Dissociates from dna lecture 5 transcription

dissociates from DNA
Lecture 5
- Transcription initiation
o RNA polymerase (big) binds to promoter sequence in duplex DNA; start site near 3’ end, stop
site further along; this is called the CLOSED COMPLEX because DNA is not yet denatured
 Eukaryotic RNA polymerases require associated proteins called general transcription
factors to find promoters and initiate transcription
o Polymerase melts duplex DNA near transcription start site, forming a transcription bubble that is
about 14 base pairs, allows rNTPs to begin RNA synthesis; OPEN complex
o Polymerase catalyzes phoshosphodiester linkage of 2 initial rNTPs
- Transcription elongation
o Polymerase advances 3’  5’ down template strand, melting duplex DNA and adding rNTPs to
growing RNA; within bubble there is DNA-RNA hybrid region; nascent RNA, the chain being
synthesized, comes off
o Multiple polymerase molecules can transcribe the same template DNA strand at the same time
o Elongation complex is very stable; polymerase does not fall off until it reaches the stop site

o Speed of elongation around 1000 nt/min, so small genes are transcribed in a few minutes but big
genes (with big introns) can take hours
- Transcription termination
o At transcription stop site, polymerase releases completed RNA and dissociates from DNA
o Primary transcript: completed RNA molecule (mRNA not directly synthesized from DNA)
o A specific sequence in the template DNA signals the bound RNA polymerase to terminate
transcription
- Image 1: E. coli RNA polymerase has 5 subunits; in diagram, DNA bends sharply upon entering the
enzyme; inflection site is around the transcription start site, favors local denaturation
- Organization of genes is different in prokaryotes and eukaryotes
o Prokaryotic genome is very compact; genes with a common function are often arranged linearly
in operons & transcribed together on a single mRNA; there are very few non-coding gaps of
DNA in prokaryotic genomes (no introns, no RNA processing)
o Eukaryotic genome such as in yeast have genes scattered on several chromosomes; that means
that coregulation is not achieved simply by physical linkage
o In prokaryotes, mRNAs are directly transcribed from DNA; but in eukaryotes, transcripts must
go through several processing steps before becoming mRNAs
- RNA processing
o As the 5’ end of a nascent RNA chain emerges from RNA polymerase, the 5’ cap structure ( 7-
methyl-G ) is added to it by several enzymes (5’ to 5’ linkage)
o Polyadenylation: addition of 100 to 250 A residues by poly(A) polymerase on 3’ end of mRNA
enzymatically
o Intron excision, exon ligation (first exon will always include 5’ UTR, last exon will always
include 3’ UTR)
o mRNAs retain untranslated regions (UTRs) at the 5’ and 3’ ends but they do not encode for
proteins; UTRs contain elements that regulate translation of mRNA & recruit ribosome to RNA
o open reading frame: part of the RNA that encodes for the protein
o the same primary transcript can be alternatively spliced in different tissues; liver cells remove