Tinjauan Umum Transkripsi

Dra. Exsyupransia Mursyanti

(b) Eukaryotic cell

Tinjauan Umum Transkripsi

Transkripsi: proses pembentukan molekul RNA berdasarkan kode genetik yang terdapat dalam DNA

Dikatalis: RNA polimerase

Hasil: berupa prekursor RNA ------ prosesing secara biokemis meliputi:1. penghilangan kelebihan nukleotida2. penambahan nukleotida pada ujung prekursor3. Modifikasi basa

Fungsi &Struktur RNA

Fungsi:1. mRNA = membawa kode genetik untuk protein

yang akan disintesis2. rRNA = bagian struktural & fungsional ribosom3. tRNA= mengikat asam amino dan diberikan pada

ribosomStruktur:• Merupakan rantai tunggal• Mengandung urutan berulang (pada tRNA 90%)

---- struktur hairpin loop

Komponen Transkripsi

1. RNA polimerase

2. DNA acuan (arah 3’-5’)

3. Mg2+ /Mn2+

4. 4 macam RNA nukleosida trifosfat: ATP, GTP, CTP, UTP

5. Faktor-faktor transkripsi

Fungsi RNA pol

1. Mengkatalisis reaksi transkripsi (arah 5’ ke 3’)2. Mempunyai sisi pengenalan untuk promoter3. Dapat berinteraksi dengan faktor inisiasi,

elongasi dan terminasi transkripsi4. Mengenal nukleosida RNA untuk pasangan yang

cocok, mengikat dan menghidrolisis untuk dipasangkan

5. Membentuk ikatan fosfodiester6. Menghentikan transkripsi7. Mengurai duplek DNA dan mengembalikan

pada posisi semula

Tahap Umum Transkripsi

I. Inisiasi

II. Elongasi

III. Terminasi

Tahap Inisiasi1. Pembentukan komplek tertutup (Closed

complex)

2. Transisi ke komplek terbuka (Open complex)

3. Promoter escape

Tahap Elongasi1. Peruraian DNA pada lokasi didepan

enzim RNA pol

2. Sintesis RNA

3. RNA proofreading

4. Disosiasi RNA

5. Reannealing DNA dibelakang enzim

ConceptConcept : Transcription and Translation : Transcription and Translation in Cellsin Cells

In a prokaryotic cell, transcription and translation are coupled; In a prokaryotic cell, transcription and translation are coupled; that is, translation begins while the mRNA is still being that is, translation begins while the mRNA is still being synthesized. In a eukaryotic cell, transcription occurs in the synthesized. In a eukaryotic cell, transcription occurs in the nucleus, and translation occurs in the cytoplasmnucleus, and translation occurs in the cytoplasm

Because there is no nucleus to separate the processes of Because there is no nucleus to separate the processes of transcription and translation, when bacterial genes are transcription and translation, when bacterial genes are transcribed, their transcripts can immediately be translated.transcribed, their transcripts can immediately be translated.

Concept : Transcription and Translation in Concept : Transcription and Translation in CellsCells

Transcription and translation are spatially and temporally Transcription and translation are spatially and temporally separated in eukaryotic cells; that is, transcription occurs in separated in eukaryotic cells; that is, transcription occurs in the nucleus to produce a pre-mRNA moleculethe nucleus to produce a pre-mRNA molecule

The pre-mRNA is typically processed to produce the mature The pre-mRNA is typically processed to produce the mature mRNA, which exits the nucleus and is translated in the mRNA, which exits the nucleus and is translated in the cytoplasmcytoplasm..

Concept 3: Different Genes for Different Concept 3: Different Genes for Different RNAsRNAs

There are 4 types of RNA, each encoded by its own type of gene.There are 4 types of RNA, each encoded by its own type of gene. The genomic DNA contains all the information for the structure and The genomic DNA contains all the information for the structure and

function of an organism.function of an organism. In any cell, only some of the genes are expressed, that is, transcribed In any cell, only some of the genes are expressed, that is, transcribed

into RNAinto RNA There are 4 types of RNA, each encoded by its own type of gene:There are 4 types of RNA, each encoded by its own type of gene:

mRNAmRNA - Messenger RNA: Encodes amino acid sequence of a polypeptide - Messenger RNA: Encodes amino acid sequence of a polypeptide tRNAtRNA - Transfer RNA: Brings amino acids to ribosomes during translation - Transfer RNA: Brings amino acids to ribosomes during translation rRNArRNA - Ribosomal RNA: With ribosomal proteins, makes up the - Ribosomal RNA: With ribosomal proteins, makes up the

ribosomes, the organelles that translate the mRNAribosomes, the organelles that translate the mRNA snRNAsnRNA - Small nuclear RNA: With proteins, forms complexes that are - Small nuclear RNA: With proteins, forms complexes that are

used in RNA processing in eukaryotes. (Not found in prokaryotes.)used in RNA processing in eukaryotes. (Not found in prokaryotes.)

Concept : The RNA MoleculeConcept : The RNA MoleculeRNA is structurally similar to DNARNA is structurally similar to DNA

                                                         

Concept : The Transcription Concept : The Transcription ProcessProcess

RNA synthesis involves separation of the DNA RNA synthesis involves separation of the DNA strands and synthesis of an RNA molecule in the 5' strands and synthesis of an RNA molecule in the 5' to 3' direction by RNA polymerase, using one of the to 3' direction by RNA polymerase, using one of the DNA strands as a template.DNA strands as a template.

In complementary base pairing, A, T, G, and C on In complementary base pairing, A, T, G, and C on the template DNA strand specify U, A, C, and G, the template DNA strand specify U, A, C, and G, respectively, on the RNA strand being synthesized.respectively, on the RNA strand being synthesized.

Concept : Complete Transcription of an Concept : Complete Transcription of an RNA MoleculeRNA Molecule

Transcription begins at the promoter, proceeds Transcription begins at the promoter, proceeds through the coding region, and ends at the through the coding region, and ends at the terminator.terminator.

coding region, and ends at the terminator.

                                                  

Concept: mRNA in ProkaryotesConcept: mRNA in Prokaryotes

The sequence of a prokaryotic protein-coding The sequence of a prokaryotic protein-coding gene is colinear with the translated mRNA; that gene is colinear with the translated mRNA; that is, the transcript of the gene is the molecule that is, the transcript of the gene is the molecule that is translated into the polypeptide.is translated into the polypeptide.

Most eukaryotic protein-coding genes contain Most eukaryotic protein-coding genes contain segments called introns, which break up the segments called introns, which break up the amino acid coding sequence into segments called amino acid coding sequence into segments called exons.exons.

The transcript of these genes is the pre-mRNA The transcript of these genes is the pre-mRNA (precursor-mRNA).(precursor-mRNA).

The pre-mRNA is processed in the nucleus to The pre-mRNA is processed in the nucleus to remove the introns and splice the exons together remove the introns and splice the exons together into a translatable mRNA. That mRNA exits the into a translatable mRNA. That mRNA exits the nucleus and is translated in the cytoplasm.nucleus and is translated in the cytoplasm.

Concept : mRNA in EukaryotesConcept : mRNA in EukaryotesThe sequence of a eukaryotic protein-coding gene is The sequence of a eukaryotic protein-coding gene is

typically not colinear with the translated mRNA; typically not colinear with the translated mRNA; that is, the transcript of the gene is a molecule that that is, the transcript of the gene is a molecule that

must be processed to remove extra sequences must be processed to remove extra sequences (introns) before it is translated into the polypeptide.(introns) before it is translated into the polypeptide.

Concept : Pre-mRNA Processing Concept : Pre-mRNA Processing (Splicing)(Splicing)

Eukaryotic pre-mRNAs typically include introns. Introns are Eukaryotic pre-mRNAs typically include introns. Introns are removed by RNA processing in which the intron is looped removed by RNA processing in which the intron is looped out and cut away from the exons by out and cut away from the exons by snRNPssnRNPs, and the , and the exons are spliced together to produce the translatable exons are spliced together to produce the translatable mRNA.mRNA.

The steps of pre-mRNA splicing (intron removal) are as The steps of pre-mRNA splicing (intron removal) are as follows:follows: The intron loops out asThe intron loops out as snRNPs snRNPs (small nuclear (small nuclear

ribonucleoprotein particles, complexes of ribonucleoprotein particles, complexes of snRNAs snRNAs and and proteins) bind to form the proteins) bind to form the spliceosomespliceosome

The intron is excised, and the exons are then spliced togetherThe intron is excised, and the exons are then spliced together The resulting mature mRNA may then exit the nucleus and be The resulting mature mRNA may then exit the nucleus and be

translated in the cytoplasmtranslated in the cytoplasm