Lecture 10 - Transcription

8/3/2019 Lecture 10 - Transcription

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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.

Lecture 10 - Transcription

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