Nucleic Acids

What you’ll learn to do: Discuss nucleic acids and the role they play in DNA and RNA

Humans have two types of nucleic acids in their bodies: DNA and RNA. These molecules contain the set of instructions for our cells: they determine who and what we are. But what makes up our DNA?

A strand of RNA next to DNA. RNA is a single helix composed of cytosine, guanine, adenine, and uracil. DNA is a double helix composed of cytosine, guanine, adenine, and thymine.

Figure 1. Spot the differences between DNA and RNA

In this outcome, we’ll learn about the components of DNA and RNA and get a brief introduction to how they work.

Learning Outcomes

  • Describe the basic structure of nucleic acids
  • Compare and contrast the structure of DNA and RNA

Structure of Nucleic Acids

Nucleic acids are key macromolecules in the continuity of life. They carry the genetic blueprint of a cell and carry instructions for the functioning of the cell.

Structure of a nucleotide.

Figure 2. A nucleotide is made up of three components: a nitrogenous base, a pentose sugar, and a phosphate group.

The two main types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is the genetic material found in all living organisms, ranging from single-celled bacteria to multicellular mammals.

The other type of nucleic acid, RNA, is mostly involved in protein synthesis. The DNA molecules never leave the nucleus, but instead use an RNA intermediary to communicate with the rest of the cell. Other types of RNA are also involved in protein synthesis and its regulation.

DNA and RNA are made up of monomers known as nucleotides. The nucleotides combine with each other to form a polynucleotide, DNA or RNA. Each nucleotide is made up of three components: a nitrogenous base, a pentose (five-carbon) sugar, and a phosphate group (Figure 2). Each nitrogenous base in a nucleotide is attached to a sugar molecule, which is attached to a phosphate group. The nucleotides link together by phosphodiester bonds to form the polynucleotide.

DNA Double-Helical Structure

Double helix of DNA.

Figure 3. The double-helix model shows DNA as two parallel strands of intertwining molecules. (credit: Jerome Walker, Dennis Myts)

DNA has a double-helical structure (Figure 3). It is composed of two strands, or polymers, of nucleotides. The strands are formed with covalent bonds between phosphate and sugar groups of adjacent nucleotides.

The two strands are bonded to each other at their bases with hydrogen bonds, and the strands coil about each other along their length, hence the “double helix” description, which means a double spiral.

The alternating sugar and phosphate groups lie on the outside of each strand, forming the backbone of the DNA. The nitrogenous bases are stacked in the interior, like the steps of a staircase, and these bases pair; the pairs are bound to each other by hydrogen bonds. The bases pair in such a way that the distance between the backbones of the two strands is the same all along the molecule.

DNA and RNA

While DNA and RNA are similar, they have very distinct differences. Table 1 summarizes features of DNA and RNA.

Table 1. Features of DNA and RNA
DNA RNA
Function Carries genetic information Involved in protein synthesis
Location Remains in the nucleus Leaves the nucleus
Structure DNA is double-stranded “ladder”: sugar-phosphate backbone, with base rungs. Usually single-stranded
Sugar Deoxyribose Ribose
Pyrimidines Cytosine, thymine Cytosine, uracil
Purines Adenine, guanine Adenine, guanine

One other difference bears mention. There is only one type of DNA. DNA is the heritable information that is passed along to each generation of cells; its strands can be “unzipped” with small amount of energy when DNA needs to replicate, and DNA is transcribed into RNA. There are mutliple types of RNA: Messenger RNA is a temporary molecule that transports the information necessary to make a protein from the nucleus (where the DNA remains) to the cytoplasm, where the ribosomes are. Other kinds of RNA include ribosomal RNA (rRNA), transfer RNA (tRNA), small nuclear RNA (snRNA), and microRNA.

Even though the RNA is single stranded, most RNA types show extensive intramolecular base pairing between complementary sequences, creating a predictable three-dimensional structure essential for their function.

As you will learn later, information flow in an organism takes place from DNA to RNA to protein. DNA dictates the structure of mRNA in a process known as transcription, and RNA dictates the structure of protein in a process known as translation. This is known as the Central Dogma of Life, which holds true for all organisms; however, exceptions to the rule occur in connection with viral infections.

In Summary: DNA and RNA

Nucleic acids are molecules made up of nucleotides that direct cellular activities such as cell division and protein synthesis. Each nucleotide is made up of a pentose sugar, a nitrogenous base, and a phosphate group. There are two types of nucleic acids: DNA and RNA. DNA carries the genetic blueprint of the cell and is passed on from parents to offspring (in the form of chromosomes). It has a double-helical structure with the two strands running in opposite directions, connected by hydrogen bonds, and complementary to each other. RNA is single-stranded and is made of a pentose sugar (ribose), a nitrogenous base, and a phosphate group. RNA is involved in protein synthesis and its regulation. Messenger RNA (mRNA) is copied from the DNA, is exported from the nucleus to the cytoplasm, and contains information for the construction of proteins. Ribosomal RNA (rRNA) is a part of the ribosomes at the site of protein synthesis, whereas transfer RNA (tRNA) carries the amino acid to the site of protein synthesis. microRNA regulates the use of mRNA for protein synthesis.

Check Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.

Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.