still be around as water. We can use it to deprotonate this Nitrogen. We're removing this H, and we can say either the lone pair on the Nitrogen or this bond being broken comes down to form a double bond here, which is gonna kick this pi bond onto this nitrogen. And we're going to have our reversible arrows again, And when we draw this out, we're going to have this nitrogen here which still has one H. It now has a double bond on it. We have this double bond, we have this NH group still present. Our carbonyl is still intact, and we're going to say we have this nitrogen here now with a lone pair and it's making 3 bonds so it's neutral again. So, this will represent 1 of our tautomers. What's another thing we could do? Well, another thing I can do is I could get the carbonyl oxygen involved. So we're going to draw this same exact structure again, and what we're going to do now is going to be the carbonyl oxygen that can act as a deprotonator of our H3O+ catalyst. So if we draw all this out, remember this carbonyl oxygen has lone pairs on it. So that lone pair can be used to deprotonate this H3O+. It's gonna come here. It's going to remove an H. And what we're going to have initially is this structure where oxygen now is positively charged because it's making 3 bonds, and it has a lone pair here. So this is our initial structure. Now, that H2O is now water. Water can come into play and it's gonna deprotonate this. Nitrogen is gonna remove its H. That bond here falls to make a double bond here, which breaks this double bond and goes to the oxygen, and thereby making our second tautomer. So we're going to have here and so if we work this out, we're going to say this is an NH2. This pi bond is still here. This nitrogen now is double bonded to this carbon, which is connected to this OH group, which is connected here. So this would represent our other possible tautomers. There'd be 2 tautomers involved because there are 2 places where we could have deprotonation of our H3O+. We can either use that nitrogen initially in the top one, or we can use the carbonyl oxygen in the bottom one. Both of these represent the other 2 tautomers of our cytosine molecule.