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Transcription and RNA Processing
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DNA is found mostly in the nucleus.
RNA is common in the cytoplasm.
Transfer of Genetic Information:The Central Dogma
the DNA message in the nucleus and carry it out tothe cytoplasm, where proteins are synthesized.
Crick predicted the existence of adaptor molecules
that reads the genetic code and selects theappropriate amino acids to add to a growingpolypeptide chain. The Central Dogma
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The Central Dogma
The central dogma of biology is that
information stored in DNA is
transferred to RNA molecules duringtranscription and to proteins during
translation.
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The Central Dogma
Transcription involves the synthesis of anRNA transcript complementary to one strandof DNA of a gene.
Translation is the conversion of informationstored in the sequence of nucleotides in theRNA transcript into the sequence of aminoacids in the polypeptide gene product,according to the specifications of the geneticcode.
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The Central Dogma
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Transcription and Translationin Prokaryotes
The primarytranscript isequivalent to the
.
The mRNA codonson the mRNA aretranslated into an
amino acidsequence by theribosomes.
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Transcription and Translationin Eukaryotes
The primary transcript(pre-mRNA) is aprecursor to the mRNA.
The pre-mRNA is
modified at both ends,and introns areremoved to produce themRNA.
After processing, themRNA is exported tothe cytoplasm fortranslation by
ribosomes.
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Types of RNA Molecules
Messenger RNAs (mRNAs)intermediates thatcarry genetic information from DNA to the ribosomes.
Transfer RNAs (tRNAs)adaptors between amino.
Ribosomal RNAs (rRNAs)structural and catalytic
components of ribosomes.
Small nuclear RNAs (snRNAs)structural
components of spliceosomes.Micro RNAs (miRNAs)short single-stranded RNAs
that block expression of complementary mRNAs.
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The Process of GeneExpression
Information stored in the nucleotide
sequences of genes is translated intothe amino acid sequences of proteins
through unstable intermediaries called
messenger RNAs.
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General Features of RNA Synthesis
Similar to DNA Synthesis except The precursors are ribonucleoside triphosphates.
Only one strand of DNA is used as a template.
RNA chains can be initiated de novo(no primer required).
The RNA molecule will be complementary to the DNAtemplate (antisense) strand and identical to the DNAnontemplate (sense) strand (except that uridinereplaces thymidine).
RNA synthesis is catalyzed by RNA polymerases andproceeds in the 53 direction.
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Template: This strand provides thepattern for transcription. (antisense)
Nontemplate: This strand is theoriginal message thats actually being
transcribed (sense).
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The Transcription Unit in Eukaryotes
The transcription unit is made up of promoter, gene, and terminator.The presence of TATA tells the transcription-starting enzyme that thegene to transcribe is about 30 to 50 base pairs away and also locatesthe template strand..
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Transcription Promoter Sequences
RNA polymerase recognizes a specific base sequencein the DNA called a promoter and binds to it.
Many eukaryotic promoters contain a TATA box(sequence TATAAA, often within 50 bases of the start
site), where a TATA binding protein binds assisting in
e orma on o e po ymerase ranscr p onacomplex.
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A promoter is defined as the region upstreamof a gene containing the binding site for RNA
ol merase II that initiates transcri tion of the
Promoter: Technical Definition
DNA. It contains a TATA box, a CAAT box oran AGGA box, and the CAP site.
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Stages of Transcription
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The early stages of transcription in prokaryotes, showing the components of the
process.
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Initiation of RNA Chains
1. Binding of RNA polymerase holoenzyme toa promoter region in DNA
2. Localized unwinding of the two strands of
DNA by RNA polymerase to provide asingle-stranded template
3. Formation of phosphodiester bonds
between the first few ribonucleotides in thenascent RNA chain
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The early stages of transcription in prokaryotes, showing template binding at the -10site involving the subunit of RNA polymerase and subsequent initiation of RNA
synthesis.
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The early stages of transcription in prokaryotes, showing chain elongation, after the subunit has dissociated from the transcription complex and the enzyme moves
along the DNA template.
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Elongation
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Termination Signals in E. coli
Rho-dependent terminatorsrequire aprotein factor ()
-
require
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1. Rho-independent transcription terminationinvolves sequences within the RNA (poly U residues)that signal the RNA polymerase to stop. The
terminator sequences form a stem-loop hairpinstructure that leads to the dissociation of the RNAPfrom the DNA tem late.
Transcription Terminator Sequence (Prokaryotes)
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2. Rho-dependent termination - uses a terminationfactor called factor (rho factor) which is a protein tostop RNA synthesis at specific sites.
Rho protein binds at a rho utilisation site on the
RNA strand and runs along the mRNA towards theRNAP.
region pauses the RNAP, when -factor reachesthe RNAP, it causes RNAP to dissociate from theDNA, terminating transcription.
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Transcription Terminator Sequence (Eukaryotes)
Eukaryotic genes contain a poly-A signal
located downstream of the last exon. This
signal is used to add a series of adenylateresidues during RNA processing. Transcription
. -
poly-A signal, but the mechanism is unclear.
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Transcription in Eukaryotes
Three different enzymes catalyze transcription
in eukaryotes, and the resulting RNA transcripts
undergo three important modifications, includingthe excision of noncoding sequences called
introns. The nucleotide sequenced of some
RNA transcripts are modifiedposttranscriptionally by RNA editing.
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RNA Processing inEukaryotes
Eukaryotic gene transcripts usually undergothree major modifications:
-
termini,(2) The addition of poly(A) tails to 3 ends, and
(3) Editing of the RNA sequence
(4) The excision of noncoding intron sequences.
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The 3 Poly(A) Tail
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RNA Editing
Usually the genetic information is notaltered in the mRNA intermediary.
Sometimes RNA editing changes theinformation content of genes by Changing the structures of individual bases
Inserting or deleting uridinemonophosphate residues.
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Interrupted Genes in Eukaryotes:Exons and Introns
Most eukaryotic genes contain noncoding
sequences called introns that interrupt thecoding sequences, or exons. The introns areexcised from the RNA transcripts prior to their
transport to the cytoplasm.
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IntronsIntrons (or intervening sequences) are
noncoding sequences located between codingsequences.
Introns are removed from the pre-mRNA and
are not present in the mRNA.Exons (both coding and noncoding
sequences) are composed of the sequences
that remain in the mature mRNA after splicing.Introns are variable in size and may be very
large.
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Types of Intron Excision
The introns of tRNA precursors are excised by precise
endonucleolytic cleavage and ligation reactionscatalyzed by special splicing endonuclease and ligaseactivities.
The introns of some rRNA precursors are removedautocatalytically in a unique reaction mediated by the
RNA molecule itself.
The introns of nuclear pre-mRNA transcripts are splicedout in two-step reactions carried out by spliceosomes.
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Excision of Intron Sequences
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Splicing mechanism of pre-rRNA involving group I introns that are removed fromthe initial transcript. The process is one of self-excision involving two
transesterification reactions.
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Introns can be spliced out leaving all the exons in their original order, or
introns and exons can be spliced out to create a new sequence of
exons (also called alternative splicing). Alternative splicing results in the
possibility for one gene to be expressed in different ways.
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