A gene is often defined as a sequence of DNA nucleotides that is transcribed into a single RNA molecule.
Introns—noncoding sequences that interrupt the coding sequences (exons) of genes—are common in eukaryotic cells but rare in bacterial cells.
An mRNA molecule has three primary parts: a 5′ untranslated region, a protein-coding sequence, and a 3′ untranslated region.
Bacterial mRNA is translated immediately after transcription and undergoes little processing. The pre-mRNA of a eukaryotic protein-encoding gene is extensively processed: a modified nucleotide and methyl group, collectively termed the cap, are added to the 5′ end of pre-mRNA; the 3′ end is cleaved and a poly(A) tail is added; and introns are removed. Introns are removed within a structure called the spliceosome, which is composed of several small nuclear RNAs and proteins.
Some introns found in rRNA genes and mitochondrial genes are self-splicing.
Some pre-mRNAs undergo alternative splicing, in which different combinations of exons are spliced together or different 3′ cleavage sites are used.
Messenger RNAs may be altered by the addition, deletion, or modification of nucleotides in the coding sequence, a process called RNA editing.
Transfer RNAs, which attach to amino acids, are short molecules that assume a common secondary structure and contain modified bases.
Ribosomes, the sites of protein synthesis, are composed of several ribosomal RNA molecules and numerous proteins.
Small interfering RNAs, microRNAs, Piwi-interacting RNAs, and CRISPR RNAs play important roles in gene silencing and in a number of other biological processes.
Long noncoding RNAs are RNA molecules that do not encode proteins. Evidence increasingly suggests that many of these molecules function in the control of gene expression.