Dasar-dasar Ekspresi GenDasar-dasar Ekspresi Gen

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

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.

PROTEINDNA RNA

Replikasi DNA Replikasi RNA

Transkripsi balik

(Reverse transcription)

Transkripsi Transla

i

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

transfer

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

DNA

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

genes

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

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 dibantu oleh EF-G

GTPGDP + Pi

Proses pemanjangan rantai polipeptida selama sintesis protein