molecular biology central dogma (ii)
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The first stage in the flow of informationfrom DNA to polypeptide is thetranscription of a DNA nucleotidesequence into an RNA nucleotidesequence.
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In transcription, theDNA helix unzips
RNA nucleotides
line up along onestrand of the DNA
following the base-
pairing rules
The single-stranded
messenger RNA
peels away and theDNA strands rejoin
RNA polymerase
DNA of gene
Promoter
DNA TerminatorDNA
Initiation
Elongation
TerminationGrowing
RNA
RNApolymeraseCompleted RNA
Three main kinds of RNA molecules aretranscribed:
messenger RNA
transfer RNA
ribosomal RNA
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Messenger RNA (mRNA)
is a single strand of RNA that carries theinformation for making a protein.
Transfer RNAs (tRNAs)
is a single strand of RNA that folds back on itselfto form a specific shape.
Ribosomal RNA (rRNA)
is in a globular form, is an important part of thestructure of ribosomes and has catalyticfunctions needed during protein synthesis.
Most RNA is synthesized by one of threeRNA polymerases, enzymes present in allcells.
The three RNA polymerases differ in thekinds of RNA they synthesize.
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RNA polymerase I catalyzes the synthesis of several kinds of
rRNA molecules that are components of theribosome;
RNA polymerase II catalyzes the production of the protein-
coding mRNA;
RNA polymerase III catalyzes the synthesis of tRNA and one of
the rRNA molecules.
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Promoter : a region of DNA where RNA polymerase bindsto initiate transcription
Promoters contain specific, highly-conserved sequences of DNA Transcription factors contain DNA-binding domains that
recognize these specific sequences Common promoter elements are shown below; many eukaryotic
genes have these (though not all) TATA box is important
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A characteristic feature of atranscriptional terminator is the presenceof a short sequence that iscomplementary to the sequence justpreceding it.
and so can anneal together resulting inthe formation of a stem-loop structure.
In most terminator sequences, the stem-loop structure is followed by a run of Uresidues.
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1. Pre-mRNA containing introns and exons
2. 5 end of premRNA capped with modified
nucleotide
3. Poly-A tail added to 3 end
4. Introns removed and exons spliced
together
5. Mature mRNA transported into cytosol
6. Translation at ribosome
Each end of a pre-mRNA molecule ismodified in a particular way
The 5 end receives a modified nucleotide cap
The 3 end gets a poly-A tail
Figure 17.9
A modified guanine nucleotideadded to the 5 end
50 to 250 adenine nucleotidesadded to the 3 end
Protein-coding segment Polyadenylation signal
Poly-A tail3UTRStop codonStart codon
5 Cap 5UTR
AAUAAA AAAAAA
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
mRNA
TRANSLATIONRibosome
Polypeptide
G P P P
5 3
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RNA splicing
Removes introns and joins exons
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
mRNA
TRANSLATION
Ribosome
Polypeptide
5 Cap
Exon Intron
1
5
30 31
Exon Intron
104 105 146
Exon 3Poly-A tail
Poly-A tail
Introns cut out andexons spliced together
Coding
segment
5 Cap1 146
3UTR3UTR
Pre-mRNA
mRNA
Is carried out by spliceosomes in some casesRNA transcript (pre-mRNA)
Exon 1 Intron Exon 2
Other proteins
Protein
snRNA
snRNPs
Spliceosome
Spliceosomecomponents
Cut-outintron
mRNA
Exon1 Exon2
5
5
5
1
2
3
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Ribozymes
Are catalytic RNA molecules that function as
enzymes and can splice RNA
The presence of introns
Allows for alternative RNA splicing
Proteins often have a modular architecture
Consisting of discrete structural and functional
regions called domains
In many cases
Different exons code for the different domains
in a protein
GeneDNA
Exon 1 Intron Exon 2 Intron Exon 3
Transcription
RNA processing
Translation
Domain 3
Domain 1
Domain 2
Polypeptide
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The mRNA has a non-coding leadersequence at its 5 end.
The leader has recognition sites forribosome binding, which properlyposition the ribosomes to translate the
message.
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The start codon follows the leadersequence and signals the beginning ofthe coding sequence that contains theactual message for the polypeptide
At the end of each coding sequence, astop codon signals the end of the protein
The stop codonsUAA, UGA, andUAGend both bacterial andeukaryotic messages.
They are followed by non-coding 3trailing sequences, which vary in length.
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Following transcription, the transcribedinformation in the mRNA is used tospecify the amino acid sequence of apolypeptide
This process is called translation becauseit involves conversion of the nucleicacid language in the mRNA molecule
into the amino acid language ofprotein.
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A specific enzyme called an aminoacyl-tRNA synthetase
Joins each amino acid to the correct tRNA
Ribosome Structure
Ribosomes are complexes of RNAs and proteins.
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Ribosomal RNARibosomal RNA
Ribosomal RNA (rRNA) is
single stranded RNA thatfolds into a complexstructure due to hydrogenbonding.
The peptidyl transferaseactivity of the ribosomeappears to be associatedwith the RNA in theribosome rather than theproteins.
Ribosome Binding SiteRibosome Binding Site
The Ribosome Binding Site (RBS) orShine-Dalgarno sequence(which was characterized by John Shine and Lynn Dalgarno)is a short sequence that base pairs with the 16S rRNA.
The RBS will be near the start codon of prokaryotic genes.
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The process of protein synthesis has threedistinct stages:
initiation,
repeating cycles of elongation,
termination.
Initiation of protein synthesis is essentiallythe same in all organisms
In Escherichia coli, the ribosome bindingsite has the consensus sequence 5 -AGGAGGU-3 and is located between 3and 10 nucleotides upstream of theinitiation codon.
Shine Dalgarno sequence: The ribosomebinding site upstream of an E. coli gene
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Initiation factors bind to the smallribosomal subunit, which then binds tomRNA in the region of AUG, the startcodon.
The initiator tRNA binds to the startcodon, followed by binding of the largeribosomal subunit.
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Elongation is a cyclic process in whichamino acids are added one by one tothe growing polypeptide chain.
Elongation proceeds in the 5 3direction along the mRNA.
The polypeptide chain grows from itsamino end to its carboxyl end.
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Termination, the final stage of translation,occurs when the ribosome reaches oneof three stop codons.
The A site binds to a release factor,which triggers the release of thecompleted polypeptide chain anddissociation of the translation complex.
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An overview of transcription and translation
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The genetic code was assumed to be universal,meaning that each codon specifies the sameamino acid in all organisms, with a few exceptions.
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A gene
Is a region of DNA whose final product is either
a polypeptide or an RNA molecule