Question

Describe splicing and the role of RNA as a “ribozyme” in the process. Be sure to mention the proteins and RNAs involved.

Answer 1

Splicing — RNA transcription is a continuous process, yet the DNA sequence that codes for protein (exons) is often interrupted by intervening non-coding sequences (introns). These intron segments must be removed or spliced out of the RNA prior to translation.

The splicing process can lead to production of several different proteins from one gene. The following factors are involved:

●Spliceosomes are enzymatic ribonucleoprotein complexes that remove introns from the primary RNA transcript (pre-mRNA) to produce mature mRNA.

●Intron boundaries are marked by conserved splice donor (end of an exon/start of an intron) and splice acceptor (end of an intron/start of an exon) sites. These splice sites provide sequence recognition sites for the spliceosomes. Mutations that alter splice sites can impair normal splicing and can cause disease.

●Tissue-specific splicing is regulated by the binding of enhancers and suppressors to DNA. Differential exon splicing leads to formation of related but unique mRNA sequences that encode different proteins, known as protein isoforms.

●Differential splicing of coding exons results in differences in the ultimate protein structure. Hence, genes often encode more than one protein

A ribozyme is a ribonucleic acid (RNA) enzyme that catalyzes a chemical reaction. The ribozyme catalyses specific reactions in a similar way to that of protein enzymes.

Also called catalytic RNA, ribozymes are found in the ribosome where they join amino acids together to form protein chains. Ribozymes also play a role in other vital reactions such as RNA splicing, transfer RNA biosynthesis, and viral replication.

The first ribozyme was discovered in the early 1980s and led to researchers demonstrating that RNA functions both as a genetic material and as a biological catalyst. This contributed to the worldwide hypothesis that RNA may have played a crucial role in the evolution of self-replicating systems. This is referred to as the RNA World Hypothesis and today, many scientists believe that ribozymes are remnants of an ancient world that existed before the evolution of proteins. It is thought that RNAs used to catalyse functions such as cleavage, replication and RNA molecule ligation before proteins evolved and took over these catalytic functions, which they could perform in a more efficient and versatile way.