Rna Splicing
RNA Splicing is a biological reaction in which introns are removed from a transcribed RNA to create mRNA. This process occurs in conjunction with the transcription of DNA to mRNA.
Before 1977, scientists were not aware that eukaryote genes were dramatically different than prokaryotes. It was known, however, that eukaryotes had significantly more DNA than prokaryotes. This difference, called the C-value paradox, led to the discovery that eukaryotes had interrupted genes. These are genes containing exons and introns; nucleotide sequences that are both coding and non-coding. Evidence for RNA splicing was obtained when nuclear RNA was compared to mRNA. It was found that nuclear RNA was much longer than mRNA suggesting mRNA was further processed before being transported to the cytoplasm.
RNA splicing is one step in the overall process in which the genetic code is transcribed into mRNA and then translated into proteins. It is known to occur in the nucleus of the cell where DNA transcription takes place. During the process of transcription, each nucleotide from the specific gene is translated into RNA. This results in an RNA molecule that contains sequence of both the exons and the introns. Since the introns do not code for proteins, they are removed by RNA splicing.
There are several types of known splicing systems. One system involves a spliceosome which is an array of proteins that function together. The human spliceosome has been found to contain 44 different components. Another type of system involves excision of introns by the RNA itself. Still another type involves the removal of introns by tRNA.
The spliceosome system has been one of the most thoroughly studied splicing system. It is responsible for removing an intron that is located between two exons. The process begins with a set of enzymes that recognize and bind to the splice sites. These sites are located at the exon-intron boundaries and are made up of four nucleotides, two at each end of the intron. The bond between the left exon and intron is cut by enzymes in the spliceosome. This is a transesterification chemical reaction. Next, the intron is folded over on itself to the right, forming a molecular lariat structure. Another set of enzymes cuts the right end of the intron and finally joins the two exons.
The existence of a processing system for changing nuclear RNA to mRNA was surprising to scientists at first. Scientists considered it strange that a cell would waste the energy required to maintain an amount of genetic material that does not directly aid in the production of proteins. To explain this phenomena, researchers have attempted to find a function for introns. Current theories suggested that introns play a regulatory role in gene expression. Also, they may help cells produce multiple proteins from a single gene.
This is the complete article, containing 458 words
(approx. 2 pages at 300 words per page).
