Transcription (Part-II)

Summary of Previous Lecture: In previous lecture we discussed about structure of gene which includes promoter, coding and terminator region. We also illustrated the differences between Transcription in Prokaryotes and Eukaryotes system. There was elaborate description of transcription in prokaryotes and eukaryotes. However, transcription in general is divided in 3 steps:     1) Initiation    2) Elongation and (3) Termination. In this lecture we will discuss about post-transcriptional modification.

POST-TRANSCRIPTIONAL MODIFICATION: In prokaryotes, transcription and translation both processes are carried out simultaneously. The RNA copy of a gene is m- RNA, and it is ready for translation into protein.  In fact, translation starts even before transcription is finished. In eukaryotes, the primary RNA transcript of a gene are further processed prior to its translation into protein.  This step is called “RNA processing”. Also, Post RNA processing, it needs to be transported out of the nucleus into the cytoplasm through the nuclear pores. Many of the RNA molecules in bacteria and virtually all molecules in eukaryotes are processed to some degree after synthesis as a newly synthesised RNA molecule is immature and is called primary transcript. Most elaborated form of processing occurs for all eukaryotic mRNAs and tRNAs of both bacteria and eukaryotes.

A eukaryotic gene has several non-coding region (introns) interspersed with coding region (exons). When an m-RNA is transcribed, primary transcript has both introns and exons. Introns are removed from primary transcript via a process called splicing. In splicing, introns are removed and exons are joined together in a continuous sequence forming mature m-RNA transcript ready to be transported out of the nucleus and translated. Apart from splicing, eukaryotic m-RNA, also undergoes 5’ capping and polyadenylation at 3’ end.All three processes of splicing, capping and adenylation occurs inside nucleus. Elaborate protein complexes are involved in these three RNA processing which act in association with each other and with phosphorylated CTD of Pol II. The primary transcript of bacterial or eukaryotic tRNAs are processed by either cleavage which removal of bases from each end or in some cases splicing. Furthermore, many unusual bases, not found in any nucleic acid are also added to tRNA.