Life on Earth is categorized into two fundamental groups: prokaryotes and eukaryotes. Both rely on genes, segments of DNA, to carry instructions for their existence. Genes contain the blueprint for building proteins or functional RNA molecules. A central question is whether prokaryotes, like eukaryotes, possess specific gene segments known as introns and exons.
What Are Introns and Exons?
Genes are not always continuous stretches of coding information. Many genes contain exons, which are coding regions with instructions for building proteins or functional RNA. Interspersed between exons are introns, which are non-coding regions.
When a gene with introns and exons is transcribed into an RNA molecule, this primary transcript includes both. Before this RNA can be used to make a protein, introns must be removed. This ensures only coding information from exons remains, forming a mature, functional RNA molecule.
Gene Organization in Eukaryotes
Genes in eukaryotes, including animals, plants, fungi, and protists, typically have a fragmented structure. They are composed of multiple exons interrupted by introns. Intron lengths can vary significantly, often being much longer than exons.
After a eukaryotic gene is transcribed into a precursor messenger RNA (pre-mRNA), RNA splicing occurs. During splicing, cellular machinery excises intron sequences from the pre-mRNA. The remaining exon sequences are then joined, forming a continuous, mature messenger RNA (mRNA) molecule. This processed mRNA carries protein-coding instructions to ribosomes for translation.
Gene Organization in Prokaryotes
In contrast to eukaryotes, the genetic organization of prokaryotes, such as bacteria and archaea, is generally more streamlined. Prokaryotic genes typically do not contain introns or exons. Their genes are usually compact and continuous, meaning the entire sequence directly codes for a protein or functional RNA.
This continuous structure allows for an efficient process known as coupled transcription-translation. As DNA is transcribed into mRNA, ribosomes can immediately begin translating the mRNA into protein, often before transcription is complete. Many prokaryotic genes are also organized into operons, which are clusters of genes with related functions transcribed together as a single, long mRNA molecule. This strategy allows prokaryotes to coordinate the expression of multiple genes for specific metabolic pathways or cellular processes.
Rare Intron-like Elements in Prokaryotes
While prokaryotes generally lack introns, rare exceptions exist where intron-like elements have been identified. These elements differ from eukaryotic spliceosomal introns and use different removal mechanisms. Some prokaryotes contain self-splicing introns, like Group I and Group II introns, which are RNA molecules that can catalyze their own removal without protein machinery.
These have been found in specific genes encoding proteins, transfer RNA, and ribosomal RNA across various prokaryotic lineages. Another intron-like element in some prokaryotes are inteins, protein segments that can excise themselves from a larger precursor protein. These rare occurrences highlight genetic diversity but do not negate the fundamental difference in gene organization between most prokaryotic and eukaryotic organisms.