RNA molecules perform a variety of functions in the cell. Ribosomal RNA (rRNA), along with ribosomal protein subunits, makes up the ribosome, the site of protein assembly. We’ll take a more detailed look at the ribosome later in the chapter. Messenger RNA (mRNA) carries the coding instructions for polypeptide chains from DNA to the ribosome. After attaching to a ribosome, an mRNA molecule specifies the sequence of the amino acids in a polypeptide chain and provides a template for the joining of those amino acids. Large precursor molecules, which are termed pre-messenger RNAs (pre-mRNAs), are the immediate products of transcription in eukaryotic cells. Pre-mRNAs (also called primary transcripts) are modified extensively before becoming mRNA and exiting the nucleus for translation into protein. Bacterial cells do not possess pre-mRNA; in these cells, transcription takes place concurrently with translation.

Transfer RNA (tRNA) serves as the link between the coding sequence of nucleotides in an mRNA molecule and the amino acid sequence of a polypeptide chain. Each tRNA attaches to one particular type of amino acid and helps to incorporate that amino acid into a polypeptide chain.

Additional classes of RNA molecules are found in the nuclei of eukaryotic cells. Small nuclear RNAs (snRNAs) combine with small protein subunits to form small nuclear ribonucleoproteins (snRNPs, affectionately known as “snurps”). Some snRNAs participate in the processing of RNA, converting pre-mRNA into mRNA. Small nucleolar RNAs (snoRNAs) take part in the processing of rRNA. Two types of very small and abundant RNA molecules found in the cytoplasm of eukaryotic cells, termed microRNAs (miRNAs) and small interfering RNAs (siRNAs), carry out RNA interference (RNAi), a process in which these small RNA molecules help trigger the degradation of mRNA or inhibit its translation into protein. Another class of small RNA molecules, called Piwi-interacting RNAs (piRNAs; named after Piwi proteins, with which they interact), has a role in suppressing the expression of transposable elements (see Chapter 13) in reproductive cells. Long noncoding RNAs (lncRNAs) are relatively long RNA molecules found in eukaryotes that do not code for proteins. They provide a variety of functions, including regulation of gene expression. Recently, an RNA interference–like system has been discovered in prokaryotes, in which small CRISPR RNAs (crRNAs) assist in the destruction of foreign DNA molecules. Some of the different classes of RNA molecules are summarized in Table 10.2.

CONCEPTS