Page 13 - CH6 (1)

Complexities Associated with Transcription

● In prokaryotes:

○ There is no clear demarcation between cytosol and nucleus.
Therefore, translation can begin even before transcription is

completed. Thus, in prokaryotes, transcription and translation
are coupled.
● In eukaryotes:

○ Three different kinds of RNA polymerases are present.
RNA polymerase I transcribes rRNA.

RNA polymerase II transcribes hnRNA (mRNA precursor).
RNA polymerase III transcribes tRNA, snRNA, and srRNA.

○ The precursor of mRNA, i.e. hnRNA, contains both introns and
exons. Introns are removed and exons are joined by a process

called splicing.
○ Capping − In this, methyl guanosine triphosphate is added to
the 5′ end of hnRNA.

○ Tailing − In this, adenylate residues are added to the 3′ end of
hnRNA.

○ When hnRNA is fully processed, it is known as mRNA, which is