There are two types of nucleotides that are used to create strands of DNA and RNA: purines and pyrimidines. Based on their structure, the five nucleotides are classified as either purine or pyrimidine. The nucleotides cytosine, thymine and uracil are pyrimidines, and they are produced through pyrimidine biosynthesis.
All nucleotides have a similar basic structure in that they are made up of a five carbon sugar molecule that is bound to a nitrogen-containing base and a phosphate group. The structure of the nitrogenous base is what differentiates pyrimidines from purines. They also differ in the way that they are synthesized.
Pyrimidine biosynthesis can occur both inside a living organism and outside, or in vivo and in vitro. There are two different pathways for both purine and pyrimidine biosynthesis, which are de novo and salvage pathways. During de novo biosynthesis, the nucleotide is synthesized from scratch, or a new pyrimidine is created from the molecules that make it up. Pyrimidines that have already been formed are used, or recycled, during salvage biosynthesis. In both cases, the final step of the process is to attach the pyrimidine to a ribose sugar.
The main way that pyrimidine biosynthesis differs from purine biosynthesis is how the pyrimidine or purine is assembled. During pyrimidine biosynthesis, the pyrimidine is built first and then attached to the ribose sugar. In contrast, purines are built right on the ribose sugar.
The pyrimidine nitrogenous base is made up of a six member ring that contains two nitrogen atoms at positions one and three within the ring. This is the part of the pyrimidine that is completed before it is attached to the ribose sugar. There are six steps that lead to the formation of a pyrimidine from the two precursor molecules, which are carbamoyl phosphate (carbamoyl-P) and aspartic acid.
Depending on the type of organism, different numbers of enzymes are used to carry out the six steps of pyrimidine biosynthesis. Within bacteria, there are six distinct enzymes, or one for each step of the process. Only three enzymes are necessary within mammals.
Several different chemical reactions are involved in creating a pyrimidine. The first two steps involve the production of carbamoyl-P, which is then joined to an amine group (-NH2) that contains one nitrogen atom and two hydrogen atoms. At this point, the ring is closed and provides the basic structure of the nitrogenous base. The last three steps result in the pyrimidine ring being completed and attached to the five carbon ribose sugar.