Unit 3 Synthesis of RNA

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1

RNA Polymerase

Conserved structure, do NOT require primer, lack 3'-5' exonuclease activity; initiation, elongation & termination

2

Why Transcription?

protect DNA from cytoplasmic environment, not enough DNA to serve metabolic needs, regulate protein synthesis through # and lifetime of mRNA, & RNA binding factors

3

Types of RNA Pol

Bacteria: single pol transcribes all RNA, core enzyme alpha2, beta, beta', sigma binds to core directs binding to promoter sigma 70; Eukaryotic: Pol I- rRNA, Pol II- mRNA, Pol III- tRNA & 5S rRNA; all have same mech but recognize diff. promoters

4

Template and Coding DNA Starnds

Template: complementary and antiparallel to RNA & coding strand; Coding strand: analogous to produced RNA- determines AA sequence of protein.

5

Codon

sets of three AA read from 5' to 3' of mRNA

6

Upstream Elements

AT-rich sequence determines start of transcription: Pribnow box (TATAAT- Prokaryote) & TATA box (TATA(A/T)A- Eukaryote), CAAT boxes, GC-rich sequences, and enhancers in Eukaryotes

7

Bacterial RNA Pol

holoenzyme, recognizes & binds promoter, unwinds and separates DNA, transcribes DNA

8

Rho

*Rho-independent: hairpin loop formation by transcript; *Rho-dependent: binding of protein to release pol and transcript.

9

Bacterial Operon

polycistronic transcripts- multiple genes on one RNA coding for proteins within same metabolic pathway.

10

Coupling of Transcription & Translation

in prokaryotes only due to the absence of a nucleus

11

Transcription of Eukaryotic Genes

Differences from Prokaryotes: occurs in nucleus and in chromatin, three RNA pol, more elaborate processing, 1mRNA-1 polypeptide

12

Regulation of Transcription

more transcription factors, nucleosome-modifying complexes, chromatin modeling complexes

13

General Transcription Factors (GTFs)

promoters bind to GTFs, GTFs interact with each other & RNA pol II to initiate trancription

14

Enhancers

sequences that stimulate transcription initiation, upstream or downstream, close or far, contain response elements (sequences that bind activators)

15

Synth and Processing of Euk. *mRNA

Pol II synth primary transcript, capped at 5' during transcription, poly(A) tail at 3' after mRNA cleaved from transcript (by poly(A) pol), introns and exons, mature mRNA exported to cytoplasm

16

Toxin alpha-amanitin

inhibits RNA pol II & fatal at low doeses

17

Synth of Euk. *rRNA

RNA pol I, from set of tandemly repeated genes (1000 copies in genome), cleaved to produce 18S, 28S, & 5.8S rRNAs, XMas tree appearance, form ribonucleoprotein complex (ribosome)

18

Synth of Euk. *tRNA

RNA pol III, mature produced through: cleavage of 5' & 3' ends, removal of intron, replacement of 3' terminal UU w/ CCA, modifications of bases

19

Thalassemias

hereditary anemias most common gene disorder in the world (carrier rate ~7%); Beta + mutation in TATA box (A to G or C -28 to -31 region) reduce accuracy of start point, only 20-25% normal beta-globin synthesized, or other mutations further upstream (-87 C to G and -88 C to T) cause same effect