TRANSKRIPSI

Andriani Departemen Biokimia dan Biologi Molekuler FK UNTAN

TRANSKRIPSI : Sintesis RNA dari DNA pada gen

PERSAMAAN TRANSKRIPSI DAN REPLIKASI

• Both processes use DNA as the template.

• Phosphodiester bonds are formed in both cases.

• Both synthesis directions are from 5´ to 3´.

PERBEDAAN REPLIKASI DAN TRANSKRIPSI

REPLICATION TRANSCRIPTION

Template Double strands Single strand

Substrat dNTP NTP

Primer yes no

Enzyme DNA Polimerase RNA polimerase

Product dsDNA ssRNA

Base pair A-T, G-C A-U,, G-C

Efisiensi transkripsi sebuah gen tidak sama dengan gen yang lain → Regulasi ekspresi gen

Satu bagian DNA di transkripsikan menjadi RNAPerbedaan RNA dan DNA

5'

3'

3'

5'

TRANSKRIPSI menghasilkan untai RNA yang komplementer dengan DNA template/cetakan Enzim : RNA Polimerase

Perbedaan RNA Polimerase dari D NA Polimerase 1. Menggabungkan untai yang

mngandung Ribosa2. Bisa memulai sintesis tanpa

menggunakan primer

Transkripsi tidak seakurat replikasi → RNA polymerases make about one mistake for every 104 nucleotides copied into RNA (error rate for direct copying by DNA polymerase of about one in 107 nucleotides)

RNA POLIMERASE

TAHAP-TAHAP TRANSKRIPSI (prokariota) 1. Inisiasi → pengenalan gen yang

akan ditranskripsi oleh RNA polimerase core enzyme → holoenzyme RNA polymerase (σ factor & core enzyme RNA polimerase)

- RNA polimerase holoenzim melekat pada promotor (titik awal dimulainya transkripsi)

- Untai DNA membuka sebagian - Transcription Bubble

2. Elongasi : - Terlepasnya σ factor- RNA polimerase core enzyme bergeser menyusuri DNA

- Sintesis RNA komplementer dng DNA template

3. Terminasi - apabila sudah bertemu

dengan sinyal terminasi (struktur RNA hairpin)

SIKLUS TRANSKRIPSI PROKARIOTA

TRANSKRIPSI EUKARIOTA DAN PROKARIOTA

TAHAP –TAHAP TRANSKRIPSI (eukariot)

1. INISIASI - RNA polimerase

bergabung dengan generel transcription factor

- general TF mengikat TATA Box → TATA Binding protein (TBP)

- Pembentukan Komplek Inisiasi Transkripsi

- Sintesis RNA untai pendek - Penambahan gugus

phosphat pada “ekor” RNA polimerase (C- Terminal Domain, CTD)

2. ELONGASI - general transcription

factor lepas dari RNA polimerase II

- memerlukan faktor elongasi

3. TERMINASI • sequence terminasi AATAAA

diikuti GT repeats. • The termination is closely

related to the post-transcriptional modification

• Primary transcripts of mRNA are called as heteronuclear RNA (hnRNA).

• hnRNA are larger than matured mRNA by many folds.

• Modification includes – Capping at the 5- end – Tailing at the 3- end– mRNA splicing– RNA edition

Modification of hnRNA

CH3

O

O OH

CH2

PO

O

O

N

NHN

N

O

NH2

AAAAA-OH

O

Pi

5'

3'

O

OHOH

H2CN

HNN

N

O

H2N O P

O

O

O P

O

O

O P

O

O

5'

a. Capping at the 5- end

m7GpppGp----

• The 5- cap structure is found on hnRNA too. The capping process occurs in nuclei.

• The cap structure of mRNA will be recognized by the cap-binding protein required for translation.

• The capping occurs prior to the splicing.

b. Poly-A tailing at 3 - end

• There is no poly(dT) sequence on the DNA template. The tailing process dose not depend on the template.

• The tailing process occurs prior to the splicing.

• The tailing process takes place in the nuclei.

Exon and intron

Exons are the coding sequences that appear on split genes and primary transcripts, and will be expressed to matured mRNA.

Introns are the non-coding sequences that are transcripted into primary mRNAs, and will be cleaved out in the later splicing process.

mRNA splicing

Splicing mechanism

Performed by units called Spliceosome

• Taking place at the transcription level

• One gene responsible for more than one proteins

• Significance: gene sequences, after post-transcriptional modification, can be multiple purpose differentiation.

d. mRNA editing

TERIMA KASIH