3.4 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. 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. We will be going into more detail about nucleic acids in the next section.

DNA and RNA are made up of monomers known as nucleotides. The nucleotides combine with each other to form a chain, which is known as DNA or RNA. Each nucleotide is made up of three components: a nitrogenous base, five-carbon sugar, and a phosphate group (Figure 7). The nitrogenous base in one nucleotide is attached to the sugar molecule, which is attached to the phosphate group.

the three parts of a nucleotide
Figure 7: A nucleotide is made up of three components: a nitrogenous base, a pentose sugar, and a phosphate group.

DNA Double-Helical Structure

DNA has a double-helical structure (Figure 8). It is composed of two strands, or chains, of nucleotides. The double helix of DNA is often compared to a twisted ladder. The strands (the outside parts of the ladder) are formed by linking the phosphates and sugars of adjacent nucleotides with strong chemical bonds, called covalent bonds. The rungs of the twisted ladder are made up of the two bases attached together with a weak chemical bond, called a hydrogen bonds. Two bases hydrogen bonded together is called a base pair. The ladder twists along its length, hence the “double helix” description, which means a double spiral.

picture of double helix
Figure 8: The double-helix model shows DNA as two parallel strands of intertwining molecules. (credit: Jerome Walker, Dennis Myts).

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 basespair; 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.


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Mt Hood Community College Biology 102 by Lisa Bartee is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.


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