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gene
– either an RNA or a polypeptide
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• Transcription : (Verb) The act or process of making a copy
– Example: Court reporter hears the witness speaking in English and types a written copy, in English, of the witness’ statements.
• Translation : Express the meaning of words or text in another language
• Dogma : A principle or set of principles laid down by an authority as incontrovertibly true
Court reporter
transcribing court testimony
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• In genetics , the term refers to the copying of a
DNA sequence into an RNA sequence
– Only one strand is copied
• The structure of DNA is not altered as a result of this process
– It continues to store information and can be transcribed again and again and again
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1. Check out
3. Return unaltered
4. Distribute and incite a riot!
2. Make many copies of the same page
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
Structural genes encode the amino acid sequence of a polypeptide

Transcription of a structural gene produces messenger RNA , usually called mRNA

The mRNA nucleotide sequence determines the amino acid sequence of a polypeptide during translation

The synthesis of functional proteins determines an organisms traits

This path from gene to trait is called the central dogma of genetics

Refer to Figure 12.1
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DNA replication: makes DNA copies that are transmitted from cell to cell and from parent to offspring.
Gene Chromosomal DNA: stores information in units called genes.
Transcription: produces an RNA copy of a gene.
Figure 12.1
Messenger RNA: a temporary copy of a gene that contains information to make a polypeptide.
Translation: produces a polypeptide using the information in mRNA.
Polypeptide: becomes part of a functional protein that contributes to an organism's traits.
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• Gene expression is the overall process by which the information within a gene is used to produce a functional product which can, in concert with environmental factors, determine a trait
– Or: How does a book result in a riot?
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
Transcription occurs in three stages

Initiation


Elongation
Termination

These steps involve protein-DNA interactions

Proteins such as RNA polymerase interact with DNA sequences
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Promoter
5′ end of growing
RNA transcript
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DNA of a gene
Transcription
Terminator
Initiation: The promoter functions as a recognition site for transcription factors (not shown). The transcription factor(s) enables RNA polymerase to bind to the promoter.
Following binding, the DNA is denatured into a bubble known as the open complex.
Open complex
RNA polymerase Elongation/synthesis of the RNA transcript:
RNA polymerase slides along the DNA in an open complex to synthesize RNA.
Termination: A terminator is reached that causes RNA polymerase and the RNA transcript to dissociate from the DNA.
Completed RNA transcript
Figure 12.3
RNA polymerase
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
Once they are made, RNA transcripts play different functional roles

Refer to Table 12.1

Well over 90% of all genes are structural genes which are transcribed into mRNA

Final functional products are polypeptides

The other RNA molecules in Table 12.1 are never translated

Final functional products are RNA molecules
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
The RNA transcripts from nonstructural genes are not translated

They do have various important cellular functions

They can still confer traits

In some cases, the RNA transcript becomes part of a complex that contains protein subunits

For example

Ribosomes

Spliceosomes

Signal recognition particles
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You don’t need to memorize this slide – however, note how many different types of functional RNA molecules exist and how many different types of functions they perform!
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• Our molecular understanding of gene transcription came from studies involving bacteria and bacteriophages
• Indeed, much of our knowledge comes from studies of a single bacterium
– E. coli , of course
• In this section we will examine the three steps of transcription as they occur in bacteria
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
Promoters are DNA sequences that “promote” gene expression

More precisely, they direct the exact location for the initiation of transcription

Promoters are typically located just upstream of the site where transcription of a gene actually begins

The bases in a promoter sequence are numbered in relation to the transcription start site

Refer to Figure 12.4
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Most of the promoter region is labeled with negative numbers
Bases preceding the start site are numbered in a negative direction
There is no base numbered 0
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Coding strand
Promoter region
–35 sequence 16 –18 bp –10 sequence
+1
5′
Transcriptional start site
3′
T
A
T
A
G A
C T
C
G
A
T
T
A
A
T
T
A
A
T
A
T
T
A
A
T
3′ 5′
Template strand
Bases to the right are numbered in a positive direction
5′
A
RNA
Transcription
Figure 12.4 The conventional numbering system of promoters
3′
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Sequence elements that play a key role in transcription
The promoter may span a large region, but specific short sequence elements are particularly critical for promoter recognition and activity level
Transcriptional start site
Coding strand
Promoter region
–35 sequence 16 –18 bp –10 sequence
5′
T
A
T
A
G A
C T
C
G
A
T
T
A
A
T
T
A
A
T
A
T
T
A
3′
+1
A
T
3′
5′
Template strand
Sometimes termed the
Pribnow box, after its discoverer
5′
A
RNA
3′
Transcription
Figure 12.4 The conventional numbering system of promoters
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
RNA polymerase is the enzyme that catalyzes the synthesis of RNA

In E. coli , the RNA polymerase holoenzyme is composed of


Core enzyme

Five subunits = a
2 bb ’ 
Sigma factor

One subunit = s

These subunits play distinct functional roles
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
The RNA polymerase holoenzyme binds loosely to the DNA

It then scans along the DNA, until it encounters a promoter region

When it does, the sigma factor recognizes both the –35 and –10 regions

A region within the sigma factor that contains a helix-turn-helix structure is involved in a tighter binding to the DNA

Refer to Figure 12.6
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s
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α helices binding to the major groove
Amino acids within the a helices hydrogen bond with bases in the
-35 and -10 promoter sequences
Turn
Figure 12.6
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
The binding of the RNA polymerase to the promoter forms the closed complex

Then, the open complex is formed when the
TATAAT box in the -10 region is unwound

A short RNA strand is made within the open complex

The sigma factor is released at this point

This marks the end of initiation

The core enzyme now slides down the DNA to synthesize an RNA strand

This is known as the elongation phase
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Figure 12.7
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
RNA polymerase
σ factor
Promotor region
–35 –10
RNA polymerase holoenzyme
After sliding along the DNA, σ factor recognizes a promoter, and
RNA polymerase holoenzyme forms a closed complex.
–35
–10
Closed complex
An open complex is formed, and a short RNA is made.
–35
–10
Open complex
σ factor is released, and the core enzyme is able to proceed down the DNA.
RNA polymerase
–35
–10 core enzyme
σ factor
RNA transcript
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Character
A shy female college student
Played By
A cute dude
A helpful friend Dr. Ballard
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