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Chapter 16

BLG 143 Chapter 16: Biology Study Notes Chapter 16


Department
Biology
Course Code
BLG 143
Professor
Vadim Bostan
Chapter
16

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Biology Study Notes Chapter 16
16.1 An Overview of Transcription
The strand that is read by the enzyme is called the template strand.
The other strand is called the non template strand or coding strand.
The coding strand is particularly appropriate name, eause the o teplate stad’s
sequence matches the sequence of the RNA that is transcribed from the template
strand and codes for a poly peptide.
Characteristics of RNA Polymerase
An RNA polymerase performs a template dieted sthesis i the 5’ to 3’ dietio.
But unlike DNA polymerases, RNA polymerases do not require a primer to begin
transcription.
Name of Enzyme
Type of Gene Transcribed
RNA Polymerase I
Genes that code for most of the large RNA
molecules (rRNAs) found in ribosomes
RNA Polymerase II
Protein coding genes (produce mRNAs);
also, genes that code for RNAs that function
in ribosome assembly and in processing and
regulation of mRNAs
RNA Polymerase III
Genes that code for transfer RNAs (tRNAs),
for one of the small rRNAs found in
ribosomes, and for noncoding RNAs; also,
genes that code for RNAs that function in
ribosome assembly and in processing and
regulation of mRNAs.
Initiation: How Does Transcription Begin?
A detachable protein subunit called sigma must bind to
the polymerase before transcription can begin.
Bacterial RNA polymerase and sigma form what
biologist call a holoenzyme.
A holoenzyme consists of a core enzyme, which
contains the active site for catalysis, and other required
proteins.
The binding sites where holoenzyme bound to were
called promoters. (Transcription Begins)
Bacterial and Eukaryotic Promoters
Promoters have one section in common that is about 6 base pair sequence of TATAAT.
This is known as the -10 box, it is centered about ten bases from where transcription
begins.
DNA that is located in the direction RNA polymerase moves during transcription is said
to be downstream from the point of reference. Vise versa for upstream.
The place where transcription starts is said to be called the 1+ site.
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In the same promoters, a sequence of another 6
bases was found (TTGACA) but was found about
35 bases upstream from 1+ (-35 box).
Many eukaryotic promoters include a unique
sequence called the TATA box, centered about 30
base pairs upstream of the transcription start site.
The Role of Sigma Subunits and Basal Transcription
Factors
Transcription starts when sigma binds to the -10
and -35 boxes.
Sigma makes initial contact with DNA that starts
transcription and not RNA polymerase, this observation supports the hypothesis that
sigma is a regulatory protein.
Each type of sigma protein allows RNA polymerase to bind to a different type of
promoter and therefore a different kind of gene.
Sigma protein will determine which type of genes will be transcribed.
Controlling which sigma proteins are active is one of the ways that bacterial cells control
which genes are expressed.
Proteins that came to be called basal transcription
factors initiate eukaryotic transcription by binding to
the appropriate promoter region in DNA.
Basal transcription factors and sigma proteins are
analogous.
Events inside the Holoenzyme
Step 1 Sigma binds to the promoter region.
Step 2 the template strand is threaded through a
channel that leads to the active site inside the RNA
polymerase. Ribonucleotides triphosphates the
monomers that will be strung together into RNA
enter a channel at the bottom of the enzyme and
diffuse to the active site.
When an incoming NTP pairs with a complementary
base on the template strand of DNA, RNA
polymerization begins.
Step 3 Sigma is released once RNA synthesis is
under way. The initiation phase of transcription is
complete.
Elongation and Termination
Once RNA polymerase begins moving along the DNA
teplate i the 3’ 5’ dietio, sthesizig RNA i
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the 5’ 3’ dietio. The elogatio phase of tasiptio is ude a.
The rudder helps steer the template and non
template strand through channels inside the enzyme.
Eze’s atie site atalzes the additio of
uleotides to the 3’ ed of the goig RNA
molecule.
The eze’s zippe the helps sepaate the el
synthesized RNA from the DNA template.
Transcription ends with the termination phase.
In most cases, transcription stops when RNA
polymerase reaches a DNA sequence that functions as
a transcription termination signal.
As soon as the code was synthesized the RNA
sequence folds back on itself and forms a short
double helix that is held together by complementary
base pairing.
The secondary structure is called a hairpin.
16.2 RNA Processing in Eukaryotes
In bacteria, the information in DNA is converted to
RNA dietl. I eukaotes, it is’t.
The result of transcription in bacteria is a mRNA that
is ready to be translated into a protein.
In eukaryotes, the product is an immature primary
RNA transcript. Before primary transcripts can be
translated, they have to be processed in a complex
series of steps.
Noncoding regions must be removed before the
mRNA can carry intelligible message to the
translation machinery.
Eukaryotic genes that are part of the final mRNA be
referred to as exons and sections of primary
transcript not in mRNA be referred to as introns.
Introns are the reason why Eukaryotic genes are
much longer than their corresponding mature RNA
transcripts.
RNA Splicing
As transcription process, the introns are removed
from the growing RNA strand by a process known as
splicing.
Pieces of primary transcript are removed and the
remaining segments are joined together.
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