Nucleotide biosynthesis is the process whereby nucleotides are created or synthesized. This process can occur both within living organisms, as well as within a lab. If occurring within living cells, the process takes place in the cytoplasm of the cell and not within a particular organelle. Nucleotides are particularly important molecules within the cells of all living organisms because they are the molecules that are used to produce DNA and RNA. Also, nucleotides are used to form energy storage molecules and molecules that are necessary for passing signals between cells and between organelles within cells.
There are five different nucleotides; adenine, cytosine, and guanine are found in both DNA and RNA, thymine is found solely within DNA molecules, and uracil is only in RNA. All nucleotides have a similar basic structure, which is a nitrogenous base attached to a sugar molecule and a phosphate group. They are categorized into two groups based on the structure of this base. The nitrogenous base for purines — adenine and guarnine — contains a double ring structure, while the base found in pyrimidines — cytosine, thymine and uracil — has only a single ring structure.
Two different methods of nucleotide biosynthesis occur within cells. If a nucleotide is created from simpler compounds, it is considered de novo nucleotide biosynthesis. De novo is Latin and basically means from the beginning, or from scratch. The other way that nucleotides are formed is through salvage pathways. In this situation, parts of nucleotides that have been broken down are recycled and reused to form new nucleotides.
Each group of nucleotides undergoes de novo nucleotide biosynthesis differently. With pyrimidine nucleotides, the base structure is formed from its components and then attached to a ribose sugar molecule. In contrast, purine nucleotides are created by attaching the simpler compounds directly onto the ribose molecule. During salvage biosynthesis, a base that has already been formed is recycled and reattached to a ribose unit.
Nucleotide biosynthesis results in the creation of ribonucleotides, which are nucleotides that contain a ribose sugar. Ribonucleotides are used to create strands of RNA, while DNA is created from deoxyribonucleotides. As such, all nucleotides that are used for DNA must undergo further synthesis.
To form deoxyribonucleotides from ribonucleotides, the ribose sugar loses an oxygen molecule, or undergoes a reduction reaction. To convert uracil to thymine, for example, an additional methyl group is added to the uracil nucleotide. The reduction of the ribonucleotides occurs only after they have been fully formed.