Introns and Gene Expression Regulation

Interrupted Genes in Eukaryotes: Exons and Introns

• Eukaryotic genes contain noncoding sequences called introns that interrupt the coding sequences, or exons.
• Introns are excised from RNA transcripts prior to their transport to the cytoplasm.
• Noncoding sequences intervening between coding sequences are called introns (for intervening sequences).
• The sequences that remain present in mature mRNA molecules (both coding and noncoding sequences) are called exons (for expressed sequences).

Properties of Introns

• Introns vary in size, ranging from about 50 nucleotide pairs to thousands of nucleotide pairs in length.
• Some introns contain sequences that can regulate gene expression in either a positive or negative fashion.
• Introns may provide a selective advantage by increasing the rate at which coding sequences in different exons of a gene can reassort by recombination, thus speeding up the process of evolution.

Removal of Intron Sequences by RNA Splicing

• The noncoding introns are excised from gene transcripts by several different mechanisms.
• The splicing mechanism must be precise; it must join exon sequences with accuracy to the single nucleotide to assure that codons in exons distal to introns are read correctly.
• Splicing and intron sequences can influence gene expression.
• Mutations at these sites are sometimes responsible for inherited diseases in humans, such as hemoglobin disorders.

Three Types of Intron Excision

• The introns of tRNA precursors are excised by precise endonucleolytic cleavage and ligation reactions catalyzed by special splicing endonuclease and ligase activities.
• The introns of some rRNA precursors are removed autocatalytically in a unique reaction mediated by the RNA molecule itself.
• RNA chain elongation and the addition of 5-methyl guanosine caps occur by the same mechanism as the RNA polymerases of prokaryotes.

Termination by Chain Cleavage and the Addition of 3 Poly(A) Tails

• The 3’ ends of RNA transcripts synthesized by RNA polymerase II are produced by endonucleolytic cleavage of the primary transcripts rather than by the termination of transcription.
• After cleavage, the enzyme poly(A) polymerase adds poly(A) tails, tracts of adenosine monophosphate residues about 200 nucleotides long, to the 3’ ends of the transcripts.

Key Points

• Three to five different RNA polymerases are present in eukaryotes, and each polymerase transcribes a distinct set of genes.
• Eukaryotic gene transcripts usually undergo three major modifications: (1) the addition of 7-methyl guanosine caps to 5’ termini, (2) the addition of poly(A) tails to 3’ ends, and (3) the excision of noncoding intron sequences.
• The information content of some eukaryotic transcripts is altered by RNA editing, which changes the nucleotide sequences of transcripts prior to their translation.