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
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
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.
