1. (CHAPTER 12- Brooker Text)
Transcription
(CHAPTER 12- Brooker Text)
Sept 18 & 23, 2008
BIO 184
Dr. Tom Peavy

3. • Bacterial mRNA may be polycistronic, which
means it encodes two or more polypeptides

4. The Stages of Transcription
Transcription occurs in three stages
Initiation
Elongation
Termination
These steps involve protein-DNA interactions
Proteins such as RNA polymerase interact with DNA sequences

5. Initiation
promoter = recognition site for transcription factors
transcription factors + RNA polymerase to bind to the promoter (initial phase) = closed promoter complex
then DNA is denatured (bubble) = open promoter complex

6. Prokaryotic Transcription
E. coli RNA polymerase = holoenzyme
Core enzyme (Four subunits = a2bb’)
Sigma factor (One subunit = s)
Initiation stages involve RNA pol holoenzyme
Binding loosely to the DNA
Scaning for promoter region
Forming Open promoter complex
Synthesizing Short stretch of RNA
Releasing Sigma factor

7. Sequence elements that play a key role in transcription
Bacterial Promoter
Sequence elements that play a key role in transcription
(pribnow box)

8. The RNA transcript is synthesized during ELONGATION step
The DNA strand used as a template for RNA synthesis is termed the template or noncoding strand
The opposite DNA strand is called the coding strand
It has the same base sequence as the RNA transcript
Except that T in DNA corresponds to U in RNA

9. Termination of Bacterial Transcription
short RNA-DNA hybrid is forced to separate
= release of newly made RNA
E. coli has two different mechanisms for termination
1. rho-dependent termination
Requires a protein known as r (rho)
2. rho-independent termination
Does not require r but uses stem loop structure and U–rich
sequence region

10. Rho protein is a helicase
rho utilization site
Rho protein is a helicase
r-dependent termination
Figure 12.8

11. r-dependent termination
Figure 12.8

12. Types of RNA mRNA = messenger RNA encodes for the sequence of
amino acids within a polypeptide
tRNA = transfer RNA carries amino acids at the amino
acyl terminus for incorporation into growing
polpeptides during translation at ribosome
rRNA = ribosomal RNA is a major component of the
ribosomal complex essential for coordinating
protein sythesis using the mRNA as a template
(translation)

13. TRANSCRIPTION IN EUKARYOTES
Similar but more complex
Larger organisms
Cellular complexity
Multicellularity
Three Different RNA polymerase needed
RNA pol I = rRNA genes (except 5S)
RNA pol II = mRNAs (structural genes)
RNA pol III = tRNAs and 5S rRNA

14. - determines start point for transcription
Figure 12.11
The core promoter is relatively short = TATA box
- determines start point for transcription
- basal transcription by itself (low level)
Regulatory Elements (e.g. GC and CAAT boxes)
- Enhancers (stimulate transcription)
- Silencers (inhibit transcription)
* often found nearby (-50 to -100) but can also be
found great distances away in either direction

15. Factors that Control Gene Expression
cis-acting elements
DNA sequences that exert their effect only on nearby genes
Example: TATA box, enhancers and silencers
trans-acting elements
Regulatory proteins that bind to such DNA sequences

16. In Eukaryotes, RNA is modified:
In Bacteria:
DNA coding strand = mRNA sequence directly
Corresponds to amino acid sequence in the polypeptide
= colinearity of gene expression
In Eukaryotes, RNA is modified:
- coding sequences, called exons, are interrupted by
intervening sequences or introns (which are removed)
= RNA splicing

17. Also as heterogeneous nuclear RNA (hnRNA)
In eukaryotes, the transcription of structural genes, produces a long transcript known as pre-mRNA
Also as heterogeneous nuclear RNA (hnRNA)
RNA is altered by splicing and other modifications, before it leaves the nucleus
Spliceosome required for splicing = multicomponent structure
Figure 12.16

18. Capping
Most mature mRNAs have a 7-methyl guanosine covalently attached at their 5’ end = capping
Occurs as pre-mRNA is being synthesized (RNA pol II)
Cap structure is recognized by cap-binding proteins
Role of Cap-binding proteins
Movement of some RNAs into the cytoplasm
Early stages of translation
Splicing of introns

19. Addition of poly A tail
Most mature mRNAs have a string of adenine nucleotides at their 3’ ends = polyA tail
polyA tail is not encoded in the gene sequence
added enzymatically after the gene is completely transcribed

20. Figure 12.20 Consensus sequence in higher eukaryotes
Length varies between species
Appears to be important in the stability of mRNA and the translation of the polypeptide
From a few dozen adenines to several hundred

21. Intron Advantage? Alternative splicing:
pre-mRNA with multiple introns can be spliced in different ways
Generates mature mRNAs with different combinations of exons = different polypeptides
Organism can carry fewer genes within genome
Variation in splicing can occur in different cell types or during different stages of development