Question

Splicing of precursor messenger RNA (pre-mRNA), which involves removal of non-coding introns and the ligation of neighboring exons, occurs in all eukaryotes. Removal of the introns is carried out by the spliceosome. a) What nucleotides are almost always found at the 5’ and 3’ ends of an intron? b) Introns contain a “polypyrimidine tract” close to their 3’ end - what are the characteristics and function of this region?

Answer 1

A) GU dinucleotide is found at 5`end and dinucleotide AG is found at 3`end of introns. Upstream of AG there is a region rich in pyrimidines(C and U) also known as polypyrimidine tract. Upstream of polypyrimidine tract there is a branch point consisting of Adenine nucleotides which are involved in lariat formation.

B) Polypyrimidine tract is a region of pre messenger RNA , cis acting which is rich in pyrimidines nucleotides especially U(uracil) and is 15-20 bp long ,located at 5-60 bp upstream of 3`end of intron to be spliced.

A number of proteins which carry out the RNA splicing bind to polypyrimidine tract.