By examining the structure of DNA, it is easier and clearer to see what factors are taken into account when genes are passed down from one parent to offspring. DNA is part of an elite group of compounds called nucleic acids. Nucleic acids are polymers made up of monomers called nucleotides. DNA provides instructions for its own replication, directs the synthesis of ribonucleic acid (RNA), and also controls protein synthesis. This whole process is better known as gene expression. On a DNA molecule, each gene directs the synthesis of a particular type of RNA called messenger RNA. This mRNA molecule works in conjunction with the cell's protein synthesis machinery to direct the production of a polypeptide chain, which ultimately results in the formation of a protein. Protein synthesis occurs at the level of ribosomes. However, DNA is found in the nucleus. Nucleic acids include several components. These are macromolecules that exist in the form known as polynucleotides. As mentioned previously, the monomers of polynucleotides are nucleotides. A nucleotide is made up of three parts: a pentose sugar, a nitrogen-containing base, and a phosphate group. The sugar present in DNA is deoxyribose (an oxygen atom is missing on the second carbon of the ring) and in RNA, ribose. There are four nitrogen bases in DNA. Each nitrogen base has one or two rings that contain nitrogen