Biochemistry 2280A Lecture Notes - Lecture 17: Rna Editing, Polyadenylation, Neuron
5 May 2018
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Lecture 17: Prokaryotic Transcription
- Different cell types express different genes - ex. pattern of expression in liver cell is different from that
of a brain cell (same DNA, different expression leads to differentiation of cells)
At how many steps can gene expression can gene expression be regulated in a eukaryotic cell?
A. 2 B. 3-8 C. >8 - lots of steps to regulate genes in a eukaryotic cell
1. Transcription
- Initiation
- Elongation
- Termination
4. RNA processing (pre-mRNA-mRNA)
- RNA editing
- ’ appig
- Splicing
- ’ poladelatio
8. mRNA export from the nucleus to the cytoplasm
9. mRNA degradation - The amount of a mRNA found in a cell depends on its rate of synthesis and its
rate of decay
10. Translation (RNA-protein) same as transcription
13. Protein Modification
- phosphorylation
- Acetylation
- Cleavage (insulin)
14. Protein degradation
- The amount of a protein in a cell depends on its rate of synthesis and its rate od decay
Why is there an RNA step in gene expression?
1. The RNA step provides an amplification which contributes to differential gene expression
Dosage
A. B.
1 1 Figure 7-2
5 1
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2. Since RNA can be degraded, expression of a gene can be stopped quickly
3. RNA provides additional opportunities to regulate gene expression e.g.
- RNA processing
- RNA export from the nucleus
Transcription is the 1st step in the expression of any gene, and is often the most frequently regulated
step in gene expression (the most important)
Promoter: the DNA sequence required to initiate transcription of a gene or operon
Terminator: the DNA sequence required to stop transcription
Consensus Sequence - the most frequent base at each position in a group of functionally related DNA
elements
What is the consensus sequence for the DNA element on the left? C. TGATCA
- In this example, none of the elements are the same as the consensus sequence - consensus sequence
is just based on statistics
Bacterial Genes are organized into operons. Operon = a set of bacterial genes transcribed form a single
promoter and thus expressed from a common RNA
What are the key features of a bacterial promoter?
- The start site of transcription is +1
- +2 is to the left, -1 is to the right of +1
The -10 and -35 sequences were found by comparing the sequences of many E.Coli promoters...
You test if the -35 sequence is important by making mutations. When -35 sequence was mutated,
transcription slowed down (From 100 units to 2 units)
Bacterial RNA Pol - the enzyme that makes RNA transcripts using DNA as a template and nucleoside
triphosphates (NTPs) as substrates.
Bacterial RNA pol is a multi-subunit enzyme
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The oe eze a ake RNA i a test tue, ut does’t eogize pootes.
The promoter specificity of RNA pol is determined by the sigma subunit, it does so by...
In the context of the holoenzyme, sigma makes base-specific contacts between the -10 and -35
sequences
Steps in the Initiation of Transcription
1. RNAP holoenzyme binds the promoter
Fig 7-9
2. RNA pol unwinds the DNA strands around the start site - open complex (no auxiliary facts are
required, like DNA polymerase - RNA pol does this by itself)
3. The first NTP is brought to the Template - no primer is required (unlike DNA pol) fig. 7-9
4. Using A/G/C/UTP as substrates, hai elogatio poeeds i a ’- ’ dietio, folloig p ules; -9
RNA synthesis - phosphodiester bonds form, pyrophosphate is released
Mak Paget Biooleules
5. After addition of 5-10 nucleotides, sigma falls off the holoenzyme; fig. 7-9
. Tasiptio ule oes dostea ’-’ ith
7. RNA synthesis proceeds until a terminator is reached and RNA pol falls off
8. Sigma rebinds RNAP and the cycle is repeated
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Document Summary
Different cell types express different genes - ex. pattern of expression in liver cell is different from that of a brain cell (same dna, different expression leads to differentiation of cells) At how many steps can gene expression can gene expression be regulated in a eukaryotic cell: 2 b. 3-8 c. >8 - lots of steps to regulate genes in a eukaryotic cell: transcription. The amount of a protein in a cell depends on its rate of synthesis and its rate od decay. Why is there an rna step in gene expression: the rna step provides an amplification which contributes to differential gene expression. 5 1: since rna can be degraded, expression of a gene can be stopped quickly, rna provides additional opportunities to regulate gene expression e. g. - rna processing. Transcription is the 1st step in the expression of any gene, and is often the most frequently regulated step in gene expression (the most important)
