RNA Splicing

In the initial stage, RNA transcript introns are synthesized which are removed later on by a process called RNA splicing ( refer picture below). The junctions of intron-exon have a GU sequences at the intron’s 5’-end, and an AG sequence at its 3’OH end. These two sequences are recognized by the special RNA molecule known as small nuclear RNA (snRNA) or snurps (Steitz, 1988).

These together with proteins form small nuclear ribonucleoprotein particles called snRNPs. Some of the snRNPs recognize the splice junction, and splice introns accurately. For example, the UI-snRNP recognizes the 5’-splicing junction, and the U5 snRNP recognizes the 3’ splicing junction. Consequently pre-mRNA is spliced in a large complex called a spliceosome (Guthrie, 1991). The spliceosome consists of pre-mRNA, five types of snRNPs and non-snRNP splicing factors (Rosbash and Seraphin,1991).

Robert and Sharp, the Nobel prize winners in 1993, independently hybridized the mRNA of adenovirus with their progeny of DNA segments of virus. The mRNAs hybridized the ssDNA of virus where the complementary sequences were present. The mRNA-DNA complexes were observed under electon microscope to confirm which part of viral genome had produced the mRNA strand. It was found that mRNA did not hybridize DNA linearly but showed a discontinuous complexes pattern. Huge loops of unpaired DNA between the hybridized complexes clearly revealed the large chunk of DNA strand that carried no genetic information and did not take part in protein synthesis. The adenovirus mRNA contained four different regions of the DNA.

The B-globin genes of mice and rabbits, and tRNA genes of yeast tyrosine-tRNA consists of eight genes. Each genes contains 14 bases (ATTT-AYCAC-TACGA) as intron in the middle. In the same way the pre-tRNA genes contain introns of 18-19 bases. In all the genes introns are present near anticodon. Similarly, a few rRNA genes are also known to contain introns and some of pre-rRNA are self splicing.