Everything about RNA — Part 1

RNA, Everything about it.

Introduction
Synthesis of an mRNA molecule Part 1
Synthesis of an mRNA molecule Part 2
Termination of mRNA synthesis

The synthesis of a single strand of mRNA

The synthesis of RNA requires a DNA coding for it. DNA-directed RNA polymerase was discovered by 1959. This enzyme has clear cut prerequisites for functioning.
>Ribonucleotide triphosphates i.e ATP, GTP, UTP, CTP
>The template DNA strand
>Mg²⁺, Zn²⁺

The overall reaction is
(NMP)_n + NTP---->(NMP)_n+1 +PPi

RNA synthesis is very much similar to DNA synthesis in many aspects. The process is very efficient when double stranded DNA molecules are provided.In E.coli, the double stranded DNA is unwound and 17 base pairs are kept so at any given instant. The RNA polymerase acts at the unwound portion of the DNA, the so-called 'transcription bubble' and adds nucleotides complimentary to those found on the template strand. No primer sequence is needed for this operation. As the mRNA strand elongates, the transcription bubble also moves ahead freeing up more nucleotides for transcription. The unwinding of the DNA helix is carried out by the RNA polymerase itself.

But this action of the enzyme creates a wave of positive supercoils ahead of the transcriptionary activity site and a wave of negative supercoils behind it. All these coils are returned to the original state after transcription by topoisomerases.

Transcription proceeds at the rate of 50 nucleotides per second in prokaryotes(!).

Name of the enzyme	DNA-directed RNA polymerase
No of subunits	5+1
Proof-reading exonuclease activity	Absent
Error rate	one in about 10,000 to 1,00,000 bases
Molecular weight	390,000 Da.

All data mentioned above are for the E.coli RNA polymerase.