Nucleic Acids

Below you will find a somewhat detailed description of nucleic acids. There are two molecules that I have highlighted at the bottom that fit into this category: DNA and ATP.

Nucleic acids are made of monomers called nucleotides. Nucleotides 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:

	a five-carbon sugar (either ribose or deoxyribose)-- just meaning that it is made up of only C, H, and O atoms, and that it is NOT a monosaccharide because it has only 5 carbons, not 6.
	a phosphate group (or more than one phosphate group)
	a nitrogenous 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 first viewed using Rasmol. If you want to see it using Rasmol, you have to open up Rasmol from your Start button and then Program menu ("Raswin") and then go to the file menu and choose open. Double-click on the folder next to molecules, and you will see a choice for AMP. Double-click on AMP and you can rotate this molecule around in Rasmol on your computer. 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 nitrogenous base is represented by the "N-base." This drawing is just like the top figure in your textbook on page 47, except that it is flipped around from left to right. "N-base" in this figure represents all that is in green (on the right) in that textbook figure, for example.

The nitrogenous base is a variable group, and it can be one of five different molecules: 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.

If the nucleotides come together, they make up the polymer that is a nucleic acid. 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 they attach is by the phosphate group of one nucleotide making a new covalent bond (via a dehydration synthesis reaction) 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. A similar drawing is shown on page 53 (Figure 2.20) of your textbook.

Notice how the chain is actually a sugar-phosphate-sugar-phosphate (etc...) chain, and the nitrogenous bases stick off the side.

There are only two types of nucleic acid polymers: DNA or RNA. "DNA" stands for deoxyribonucleic acid. That just means that this particular nucleic acid has deoxyribose as its 5 carbon sugar. If it were RNA (or ribonucleic acid), the 5 carbon sugar would be ribose. The only difference between deoxyribose and ribose is just one oxygen atom. Figure 2.21 shows these two. 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 contains the genetic code for our cells whereas RNA is involved in translating that code so that our cell can make protein (it is a major component of ribosomes).

DNA:

DNA is shown in 3 figures of your textbook within chapter 4. Check out pages 118 - 119 for Figures 4.20, 4.21, and 4.22. You'll notice that DNA is "double-stranded," meaning that it has two strands that each look like the figure above with the yellow background... but these two strands face each other and twist together. This twisting gives the DNA molecule its characteristic of being a "double helix."

ATP:

One particular nucleotide all by itself (as a monomer, not a polymer) has a very important role in all cells. That nucleotide is ATP, adenosine triphosphate, the cellular form of energy. ATP is just like AMP (shown above) except for the fact that ATP has three phosphate groups instead of just one. You have a detailed picture of ATP in Figure 4.7 (page 107) in your textbook. You can also view ATP in Rasmol (go to the molecules page and download it by right-clicking it and saving the target, ATP.pdb, onto your computer). The word "adenosine" just represents the sugar and nitrogenous base together (ribose + adenine = adenosine). Note that ATP is the form our cellular energy takes, but ADP (adenosine diphosphate) is not quite an energy supplier. So in order to make ATP, we have to take ADP and add another phosphate group. This is done a great deal in the organelle called the mitochondrion.