#gene structure

Gene and Genome

Group II introns are commonly found in:

Group II introns are large self-splicing ribozymes with conserved secondary structure forming six domains radiating from central hub, excising themselves via lariat formation similar to spliceosome chemistry. They encode reverse transcriptase maturases aiding splicing and mobility. Predominantly located in mitochondria and chloroplast genomes of fungi, plants, protists and bacteria, they are considered evolutionary ancestors of spliceosomal introns due to mechanistic similarity. Nuclear genomes of higher eukaryotes lack group II introns, having replaced them with spliceosome-dependent introns.

Ref: Lambowitz and Zimmerly, Annu Rev Genetics 2004: Group II Introns in Organelle RNAs and Bacteria

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Gene and Genome

Average size of introns in eukaryotes ranges from:

Intron length varies enormously across eukaryotic genomes, reflecting balance between functional elements and splicing efficiency. Small nuclear introns in lower eukaryotes average 50-100 nucleotides, minimally containing 5' splice site, branchpoint and 3' AG. In mammals, average intron exceeds 3 kb, some spanning hundreds of kilobases such as dystrophin introns, harboring regulatory RNAs, enhancers and transposable elements. Lower size limit is dictated by spliceosome steric requirements of about 50 bases. Thus range extends from around 50 bp to several kilobases or larger, with median near 1

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Intron Size and Genome Architecture

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Gene and Genome

Which gene lacks introns?

Most vertebrate genes contain introns spliced by major U2-dependent spliceosome recognizing GU-AG boundaries. However, a small class of genes, notably many interferon family members, histone genes and some GPCR genes, lack intervening sequences, possessing continuous open reading frame from start to stop codon. Human interferon-alpha, beta and gamma genes on chromosome 9 are intronless, their mRNA sequence identical to genomic DNA except for cap and polyA addition. Absence of splicing allows rapid induction during viral infection, bypassing spliceosome assembly and enabling immediate translati

Ref: Nagata et al., Nature 1980, Human Fibroblast Interferon Gene Lacks Introns, Nucleic Acids Res.

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Gene and Genome

An interrupted gene refers to:

Eukaryotic protein-coding genes often contain non-coding intervening sequences interrupting coding potential. Philip Sharp and Richard Roberts discovered split gene organization in adenovirus and cellular genes, revealing primary transcripts contain introns removed by spliceosome. Interrupted or split gene refers to locus where exons encoding mature mRNA segments are separated by intronic DNA that is transcribed but excised. This architecture permits alternative splicing, exon shuffling and regulatory intronic elements. Bacterial genes generally lack introns, making split genes hallmark of nuc

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 7: Split Genes and Intervening Sequences, Sharp and Roberts

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