Nucleic Acids
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Structure of Nucleic Acids

Nucleotides are the monomers of nucleic acids

    Nucleotidessoundicon.gif (538 bytes) have three major regions to them, and once you learn those regions, even though nucleotides look large, they are then easy to understand. The three regions are:

bulleta five-carbon sugar (either ribose or deoxyribosesoundicon.gif (538 bytes))-- this is different from our carbohydrate building blocks because they have 6 carbons.
bulleta phosphate group (or more than one phosphate group)-- note that this is the first time since phospholipids where we have seen a phosphate group.
bulleta nitrogen base-- a ringed group of atoms with nitrogen in the ring or rings.

Here is an example of a nucleotide.  This particular nucleotide is called AMP. Smack dab in the middle of this nucleotide you can see a pentagon-shaped region. This is the five-carbon sugar. At the upper right hand corner of the nucleotide you can see the phosphate group (the yellow part represents where the phosphorus atom is, and the red parts are the oxygen atoms). To the top left of the nucleotide is the nitrogenous base-- this one is a double-ringed one called adenine. Notice that the two rings (one a hexagon and the other a pentagon) have nitrogen atoms (the blue regions) in them. This image was made using RasMol.

    A simplified version of a nucleotide can be drawn in the way shown on the right. In this drawing, the five carbon sugar is represented by the pentagon with the S, the phosphate group is represented by the P with the circle around it, and the nitrogen base is represented by the "N-base."

    The nitrogen base of nucleotides is a variable group-- kind of like the R group in amino acids, but not so variable.  Whereas there were 20 different R groups in amino acids, there are only five different nitrogen bases in nucleotides.  The five are:  adenine (as shown in the AMP image), thymine, guanine, cytosine, or uracil. Since these five all conveniently begin with different letters, they can each be represented by just their first letter in the place of "N-base" up above.   Your book defines the nitrogen bases as belonging to either the puridine or pyrimidine categories-- but I don't care if you learn that or not.  All I need you to do is to be able to recognize the nitrogen base names and to learn are the 5 letters to represent them:  A, T, G, C, and U.

Nucleotides bond together to make nucleic acids.

    This is the way that nucleotides link up with each other.  (Note: the chain of the nucleic acid is actually the connections from sugar to phosphate to sugar to phosphate, etc..., and the nitrogenous bases just stick off the side.)The way nucleotides bond is by the phosphate group of one nucleotide making a new bond with the 5-carbon sugar of the next nucleotide. You can see how that makes a long chain of nucleotides by looking at the four nucleotide chain in this image.  Your textbook has a similar drawing.

Notice how the chain is actually a sugar-phosphate-sugar-phosphate (etc...) chain, and the nitrogen bases stick off the side. That will become important when we talk in more detail about DNA later. But it is not a big deal for us right now.

    Once you have a chain of nucleotides, you have a nucleic acid!

    There are only two types of nucleic acids: DNA and RNA.  These two types differ in their structure.   "DNA" stands for deoxyribonucleic acid. That just means that DNA has deoxyribose as its 5 carbon sugar. RNA (or ribonucleic acid) has ribose as its 5 carbon sugar.  All the nucleotides that come together to make up DNA have deoxyribose for their 5 carbon sugar. Likewise, all the nucleotides that come together to make up RNA have ribose for their 5 carbon sugar.

    DNA and RNA also differ in two other ways.  One way that they differ is that DNA uses the nitrogen bases A, T, G, and C, while RNA uses the nitrogen bases A, U, G, and C.  You will never find a thymine in RNA or a uracil in DNA.  The other way that RNA and DNA differ is in the number of nucleotide chains that they have.  RNA is simply one chain.  DNA is composed of two chains that attach to one another.  I will describe this later when we discuss DNA in detail.   But that won't be for a couple weeks.

Function of Nucleic Acids

    There are three functions that we will discuss... one from a nucleotide, one from DNA, and one from RNA.

bulletcellular energy-- ATP is cellular energy.  ATP stands for adenosine triphosphate, and it is a nucleotide.  It is shown below, and you should notice that it looks just like the AMP image above, except that there are three phosphate groups in ATP.  ATP is what our cells use to do everything that they need to do-- it is generated every time we break down glucose in the presence of oxygen.

atp.jpg (34708 bytes)


genetic code-- DNA is our genetic code.  As such, what does it do?   It contains the information so that our cells know how to make all the proteins they need.  When there is a genetic problem, like with muscular dystrophy or sickle cell anemia, the problem is played out with a faulty protein (dystrophin or hemoglobin in those examples).


reading our genetic code-- RNA is used to read the DNA and provide the instructions contained within the DNA to the rest of the cell so that the cell can make the appropriate proteins.

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2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D.

Last changed: January 21, 2007