Concepts Summary

• 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.