Structure of RNA
Ribonucleic acids are made up of nitrogenous bases such as adenine, Uracil, guanine and cytosine, ribose sugar and phosphate group. RNA is normally single stranded which can have a diverse form of secondary structures. tRNA shows secondary and tertiary structure.
Types of RNA
mRNA represents about 5-10% of cellular RNA. It contains the sequence of bases coding for a particular amino acid sequence in a polypeptide chain. tRNA represents about 15-20% of cellular RNA. Each tRNA molecule is specific for one amino acid. There is an enzyme for each amino acid which recognizes the amino acid and its specific tRNA and joins the two together. The specific joining of tRNA to amino acid is the only place where the genetic code is realized. rRNA represents about 70-80% of cellular RNA. It is associated with specific set of ribosomal proteins. It functions as non-specific ‘workbench’ for the assembly of polypeptides. Many copies of genes coding for rRNA are located in nucleolar organizer regions of certain chromosomes.
RNA synthesis occurs in both prokaryotes and eukaryotes. There are three steps to RNA synthesis: Initiation, elongation and termination. In the initiation step, RNA polymerase binds to gene regulatory elements. In the elongation step, RNA polymerase unwinds DNA duplex next to a gene. RNA is transcribed 5’ to 3’ from the template of 3’ to 5’. Termination in eukaryotes is by cleavage and polyadenylation.
RNA polymerase searches DNA for initial site. It unwinds a short stretch double helical DNA to produce a single stranded DNA template from which it takes instructions. Also selects the correct ribonucleotide and catalyzes the formation of phosphodiester bond and detects the termination signals which specify where a transcript ends. RNA polymerase interacts with activated and repressor proteins that modulate the rate of transcription initiation over a wide dynamic range. In prokaryotes, only one type of RNA polymerase is present. It transcribes mRNA, tRNA and rRNA. Eukaryotes possess three RNA polymerases: RNA polymerase I, II and III.
Post transcriptional controls/ RNA splicing
mRNA undergoes significant processing within the nucleus prior to transport to cytoplasm: by removal of non coding into internal sequences called introns. It undergoes modification of the 5’ base and an addition of adenine to 3’ end (poly A tail) takes place. RNA splicing is a process that removes introns and joins exons in a primary transcript.