RNA Definition and Examples

What Is RNA?

RNA molecule
RNA is often a single-stranded molecule.

 Christoph Burgstedt / Getty Images

RNA is the acronym for ribonucleic acid. Ribonucleic acid is a biopolymer used to code, decode, regulate, and express genes. Forms of RNA include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). RNA codes for amino acid sequences, which may be combined to form proteins. Where DNA is used, RNA acts as an intermediary, transcribing the DNA code so that it can be translated into proteins.

RNA Structure

RNA consists of nucleotides made of a ribose sugar. The carbon atoms in the sugar are numbered 1' through 5'. A purine (adenine or guanine) or pyrimidine (uracil or cytosine) is attached to the 1' carbon of the sugar. However, while RNA is transcribed using only these four bases, they are often modified to yield over 100 other bases. These include pseudouridine (Ψ), ribothymidine (T, not to be confused with the T for thymine in DNA), hypoxanthine, and inosine (I). A phosphate group attached to the 3' carbon of one ribose molecule attaches to the 5' carbon of the next ribose molecule. Because the phosphate groups on a ribonucleic acid molecule carry negative charges, RNA is also electrically charged. Hydrogen bonds form between adenine and uracil, guanine and cytosine, and also guanine and uracil. These hydrogen bonds form structural domains, such as hairpin loops, internal loops, and bulges.

Both RNA and DNA are nucleic acids, but RNA uses the monosaccharide ribose, while DNA is based on the sugar 2'-deoxyribose. Because RNA has an additional hydroxyl group on its sugar, it is more labile than DNA, with a lower hydrolysis activation energy. RNA uses the nitrogenous bases adenine, uracil, guanine, and thymine, while DNA uses adenine, thymine, guanine, and thymine. Also, RNA is often a single-stranded molecule, while DNA is a double-stranded helix. However, a ribonucleic acid molecule often contains short sections of helices that fold the molecule in upon itself. This packed structure gives RNA the capacity to serve as a catalyst in much the same way as proteins can act as enzymes. RNA often consists of shorter nucleotide strands than DNA.

Types and Functions of RNA

There are 3 main types of RNA:

  • Messenger RNA or mRNA: mRNA brings information from DNA to ribosomes, where it is translated to produce proteins for the cell. It is considered to be a coding type of RNA. Every three nucleotides forms a codon for one amino acid. When the amino acids link together and are modified post-translation, the result is a protein.
  • Transfer RNA or tRNA: tRNA is a short chain of around 80 nucleotide that transfers a newly-formed amino acid to the end of a growing polypeptide chain. A tRNA molecule has an anticodon section that recognizes amino acid codons on mRNA. There are also amino acid attachment sites on the molecule.
  • Ribosomal RNA or rRNA: rRNA is another type of RNA that is associated with ribosomes. There are four types of rRNA in humans and other eukaryotes: 5S, 5.8S, 18S, and 28S. rRNA is synthesized in the nucleolus and cytoplasm of a cell. rRNA combines with protein to form a ribosome in the cytoplasm. Ribosomes then bind mRNA and perform protein synthesis.
Flow chart of transcription and translation
mRNA, tRNA, and rRNA are associated with translation of genetic information into proteins.  FancyTapis / Getty Images

In addition to mRNA, tRNA, and rRNA, there are many other types of ribonucleic acid found within organisms. One way to categorize them is by their role in protein synthesis, DNA replication and post-transcriptional modification, gene regulation, or parasitism. Some of these other types of RNA include:

  • Transfer-messenger RNA or tmRNA: tmRNA is found in bacteria and re-starts stalled ribosomes.
  • Small nuclear RNA or snRNA: snRNA is found in eukaryotes and archaea and function in splicing.
  • Telomerase RNA Component or TERC: TERC is found in eukaryotes and functions in telomere synthesis.
  • Enhancer RNA or eRNA: eRNA is part of gene regulation.
  • Retrotransposon: Retrotransposons are a type of self-propagating parasitic RNA.

Sources

  • Barciszewski, J.; Frederic, B.; Clark, C. (1999). RNA Biochemistry and Biotechnology. Springer. ISBN 978-0-7923-5862-6. 
  • Berg, J.M.; Tymoczko, J.L.; Stryer, L. (2002). Biochemistry (5th ed.). WH Freeman and Company. ISBN 978-0-7167-4684-3.
  • Cooper, G.C.; Hausman, R.E. (2004). The Cell: A Molecular Approach (3rd ed.). Sinauer. ISBN 978-0-87893-214-6. 
  • Söll, D.; RajBhandary, U. (1995). tRNA: Structure, Biosynthesis, and Function. ASM Press. ISBN 978-1-55581-073-3. 
  • Tinoco, I.; Bustamante, C. (October 1999). "How RNA folds". Journal of Molecular Biology. 293 (2): 271–81. doi:10.1006/jmbi.1999.3001