Chapter 11: Transcription & RNA Processing

mRNA

tRNA

rRNA

3 main classes of RNAs produced in both pro and eukaryotic ces

tRNA

transfer RNA

adapter between codons and amino acids

decode mRNA sequences in a protein

each codon has its own tRNA

mRNA

messenger RNA

carry genetic information from DNA to ribosomes to make proteins

rRNA

ribosomal RNA

translate mRNAs into amino acids

essential for protein synthesis

1. addition of 7-methly guanosine caps at 5' ends of primary transcripts

2. addition of poly-A tail to 3' end of transcripts, generated by cleavage, not termination

3. intron sequences spliced out of transcripts

modifications to eukaryotic pre-mRNAs before transport to cytoplasm

promoter for gene in prokaryotes

sigma factor binds at promoter region to facilitate the unwinding of DNA an thus synthesis of new RNA chain

sigma factor in RNA polymerase

anchors polymerase to promoter region TATA box

only involved in transcription initiation

rho-independent termination

GC rich region, then 6+ AT bps, A in DNA template strand, T in RNA coding strand

rho-independent termination

GC region contains inverted complementary repeats

when transcribed, produced single stranded RNA sequence that base pairs to form hairpin structure, run of Us on RNA strand facilitates release of new RNA chain from template because of weak AU bonds

polymerase II promoter in eukaryotes

transcription factors bind to promoter region to form initiation complex

polymerase II promoter in eukaryotes

promoters recognized by RNA polymerase II consist of short conserved elements located upstream from the transcription start point

polymerase II promoter in eukaryotes

-80 CAAT box: GGCCAATCT; -30 TATA bo: TATAAAA

GC box, octamer box (variable in number and location)

RNA splicing

removing intron sequences in primary RNA transcripts and exons are joined to form a mature mRNA, must join exons with accuracy of single nucleotide

RNA splicing

controlled by dinucleotide sequences at 5' and 3' ends of introns; TACTIC box about 30 its upstream from 3' splice site

spliceosome

responsible for splicing introns in nuclear pre-mRNA

autocatalytic cleavage of rRNA precursor

splicing carried out by rRNA precursor itself

doesn't involve any protein enzymes

no ATP required

transfers phosphodiester bonds

RNA is catalyst

requires guanosine with free 3'-OH group as cofactor