Transcription

Transcription is the copying of a sequence of DNA to produce a complementary strand of RNA

Act I: Initiation

• Transcription factors recognize promoter regions (TATA box) on the template and bind to the promoters.
• This signals RNA polymerase II to bind to transcription factors. Together they form a transcription initiation complex.
• Polymerase II starts transcription.

Act II: Elongation

• RNA polymerase unwinds DNA double helix and adds RNA nucleotides to the 3' end of the growing strand. RNA, same as DNA, grows from 5' to 3'.
• When adding the complimentary nucleotides to the RNA transcript, every thymine is replaced by uracil.
• As RNA polymerase II moves forward, the double helix behind re-forms, and the newly transcribed RNA molecule, RNA transcript, peels away.
• The strand that RNA reads is called the template strand/ antisense strand.
• The strand that has exactly same sequence as RNA is called coding strand/ sense strand.
• A single gene can be transcribed by multiple polymerase simultaneously.

Act III: Termination

• Transcription stops when RNA polymerase transcribes as a terminator AAUAA.
• The pre-mRNA is cut, and RNA polymerase II is released form the DNA.
• In eukaryotic cells, before the mRNA enter the cytoplasm, enzymes work on modifying them.
• G-cap (modified guanine) is assed to the 5' end of pre-mRNA.
• PolyA-tail is added to the 3' end of pre-mRNA.

• Pre-mRNA combines with snRNPs and other proteins to form a spliceosome.
• The snRNA in the snRNPs base pair with nucleotides at the end of the noncoding segments on the pre-mRNA, introns.
• Introns are excised from the pre-mRNA, then the coding regions, exons, are spliced together in the spliceosome.

• The snRNA acts as a ribozyme, and the RNA acts as an enzyme.
• The spliceosome comes apart, releasing mRNA.