Basic Units of Nucleic Acids

Nucleic acids are information-carrying molecules in living cells. They consist of nucleotide monomers made up of a ribose sugar, a phosphate group, and a nitrogenous base. A term that you should not confuse nucleotide with is nucleoside. Nucleosides have a base and sugar but no phosphate group attached.

Nucleoside versus nucleotide

There are five types of nitrogenous bases (also called nucleobases) which form the fundamental units of the genetic code. These are,

  1. A- adenine
  2. C-cytosine
  3. G-guanine
  4. T-thymine
  5. U-uracil

A, G, C and T and found in DNA while A, G, C and U are found in RNA.

Purines versus pyrimidines.

The two types of nucleic acids are,

  1. Deoxyribonucleic acid (DNA), and
  2. Ribonucleic acid (RNA)

DNA is present in the cell nucleus and mitochondria while RNA is present in the nucleus, cytosol, and mitochondria.

Structure of Nucleic Acids


The order of nucleotides is the primary structure of DNA and RNA. In this structure, nucleotides are linked by phosphodiester bonds connecting carbon 3 on one ribose sugar to carbon 5 on the next, producing a “polarity”.

Phosphodiester bond. Image source.

The secondary structure of DNA consists of two chains of nucleotides antiparallel to each other (5′ to 3′ on one side and 3′ to 5′ on the other side). Purines always pair with a pyrimidine base via hydrogen bonds, specifically,

  • G with C
  • A with T or U
DNA Secondary structure. Image source.

The two chains are wrapped in the shape of a tertiary double helix structure with two groves, a minor and a major where proteins and drugs can interact with the functional groups on the bases that are exposed in the grooves.

Major and minor grooves of DNA helix. Image source.

Structural forms of DNA may differ based on,

  • Handedness (right or left)
  • Length of the helix turn
  • Number of base pairs per turn
  • Size of major and minor grooves
Left and right hand helix. Image source.
DNA structure showing turns and grooves. Image source.


Unlike DNA, RNA is single stranded. However it is capable of base pairing across complementary regions causing it to fold into a secondary structure. Types of secondary structure include,

  1. Hairpin loop
  2. Internal loop
  3. Helix
  4. multibranched loop
  5. Bulge loop

Secondary structure of RNA. Image source.

RNA also exists in the more highly structured 3-dimentional tertiary and quaternary forms leading to a wide range of functionality. Types of RNA include:

  • Messenger RNA (mRNA): Carries the genetic code from the DNA to ribosomes
  • Transfer RNA (tRNA): Uses information encoded in the mRNA to string the right amino acids together during protein synthesis
  • Ribosomal RNA (rRNA): Complex with protein to form ribosomal subunits which provides the space for protein production
  • Heterogenous RNA (hnRNA): An unprocessed premature RNA that contains introns
  • Small Nuclear RNA (snRNA): RNA involved in the processing of hnRNA in the nucleus
Tertiary structure of RNA. Image source.
Courtney Simons
Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University.
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