Primary Structure of Nucleic Acids - Online Tutor, Practice Problems & Exam Prep

Primary Structure of Nucleic Acids Concept 1

Hey, everyone. So we're going to say here that the primary structure of a nucleic acid is the sequence of nucleotides attached through Phosphodiester bonds. Now, remember, we said that a nucleotide is made up of 3 important parts. We have our phosphate group, we have our pentose ring, and then we have our nitrogenous base. Two of these nucleotides can be connected to each other through phosphodiester bonds. When we talk about the primary structure, it's just a chain of these nucleotides connected with multiple phosphodiester bonds. Now here, the phosphodiester bonds themselves, well, these are the bonds of the phosphate group that connects two sugars in the primary structure. If we take a look here, we see a phosphate group here, and it's connecting this sugar and this sugar, so that's why we have these purple bonds. Here's this phosphate group here, we have this purple bond and this purple bond because it connects these two sugars. We're going to say the repeating Phosphate Sugar Phosphate sequence forms the new nucleic acid backbone. So we have our backbone here in the darker blue. And remember, coming off of the sugar itself is our nitrogenous base. These nitrogenous bases could be the same or they could be different. So here we'll say this is nitrogenous base 1, 2, and 3. So just remember that two nucleotides can be connected to one another through a phosphodiester bond. When we have a string of nucleotides connected by multiple phosphodiester bonds, this becomes the primary structure of our nucleic acid.

Primary Structure of Nucleic Acids Example 1

Here we're told that a pentanucleotide has a base sequence of g a u c a. Based on the given sequence, what is its likely origin? So where does this sequence originate from? And we can figure this out based on this sequence. So these are our nitrogenous bases. Because they're nitrogenous bases, that will rule out fructose which is a carbohydrate, and oleic acid because it's a fatty acid. Because these are nitrogenous bases, that means that it's going to be either DNA or RNA. Now, what are the key differences between them?

Remember, the key difference is that DNA uses thymine as one of its nitrogenous bases, abbreviated T, and RNA uses uracil, abbreviated U. Since there is a uracil involved within this sequence, it would have to be RNA that's being discussed here. So here, our final answer would be option C. This represents RNA as the origin for this nucleotide or base sequence.

Primary Structure of Nucleic Acids Concept 2

So now, remember that the primary structure of a nucleic acid is the sequence of nucleotides connected through phosphodiester bonds. With this idea, we have a new term, directionality. Now, this is the sequence of nucleotides being read, and it's read from the 5' end to the 3' end. A good way to remember which end is which is our memory tool: phosphate, phosph sounds like the letter f, equals 5. So, here, we're going to say that this is two nucleotides connected by a phosphodiester bond. If we imagine it just being elongated further by more nucleotides and more phosphodiester bonds, we'd say at this end where we have our free phosphate group, phosphate equals 5, this would be our 5' end. And if that's the 5' end, then down here where we have this free OH group, that represents our 3' end. And again, we're reading it from 5' to 3', so looking at this arrow, we're going from 5' to 3'. This is how we interpret the primary structure of nucleic acid if we're reading the chain of nitrogenous bases involved. Because here, the nitrogenous bases are where we can get differentiation in terms of our different primary structures of nucleic acids. Different orders of our nitrogenous bases, in turn, over time, could lead to different types of proteins being created from this primary structure. Right? So just keep that in mind. Phosphate equals 5. The free phosphate end is the 5' end. The free OH end is the 3' end.

Primary Structure of Nucleic Acids Example 2

In this example question, it asks which of the following statements about primary nucleic acid structure is incorrect?

• The structure and function of a nucleic acid is based on the sequence of the connected nucleotides. That there is true.
• The sequence of nitrogenous bases is read from the 5 prime to the 3 prime end. This is directionality, so this statement is also true.
• Phosphodiester bond represents a bond between the phosphate group and the sugar. That is also true.
• So here, the last one, the difference between Nucleic Acids is a result of the difference of the sugar attached to the backbone. Remember, the diversity or differences between our nucleic acids are really based on the differences in their nitrogenous bases, not the backbone. So this statement, (d), is the statement that is incorrect out of all the options given.

Draw the full structure of the DNA trinucleotide G-A-T and label its 5’ and 3’ ends.

• A.
• B.
• C.
• D.

Draw the following primary structure based on the following description:

Draw 3 nucleotides with deoxyribose sugars with dTMP bases connected by phosphodiester bonds.

Here in this practice question, it states, draw the following primary structure based on the following description. Draw 3 nucleotides with deoxyribose sugars with dTMP bases connected by phosphodiester bonds. Alright. So they are deoxyribose sugars, which means that the 2' carbon does not possess an OH group. And here it's a bunch of Thymines as our nitrogenous bases. So here we're going to draw first the backbone. We're going to draw the free phosphate end from the 5' end to the 3' end, which has our free OH group. Later on, we'll attach nitrogenous bases to them. Here, all the nitrogenous bases are thymine bases. Alright. So here, let's draw that out. We're going to have on our 5' end here, our free phosphate, okay. And that's connected to CH2 connected to our ring, that's connected to our next phosphate group, which is connected to CH2, and connected to our next ring. And then finally, our last phosphate group connected to our last ring. Remember it's a deoxyribosugar so there's an OH on the 3' carbon but not on the 2' carbon. Now what we have to do is we just have to add our 3 nitrogenous bases which are all the same. So it's just going to be a Thymine Base which remember is a 6-membered ring. It's going to look like this with a methyl group right there. So that's going to be on each one of these carbons here. So let's draw that out. So there goes one. Here comes the second one. And then we're going to draw our third one. So here goes our 3 nucleotides. We have our deoxyribosugars and dTMP. So here we have our thymine as our nitrogenous base, 3 of them, on each of these nucleotides that are all connected together by phosphodiester bonds. So this structure, so this is the 5' end, and down here is our 3' end. This would be the structure based on the description given.