molecular biology central dogma (ii)

8/2/2019 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

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