Genes that make mRNA have exon regions for translation and intron regions not for translation.
introns
RNAs typically have several introns. Introns came from bacteria or, if protein folding requires them, have always been in genes. After transcription, mechanisms {RNA splicing}| splice introns out.
exons
Exons typically are functional domains, and proteins have different functional domains. Exons can mix in different ways to make membrane-bound or secretable proteins or to make proteins for different development stages or different tissues.
process
After mRNA leaves cell nucleus, cell processes splice out introns and join exons {mRNA splicing}, to make mRNA for translation. Introns have 5' sequences and 3' sequences.
Large ribonucleotide and protein particles {spliceosome} perform splicing. Spliceosomes have U1 and U2 small nuclear ribonucleoproteins, U1 and U2 small nuclear ribonucleic acids, and SF2, U2AF, and other proteins. Intron 5' ends split first. Intermediate RNAs have lariat shapes, because introns bind to themselves with 2'-5' bonds. Enzymes cut 3' ends, and other enzymes join exon ends. Introns leave as lariats, because introns bind to themselves with 2'-5' bonds to make circles.
Proteins {splicing regulatory protein} (SR protein) can determine which exons to keep, by binding to exonic splicing enhancer (ESE) or exonic splicing suppressor (ESS). SR protein is for fruitfly sex determination.
Protein actions can block RNA 5' sites after transcription, allowing only other splicing sites {alternative splicing}. Alternative splicing results in different-size and different-function proteins.
RNAs can splice pieces together, using mechanisms {self-splicing RNA} different than spliceosomes.
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Date Modified: 2022.0225