Nucleic Acids - Structure and Function

Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.

The nucleic acids are vital biopolymers found in all living things, where they function to encode, transfer, and express genes. These large molecules are called nucleic acids because they were first identified inside the nucleus of cells, however, they are also found in mitochondria and chloroplasts as well as bacteria and viruses. The two principal nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

DNA and RNA in Cells

DNA is a double-stranded molecule organized into chromosome found in the nucleus of cells, where it encodes the genetic information of an organism. When a cell divides, a copy of this genetic code is passed to the new cell. The copying of the genetic code is called replication.

RNA is a single-stranded molecule that can complement or "match up" to DNA. A type of RNA called messenger RNA or mRNA reads DNA and makes a copy of it, through a process called transcription. mRNA carries this copy from the nucleus to ribosomes in the cytoplasm, where transfer RNA or tRNA helps to match amino acids to the code, ultimately forming proteins through a process called translation.

Nucleotides of Nucleic Acids

Both DNA and RNA are polymers made up of monomers called nucleotides. Each nucleotide consists of three parts:

• a nitrogenous base
• a five-carbon sugar (pentose sugar)
• a phosphate group (PO₄ 3- )

The bases and the sugar are different for DNA and RNA, but all nucleotides link together using the same mechanism. The primary or first carbon of the sugar links to the base. The number 5 carbon of the sugar bonds to the phosphate group. When nucleotides bond to each other to form DNA or RNA, the phosphate of one of the nucleotides attaches to the 3-carbon of the sugar of the other nucleotide, forming what is called the sugar-phosphate backbone of the nucleic acid. The link between the nucleotides is called a phosphodiester bond.

DNA Structure

Both DNA and RNA are made using bases, a pentose sugar, and phosphate groups, but the nitrogenous bases and the sugar are not the same in the two macromolecules.

DNA is made using the bases adenine, thymine, guanine, and cytosine. The bases bond to each other in a very specific way. Adenine and thymine bond (A-T), while cytosine and guanine bond (G-C). The pentose sugar is 2'-deoxyribose.

RNA is made using the bases adenine, uracil, guanine, and cytosine. Base pairs form the same way, except adenine joins to uracil (A-U), with guanine bonding with cytosine (G-C). The sugar is ribose. One easy way to remember which bases pair with each other is to look at the shape of the letters. C and G are both curved letters of the alphabet. A and T are both letters made of intersecting straight lines. You can remember that U corresponds to T if you recall U follow T when you recite the alphabet.

Adenine, guanine, and thymine are called the purine bases. They are bicyclic molecules, which means they consist of two rings. Cytosine and thymine are called the pyrimidine bases. A pyrimidine bases consists of a single ring or heterocyclic amine.

Nomenclature and History

Considerable research in the 19th and 20th centuries led to the understanding of the nature and composition of the nucleic acids.

• In 1869, Friedrick Miescher discovered nuclein in eukaryotic cells. Nuclein is the material found in the nucleus, consisting mainly of nucleic acids, protein, and phosphoric acid.
• In 1889, Richard Altmann investigated the chemical properties of nuclein. He found it behaved as an acid, so the material was renamed nucleic acid. Nucleic acid refers to both DNA and RNA.
• In 1938, the first x-ray diffraction pattern of DNA was published by Astbury and Bell.
• In 1953, Watson and Crick described the structure of DNA.

While discovered in eukaryotes, over time scientists realized a cell need not have a nucleus to possess nucleic acids. All true cells (e.g., from plants, animals, fungi) contain both DNA and RNA. The exceptions are some mature cells, such as human red blood cells. A virus has either DNA or RNA, but rarely both molecules. While most DNA is double-stranded and most RNA is single-stranded, there are exceptions. Single-stranded DNA and double-stranded RNA exist in viruses. Even nucleic acids with three and four strands have been found!