NCBI's Gene resources include collections of curated nucleotide sequences used as references, sequence clusters to predict and study homologs, and various databases and tools for the study of gene expression.

Most eukaryotic genes contain segments of coding sequences (exons) interrupted by noncoding sequences (introns). Both exons and introns are transcribed to yield a long primary RNA transcript. The introns are then removed (more...)

The traditional view has been that an entire prokaryotic genome is contained in a single circular DNA molecule. As well as this single 'chromosome', prokaryotes may also have additional genes on independent smaller, circular or linear DNA molecules called plasmids (Figure 2.3).