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Dasar-dasar Ekspresi GenDasar-dasar Ekspresi Gen

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GENGEN

sekuen DNA dengan panjang minimum tertentu yang mengkode urutanlengkap asam amino suatu polipeptida, atau RNA (mRNA, tRNA, rRNA)

Gen Struktural TerminatorPromoter

Awal transkripsi

ATG STOP

Struktur dasar gen 

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A Eukaryotic GeneHow are eukaryotic genes different from prokaryotic genes?

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Overview RNA is transcribed, using the rules of complementary

base pairing, from the template strand of DNA.

Most genes code for proteins; a smaller fractionencode untranslated functional RNAs.

The nucleotide sequence of the gene determines theorder of amino acids in a protein, which determinesshape, size, and protein function.

mRNA is translated in groups of three nucleotides(codon) at the ribosome through pairing of tRNA

anticodon with the mRNA codon.

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PROTEINDNA RNA

Replikasi DNA Replikasi RNA

Transkripsi balik

(Reverse transcription)

Transkripsi Transla

i

“Dogma” Biologi Modern

(Central “Dogma”)

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galaktosidase

 

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Transfer of Information DNA RNA polypeptide {Central Dogma}

originally believed this transfer of information was

in one direction only

RNA can go ‘back’ to DNA via reversetranscription; no transfer from protein to RNA

Complementary base pairing transfersinformation

during transcription to form RNA

during translation to form protein

The importance of weak bonds in informationtransfer

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RNA 1st step in process of information transfer is

transcription

Transcription: copying nucleotide sequence of DNAinto RNA

forms RNA transcript

DNA may be transcribed multiple times

RNA

single-stranded polynucleotide

contains ribose sugar

contains the pyrimidine uracil (U) hydrogen bonds with A

5’ and 3’ ends critically important

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Classes of RNA Informational RNA: protein encoding mRNA

primary transcript in prokaryotes

processed transcript in eukaryotes

5’ and 3’ end modification

intron removal

translated into amino acid sequence

Functional (structural) RNA tRNA: transports amino acid to ribosome; Crick’s adaptor

rRNA: structural and catalytic component of ribosomes

snRNA: structural and catalytic component of spliceosome

snRNPs snoRNA: small nucleolar RNA involved in maturation of 

rRNA

scRNA: directs protein traf fic in cytoplasm

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Transcription RNA polymerase catalyzes RNA synthesis

uses one DNA strand as template

always the same strand for a given gene

locally unwinds DNA

adds free nucleotides to growing RNA strand at 3’ end

5’ to 3’ RNA synthesis

template read 3’ to 5’

uses rules of base pairing to synthesize complementaryRNA molecule

starts RNA chain de novo

Transcript is identical in sequence to nontemplatestrand, except T’s replaced by U’s

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Transcription is asymmetric – only one strand of the DNA is

transcribed into RNA; the template strandThe RNA transcript has the same sequence as thenontemplate strand

RNA is synthesized in a 5’ to 3’ direction only

The template strand is read in the 3’ to 5’ direction

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Either strand of the DNA can be used as the template strandfor transcription

However, in any one gene only one strand of the DNA serves asthe template for transcription

DNA 

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RNA polymerases Prokaryotes: single RNA polymerase

Transcribes mRNA, rRNA and tRNA

Transcription and translation are coupled

Eukaryotes: three RNA polymerases RNA polymerase I transcribes rRNA genes

RNA polymerase II transcribes protein-encoding genes; i.e.

makes mRNA primary transcript will be processed

RNA polymerase III transcribes tRNA genes and 5S rRNAgenes

Transcription and translation occur in separatecompartments of the eukaryotic cell In organelles they occur in the same compartment

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Transcription steps

Initiation

at 5’ end of gene binding of RNA polymerase to promoter

unwinding of DNA

Elongation

addition of nucleotides to 3’ end of growing chain governed by rules of complementary base pairing

energy from NTP substrates

Termination

at 3’ end of gene terminator loop (prokaryote) or processing enzyme

coding region

5’UTR 3’UTR

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Translation mRNA is translated at the ribosome using tRNA as an

adaptor molecule

nucleotide sequence is read three nucleotides at atime

each triplet is called a codon

each amino acid has one or more codons

64 possible codons (4 ´ 4 ´ 4) = genetic code

used by all organisms with few exceptions

no punctuation except start and stop

Genetic code specifies 20 different amino acids(sometimes selenocysteine)

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Translation

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What are some featuresof the genetic code?

Each codon specifies asingle amino acid.

The code is degeneratesince there are multiplecodons for each aminoacid with 2 exceptions.

The multiple codons foreach amino acid beginwith the same twonucleotides.

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fMet

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AUGmRNA

AUGmRNA

fMet

50S subunit, H20, GTP

GDP, Pi

Pembentukan komplek inisiasi 70S

AUG CGA GCUmRNA

fMet

P A

AUG CGA GCUmRNA

fMet

P A

Arg

AUG CGA GCUmRNA

fMet

Arg

AUG CGA GCUmRNA

fMet

Arg

Penyisipan aminoasil-tRNAoleh EF-Tu

GTPGDP + Pi

Translokasi dibantuoleh EF-G

GTPGDP + Pi

Proses pemanjangan rantai polipeptida selama sintesis protein