DNA is composed of nucleotides, one of the four basic biomolecules we talked about in my previous post. Each nucleotide has a ribose 5 – carbon ring. We can label these carbons so that each carbon has a number. Assigning numbers to carbon atoms in molecular compounds is something that you will learn about in organic chemistry.
As you can see in the drawing above, the ribose sugar ring has a carbon labeled with a number 5 and another carbon labeled with a number 3. The 5′ carbon and the 3′ carbon are very important when we start discussing replication and transcription. But for now, let’s just try to remember where the 5′ carbon and 3′ carbon are located.
When two nucleotides come together, a phosphodiester bond forms. The hydroxyl group off of the 3′ carbon will attack the alpha phospho-group attached to the 5′ carbon on the next nucleotide in what organic chemists will call a classic nucleophilic substitution reaction. The result is a phospho-carbon linkage and thus the backbone of DNA.Attached to the 1′ carbon on the ribose ring of a nucleotide, we find the addition of a nitrogenous base. There are only four different types of bases that can be added to this carbon; adenine, guanine, cytosine, or thymine. (Actually, there is one more, but we will save that for our discussion of RNA). A nitrogenous base is a pretty complex molecule consisting of either a single ring or a double ring. For now, we will just identify the base by its symbol, A for adenine, G for guanine, C for cytosine, and T for thymine. Each of these bases gives the nucleotide different characteristics. These bases and the order of these bases in the long chain of a DNA molecule is the information code for our cells. That is why you have probably seen DNA written as -ATCGCGGATC-. Pretty neat huh?