intronless

Intronless refers to genes or genomic regions that lack introns, the noncoding sequences removed during RNA processing. An intronless gene typically consists of a single exon, so its primary transcript does not require splicing to generate mature mRNA.

Intronless genes occur across the tree of life but are especially common in organisms with compact genomes. Prokaryotes, whose genes generally have no introns, exemplify intronless architecture. In eukaryotes, a substantial minority of genes are intronless, and many functional gene families include intronless members. Among eukaryotes, intronless genes often encode components of basic cellular processes and housekeeping functions, such as ribosomal proteins, histones, and some metabolic enzymes, though they can occur in diverse functional categories.

Origins of intronless genes are varied. Some arise directly as single-exon genes in ancestral lineages. Others originate via retrotransposition, where an mRNA transcript is reverse-transcribed and inserted elsewhere in the genome, creating a processed intronless copy that may become functional if it acquires regulatory elements. Intron loss can also convert multi-exon genes into intronless versions, though this is less common than genesis through single-exon design or retrotransposition.

Functionally, intronless genes often display simpler gene structures, with shorter transcripts and reduced opportunities for alternative splicing. They may exhibit distinct regulatory characteristics, including promoter architecture and expression patterns that favor constitutive or tissue-specific expression. Intronless pseudogenes can arise from processed transcripts but typically lack regulatory compatibility for normal expression.

Overall, intronless genes illustrate the diversity of gene architecture and contribute to genome organization and evolution.