Transcription in Eukaryotes

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Transcription in Eukaryotes
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Differences between prokaryotes eukaryotes
transcription
Eukaryotes occurs within the nucleus under the
direction of three separate forms of RNA
polymerase
The RNA transcript is not free to associate with
ribosomes prior to the completion of
transcription the mRNA must move out of the
nucleus into the cytoplasm for translation
Eukaryotes Chromatin remodelling occurs
uncoiling of chromatin fiber (characterized by
nucleosome coiling)
Initiation and regulation more extensive
interaction between cis-acting and trans-acting
protein factors and in addition to promoters,
other control units present - enhancers
Maturation of eukaryotic mRNA from the primary
RNA transcript involves many complex stages -
processing
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Initial processing step involves the addition of
a 5 cap and a 3 tail to most transcripts
destined to become mRNAs
Other extensive modification occur to the
internal nucleotide sequence of eukaryotic RNA
transcripts that eventually serve as mRNAs
The initial transcripts are most often much
larger than those that are translated called
pre-mRNAs only found in the nucleus a group
referred to collectively as heterogeneous nuclear
RNA (hnRNPs)
Such RNA molecules are variable but large size
complexed with proteins forming heterogeneous
nuclear ribonucleoprotein particles (hnRNPs)
About 25 of hnRNA are converted to mRNA those
that are converted substantial amounts of the
ribonucleotide sequence are excised and the
remaining segments are spliced back together
prior to nuclear export and translation
concepts of split genes and splicing
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Initiation of transcription
Eukaryotic RNA polymerase exists in three unique
forms each of which transcribes different types
of genes see table 13.7)
Each enzyme is larger and more complex than the
single prokaryotic polymerase.
The RNA polymerase II (RNP II) responsible for
the production of all mRNAs in eukaryotes
The activity of RNP II is regulated by both the
cis-acting elements in the gene itself and by a
number of trans-acting factors that bind to these
DNA elements.
At least three cis-acting DNA elements that
regulate the initiation of transcription by RNP II
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First called a core-promoter element
determines where RNP II binds to the DNA and
where it begins copying the DNA into RNA
The other two called promoter and enhancer
elements influence the efficiency or the rate
of transcription by RNP II as the process
proceeds from the core promoter element
In eukaryotes transcriptional initiation is
controlled by cis-acting DNA elements
collectively referred to as the promoter
Promoter refers to both the core-promoter
sequence where RNP II binds and to the promoter
and enhancer elements in the DNA that influence
RNP II activity
Example of the cis-acting core promoter elements
Goldberg-Hogness / TATA box present in most
eukaryotic genes located 30 nucleotide pairs
upstream from the start point of transcription
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TATA box sequence TATAAAA function analogous
to that found in the prokayotic genes
TATA box site where RNP II binds to the DNA
template and transcription typically begins
approximately 25 30 bp downstream from the TATA
box
TATA box determines the exact nucleotide
position where transcription begins
transcription start site
Another example CAAT box influence the
efficiency of the promoter
The third type of cis-acting regulatory DNA
elements are enhancers locations vary
Participate not directly in RNP II binding to the
core promoter essential to highly efficient
initiation of transcription
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Various trans-acting factors facilitate RNP II
binding these proteins are referred to as
transcription factors
Two broad categories - the general
transcription factors are absolutely required for
all RNP II mediated transcription RNA
polymerase II cannot bind directly to eukaryotic
promoter sites and initiate transcription without
their presence - the specific transcription
factors influence the efficiency or the rate of
RNP II transcription
Recent discoveries concerning RNA polymerase
function See page 321 - 322
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Heterogeneous nuclear RNA and its processing caps
and tails
After the initial transcript has been produced
the genetic code is written in the ribonucleotide
sequence of mRNA
In eukaryotes complex processing of mRNA occurs
before it is transported to the cytoplasm to
participate in translation
The first step of the postranscriptional
modification of eukaryotic RNA transcripts
involves the addition of a 7-methylguanosine
(7-mG) cap to the 5 end important for further
processing to occur protect 5 end from
nuclease attack
The cap may be involved with the transport of
mature mRNAs across the nuclear membrane into the
cytoplasm and in the initiation of translation of
the mRNA into protein
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The hnRNAs and mRNAs contain at their 3 end a
stretch of adenylic acid residues
The 3 end of the initial transcript is cleaved
enzymatically at a point some 10 35
ribonucleotides from a highly conserved AAUAAA
sequence
Polyadenylation occurs by the sequential addition
of adenylic acid residues
If the poly-A tail cannot be added the RNA
transcripts are rapidly degraded
Internal DNA sequences that are present in
initial RNA transcripts but removed before the
mature mRNA is translated intervening sequences
Genes that contain them split genes
DNA sequences not represented in in the final
mRNA - introns
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Those retained and expressed are called exons
Splicing involves the removal of the
ribonucleotide sequences present in introns as a
result of an excision process and the rejoining
of exons
Splicing mechanisms page 324 - 326
The nucleotide sequence of a pre-mRNA is changed
prior to translation RNA editing
Result the ribonucleotide sequence of mature
RNA differs from the sequence encoded in the
exons of the DNA from which the RNA was transcribe
Two main types of RNA editing - Substitution
editing - Insertion / deletion editing
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Klug, Cummings Spencer, 2006
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