Nucleotides are the building blocks of the DNA and RNA molecules that make up genes. While there are several types of nucleotides in genes, we are going to represent all of them in Petri Dish with a single generic nucleotide, orotidine 5'-monophosphate (OMP). OMP is a precursor for building other nucleotides, such as cytosine, uracil, and thymine. And phosphoribosyl pyrophosphate (PRPP), an intermediate in the building of OMP in our metabolic pathway, is a precursor for building adenine and guanine.

Nucleotide synthesis begins by diverting glucose-6-phosphate and oxaloacetate from the cellular respiration pathway and glutamate from the amino acid synthesis pathway. Glucose-6-phosphate is used to build phosphoribosyl pyrophosphate (PRPP) in step 1, and oxaloacetate and glutamate are used to build aspartate in step 2, which is then used to build orotate in step 3. Phosphoribosyl pyrophosphate and orotate are then combined to form our generic nucleotide in step 4.

Step Reactants Products
1 C6H11O9P2- (glucose-6-phosphate) + 2 PO32- + H2O C5H13O14P3 (phosphoribosyl pyrophosphate) + CO2
2 C5H9NO4 (glutamate) + C4H4O5 (oxaloacetate) C4H7NO4 (aspartate) + C5H6O5 (ketoglutarate)
3 C4H7NO4 (aspartate) + NH4+ + CO2 C5H4N2O4 (orotate) + 2 H2O
4 C5H4N2O4 (orotate) + C5H13O14P3 (phosphoribosyl pyrophosphate) nucleotide (orotidine 5'-monophosphate) + 2 PO32- + H2O

The metabolic pathway for building orotidine 5'-monophosphate in Petri Dish has been simplified. Here is a more detailed version of the actual pathway:

Step Reactants Products
1 glucose-6-phosphate + NADP+ 6-phosphoglucono-δ-lactone + NADPH
2 6-phosphoglucono-δ-lactone + H2O 6-phosphoglucanate + H+
3 6-phosphoglucanate + NADP+ ribulose-5-phosphate + NADPH + CO2
4 ribulose-5-phosphate ribose-5-phosphate
5 ribose-5-phosphate + 2 HPO3- phosphoribosyl pyrophosphate
6 glutamate + oxaloacetate aspartate + α-ketoglutarate
7 NH4+ + CO2 + 2 ATP carbamoyl phosphate + 2 ADP + Pi
8 aspartate + carbamoyl phosphate carbamoyl aspartate + Pi
9 carbamoyl aspartate dihydroorotate + H2O
10 dihydroorotate + NAD+ orotate + NADH + H+
11 orotate + phosphoribosyl pyrophosphate orotidine 5'-monophosphate + PPi