Nucleic Acids

A nucleic acid is a large molecule made of nucleotides. Most, if not all, nucleic acids were discovered in the late 1860s. Swiss scientist Friedrich Miescher isolated a compound from inside the nucleus of white blood cells. He determined that the compound was not a carbohydrate, lipid, or protein. He called the substance nuclein, which scientists now know as nucleic acid.

Nucleic acids are long chains composed of smaller subunits called nucleotides. Each nucleotide consists of a five-carbon sugar, a phosphate group, and a ring compound containing nitrogen (nitrogenous base). The sugar may be ribose or deoxyribose, differing by the group attached at the 22 carbon (hydroxyl group in RNA or hydrogen in DNA). A nucleic acid that contains ribose is called ribonucleic acid. Ribonucleic acid (RNA) is an organic molecule that carries genetic messages out of the nucleus; it consists of a single strand of nucleic acids. A nucleic acid containing deoxyribose is called deoxyribonucleic acid. Deoxyribonucleic acid (DNA) is an organic molecule containing coded instructions for the life processes of an organism; it consists of nucleotides bonded together in the form of a double helix. There are other important differences in structure and function between these two types of nucleic acids, including the types of nitrogenous bases, the number of strands, and the function in protein synthesis. The nitrogenous base of a DNA nucleotide can be a single carbon-ringed molecule, such as cytosine (C) and thymine (T), or a double carbon-ringed molecule, such as guanine (G) and adenine (A). Cytosine and thymine are known as pyrimidines, which have a single carbon and nitrogen ring. Adenine and guanine are purines, which have two carbon and nitrogen rings. An RNA nucleotide has a similar set of possible nitrogenous bases (A, C, and G) with one exception. RNA has uracil (U), another pyrimidine, instead of thymine (T). In addition, DNA is most stable in an antiparallel, double-stranded form, where the nucleotides on each strand form phosphodiester bonds between the 5′ phosphate group of one nucleotide and the 3′ carbon of another nucleotide. The two sugar-phosphate repeating chains form the backbone of DNA. Between the two backbone strands, hydrogen bonds form between the nitrogenous bases of opposing nucleotides, according to complementary base-pairing rules. Adenine is always bonded to thymine by two hydrogen bonds, and cytosine is always bonded to guanine by three hydrogen bonds. However, RNA is usually single stranded, but it also utilizes phosphodiester bonds between the phosphate group of one nucleotide and the carbon of a sugar of another nucleotide. When RNA bases do bond with bases of another nucleic acid, cytosine bonds with guanine, but adenine must bond with uracil because RNA lacks thymine. The relatively weak hydrogen bonds allow the two strands of DNA to be unzipped for DNA replication as well as for the transcription portion of protein synthesis. The nucleic acids DNA and RNA are similar in many ways, but they have different functions within the cell. The nucleic acid DNA stores the hereditary information of the organism in its entirety. With a few exceptions, such as sex cells, most body cells contain the same DNA even though they carry out vastly different functions. The nucleic acid RNA, on the other hand, expresses the information stored in DNA and directs the activities that make cells different from one another. Messenger RNA, or mRNA, is built as a complementary strand of DNA that codes for a desired polypeptide. In eukaryotes, the mRNA is then processed to remove noncoding sections of the sequence. Next, the mRNA carries the sequence (instructions from DNA for making the polypeptide) out of the nucleus and into the cytoplasm. Transfer RNA, or tRNA, assists in delivering the correct amino acids to construct the polypeptide. Ribosomal RNA, or rRNA, forms the ribosomes, the structures that act as the site of translation of mRNA code into a string of amino acids, the basis of a protein. The nucleotide adenosine triphosphate (ATP) is a biological unit of energy that consists of an adenosine (an adenine group and a ribose sugar) and three phosphate groups. Adenosine triphosphate (ATP) is the energy currency of a cell. The removal of a single phosphate group releases energy and forms adenosine diphosphate (ADP), the reduced form of the biological unit of energy, ATP. Both ATP and ADP are ubiquitous in cells so that chemical energy can be stored or used whenever necessary.