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(CHAPTER 12- Brooker Text)

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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)

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

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