Transcription and RNA Modification

promoter

promoter

signals beginning of transcription

regulatory sequence

site for the binding of regulatory proteins; influence rate of transcription

transcribed region

part of this region contains the information that specifies an amino acid sequence

terminator

signals the end of transcription

3 stages of transcription

1. intitiation

2. elongation

3. termination

TATA box

portion of the promotor region about 25 nucleotides upstream of the start site (eukaryotes)

sigma factor (bacteria)

part of RNA polymerase that recognizes and facilitates binding to the promoter

initiation

transcription factors meditate the binding of RNA polymerase and initiation of transcription

completed transcription initiation complex

transcription factors and RNA polymerase

Elongation

RNA polymerase moves along the DNA strand

• untwists double helix about 20 bases at a time

what direction is RNA made?

5’ to 3’

how are the RNA strands made

reading the template strand and adding in complementary RNA nucleotides

termination

end of transcription; signaled by terminator in bacteria

termination in bacteria

terminator sequence signals the end of transcription

termination in eukaryotes

polyadenylation sequence; RNA transcript released 10-35 nucleotides past sequence

directionality of transcription

• DNA template read 3’ to 5’

• RNA transcript synthesized 5’ to 3’

RNA polymerase I

transcribes rRNA

RNA polymerase II

transcribes mRNA

RNA polymerase III

transcribes tRNA

prokaryotic transcription to translation

immediate; mRNA begins to be translated BEFORE transcription is done

eukaryotic transcription and translation timing

occurs in separate compartments

where does transcription occur in eukaryotes?

nucleus

where does translation occur in eukaryotes?

cytoplasm

pre-mRNA

product of transcription; longer RNA

mature mRNA

mRNA after undergoing modifications before exiting nucleus

3 common mRNA modifications

capping, Poly A tail, splicing

capping

modified guanosine added to 5’ end; added after first 20-25 nucleotides

5’ cap functions

• needed to exit nucleus

• prevents degradation

• enables mRNA to bind to ribosome

Poly A Tail

100-200 adenine nucleotides added to 3’ end

how is the Poly A Tail added?

enzymatically by poly-A polymerase (NOT encoded by gene)

Poly A tail function

increases stability and lifespan in cytosol

shared functions of capping and Poly A Tail

• facilitating export of mRNA into cytoplasm

• protecting mRNA from hydrolytic enzymes

splicing

removal of introns, linking of exons

introns

transcribed but NOT translated, non-coding sequences

exon

encoding sequence found in mature mRNA; EXpressed

spliceosome

composed of snRNP (small nuclear RNA and set of proteins); removes introns from pre-mRNA

alternative splicing

creates different proteins from same gene