Nucleic Acids - DNA - RNA

DNA is composed of four types of molecules known as nucleic acids or nucleotides. The four nucleotides are: adenine (A), guanine (G), cytosine (C), and thymine (T). These molecules are further classified into two families. Adenine (A) and guanine (G) are known as purine and cytosine (C) and thymine (T) are known as pyrimidine. During DNA synthesis, nucleotides are converted into nucleic acids to so that they can be linked to form strands of DNA. The assembled strand of DNA takes on the structure of a double helix.

The two strands of DNA are held together by hydrogen bonds between adjacent complementary nucleotides.

The chemical structures of the four nucleotides are planar due to the delocalized electrons in the five- and six-membered rings, each having a thickness of 3.4 angstroms. When the nucleotides form the double helix structure, A-T and G-C are joined together by a hydrogen bond to form a base pair. The base pairs are then joined together by sugar bonds to form the helix. X-ray data shows that there are 10 base pairs per turn of the helix.

The helical model of DNA also explains the theory of genetic replication. James Watson once described it as the "pretty molecule" because the method of replication is so self evident in this structure. During replication, the hydrogen bonds between nucleotides break and allow each single strand of DNA to serve as a template for replication of the other half. The two identical copies of newly synthesized of DNA are then distributed to two new daughter cells. Because during each cycle of replication half of the old DNA is preserved, DNA replication is said to be semi-conservative.

Although DNA contains the genetic blueprint of life, it requires the assistance of ribonucleic acid (RNA) to be functional. RNA also consists of strands of nucleic acids joined together by sugar-phosphate bonds. Unlike DNA, RNA substitutes the nucleotide thymine (T) with uracil (U) and exists as single strands. After DNA is converted into strands of RNA, the messenger RNA (mRNA) is sent to the ribosome to direct the synthesis of proteins.