Transkripsi: Sintesis RNA

Novi Silvia HardianyDept.Biokimia & Biologi Molekuler

FKUI

Pendahuluan

• DogmaSentral

Struktur RNA

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Overview of Transcription During transcription in the nucleus, a

segment of DNA unwinds and unzips, and the DNA serves as a template for mRNA formation

RNA polymerase joins the RNA nucleotides so that the codons in mRNA are complementary to the triplet code in DNA

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Steps in Transcription The transfer of information in the nucleus from

a DNA molecule to an RNA molecule Only 1 DNA strand serves as the template Starts at promoter DNA (TATA box) Ends at terminator DNA (stop) When complete, pre-RNA molecule is released

Transcription

Jenis RNA Polimerase• E.Coli: hanya 1, memiliki 4 subunit (α2ββ’) dan faktor

sigma pengikatan ke regio promoter spesifik pada cetakan DNA

• Eukariot: 3 RNA PolimeraseEnzim RNA yang dihasilkan

RNA Polimerase I rRNA

RNA Polimerase II mRNA

RNA Polimerase III tRNA dan RNA kecil lainnya

Pengenalan Gen oleh RNA Polimerase

• Promotor- sinyal pada DNA yang dikenali oleh RNA Polimerase- Menentukan startpoint & frekuensi transkripsi

Promotor E.Coli: - Regio -10 dengan urutan konsensus yaitu TATAAT

(boks TATA/ Prinbow)- Regio -35 dengan urutan konsensus yaitu TTGACA

• Operon: sejumlah unit gen yang dikontrol oleh sebuah promotor

• Repressor: protein yang berikatan ke suatu regio di promotor yg dikenal sebagai operator dan menghambat transkripsi dg mencegah pengikatan RNA Polimerase

• Aktivator: protein yang merangsang transkripsi dg berikatan ke regio -35 dan mempermudah pengikatan RNA Polimerase

Pengenalan Gen oleh RNA Polimerase

Promotor Eukariot: - Regio -25 dengan urutan konsensus yaitu

TATA(A/T)A boks TATA/ Hogness/Hogness-Goldberg

- Boks CAAT dan urutan kaya GC di regio antara -40 dan -110

Enhancer & silencer

• Elemen DNA yang mengatur frekuensi transkripsi gen pada eukariot

• Terletak jauh dari startpoint• Bebas orientasi (urutan dapat berjalan dalam 2 arah)• Bebas Posisi (dapat dipindahkan ke tempat yang jaraknya

berbeda-beda dari titik mulai & tetap merangsang transkripsi)

• Enhancer: transaktivator transkriptional, merangsang transkripsi

• Silencer: menghambat transkripsi

The transcription cycle of bacterial RNA Polymerase

• Video transkripsi pada prokariot

Transcription in eucaryotes

To begin transcription RNA polymerase requires several general transcription factor

A. The promoter (TATA box)B. TBP, TFIID recognizes & bind the TATA box C. adjacent binding of TFIIBD. The RNA polymerase itself, assemble the

promoterE. TFIIH uses ATP to pry apart DNA double

helix at the transcription start pointTFIIH phosphorylates RNA polymerase II changing its conformation begin the elongation phase

• Video transkripsi pada eukariot

Cells produce several types of RNA

A comparison of the structures of Procaryotic and eucaryotic mRNA molecules

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Processing Pre-mRNA

• Also occurs in the nucleus• Pre-mRNA made up of segments called

introns & exons• Exons code for proteins, while introns do

NOT!• Introns spliced out by splicesome-enzyme

and exons re-join• End product is a mature RNA molecule that

leaves the nucleus to the cytoplasm

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RNA Processing

pre-RNA molecule

intron intronexon exon exon

exon exon exon

Mature RNA molecule

exon exon exon

intron intron

splicesome splicesome

The pre-mRNA splicing reaction

Adding a 5’ cap

The major step in generating the 3’ end of a eucaryotic mRNA

- CstF & CPSF bind to spesific nucl.sequences- the RNA is cleaved- poly A-polymerase (PAP) enzymes adds 200 nucleotides to the 3’ end produced by the cleavage

poly A-tail

An ‘export ready’ mRNA molecule & its transport through the nuclear pore

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Messenger RNA (mRNA)• Carries the information for a specific

protein• Made up of 500 to 1000 nucleotides long• Sequence of 3 bases called codon• AUG – methionine or start codon• UAA, UAG, or UGA – stop codons

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Messenger RNA (mRNA)

methionine glycine serine isoleucine glycine alanine stopcodon

protein

A U G G G C U C C A U C G G C G C A U A AmRNA

startcodon

Primary structure of a protein

aa1 aa2 aa3 aa4 aa5 aa6

peptide bonds

codon 2 codon 3 codon 4 codon 5 codon 6 codon 7codon 1

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Transfer RNA (tRNA)• Made up of 75 to 80 nucleotides long• Picks up the appropriate amino acid floating

in the cytoplasm • Transports amino acids to the mRNA• Have anticodons that are complementary to

mRNA codons• Recognizes the appropriate codons on the

mRNA and bonds to them with H-bonds

A tRNA molecule

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Ribosomal RNA (rRNA)• Made up of rRNA is

100 to 3000 nucleotides long

• Made inside the nucleus of a cell

• Associates with proteins to form ribosomes