Transcribing DNA to RNA and Regulation

transcription

process of converting DNA to RNA

where does transcription occur in eukaryotes

nucleus

where does transcription occur in prokaryotes

cytoplasm

transcriptional enzyme

RNA polymerase

Transcription steps

1. Initiation

2. Elongation

3. termination

initaiton

binding of the RNA polymerase to the promoter and pries strand apart

Elongation

• RNA chain grows in 5' direction

• begins with formation of the first phosphodiester bond

Antisense strand

• negative

• template strand

Sense strand

• positive

Termination

• stops phosphodiester bond formation

• stops at hairpin RNA

• RNA polymerase releases DNA

RNA polymerase characteristics

• active center has Mg ions in a pair

• binds to promoter as holoenzyme

• has absolute requirement for divalent metal ions

RNA polymerase in eukaryotes

RNA poly 1, RNA poly 2, RNA poly 3

how does RNA polymerase 1 react to a-amanitin

insensitive to a-amanitin

how does RNA polymerase 2 react to a-amanitin

inhibited by a-amanitin

how does RNA polymerase 3 react to a-amanitin

inhibited by high concentrations of a-amanitin

Type I eukaryotic promoter

ribosomal initiator element (rInr) + upstream promoter element (UPE)

Type II eukaryotic promoter

contains enhancer element + ribosomal initiator element (rInr) + consensus sequence (TATA) or downstream promoter element (DPE)

Type III eukaryotic promoter

• Type I in 5S rRNA with short conserved sequence (A and C block)

• Type II in tRNA with A and B block

TATA box

located between -30 & -100 bp upstream of initiation site

initiator element (Inr)

located between -3 & +5 bp and is often paired with TATA box

Downstream care promoter element (DPE)

located between +28 & +35 bp and works in cooperation with Inr when TATA box is absent

GC box is common in genes with _____ expression.

continuous

RNA polymerase need ______ to interact with promoters

transcription factors

TFIID

initiates assembly of active transcription complex with TATA box binding protein — recognizes TATA box

what is the heart of the initiation complex

TBP bound to TATA box

what helps RNA poly. II bind to form basal transcription complex

TFIID, B, D, E, F, and H

PolyA tail function

• maintains stability of mRNA molecules

• move mRNA out of nucleus

• protects mRNA from exonuclease digestion

where is the polyA tail cleaved

near the 3’ end

CstF

cleavage stimulation factor

CPSF

cleavage & polyadenylation specificity factor

Splicing

removing introns from sequence. cannot alter the order of exons

processing steps of mRNA

• capping

• methylation

• ployA addition, splicing

mature mRNA characteristics

has 7-methylguanosine cap at 5’ end, spliced exons, polyA tail at 3’ end

How does slicing know where to start and stop

introns begin with GU and end with AG

splicing reactions

two transesterification reaction

splicing reaction # 1

cleavage of phosphodiester bond between exon 1 and 5’ end of intron

splicing reaction #2 aka transesterification 1

2’-5’ phosphodiester bond in formed between A residue and 5’-phosphate & generates free 3-OH on exon 1

splicing reaction #3 aka transesterification 2

3’-OH of exon 1 attacks phosphodiester bond between intron and exon 2

splicing reaction #4

exon 1 and 2 joins and lariat form of intron is released

5’ cap functions

• protection from nucleases

• enhanced translation

• enhance transport

Ribozyme

RNA molecules that can catalyze chemical reactions, similar to enzymes