Ekspresi Genetik

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  • Dasar-dasar Ekspresi GenDasar-dasar Ekspresi Gen


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

    Gen Struktural TerminatorPromoter

    Awal transkripsi


    Struktur dasar gen

  • A Eukaryotic Gene

    How are eukaryotic genes different from prokaryotic genes?

  • Overview

    RNA is transcribed, using the rules of complementarybase 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 tRNAanticodon with the mRNA codon.


    Replikasi DNA Replikasi RNA

    Transkripsi balik

    (Reverse transcription)

    Transkripsi Transla


    Dogma Biologi Modern

    (Central Dogma)

  • galaktosidase

  • 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 information


  • RNA

    1st step in process of information transfer istranscription

    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

  • 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; Cricks 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 traffic in cytoplasm

  • 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 complementary

    RNA molecule

    starts RNA chain de novo Transcript is identical in sequence to nontemplate

    strand, except Ts replaced by Us

  • Transcription is asymmetric only one strand of the DNA istranscribed into RNA; the template strand

    The 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

  • 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


  • 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 rRNA


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

  • 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 region5UTR 3UTR

  • 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)

  • Translation

  • 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.

  • fMet




    50S subunit, H20, GTP

    GDP, Pi

    Pembentukan komplek inisiasi 70S



    P A



    P A








    Penyisipan aminoasil-tRNAoleh EF-Tu

    GTPGDP + Pi

    Translokasi dibantu oleh EF-G

    GTPGDP + Pi

    Proses pemanjangan rantai polipeptida selama sintesis protein