Reading: 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 have learned, 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.

Section Summary

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 ribosmes 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.

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