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Ön mRNA İşleme: Ön mRNA Uçlarının Modifikasyonu
Ökaryotik hücrelerde, RNA polimeraz tarafından yapılan transkriptler, çekirdekten çıkmadan önce değiştirilir ve işlenir. İşlenmemiş RNA'ya, olgun mRNA…
Ökaryotik hücrelerde, yeni kopyalanan mRNA'ya öncü mRNA veya pre-mRNA denir. Her pre-mRNA iki önemli modifikasyon alır. Biri molekülün kapak adı verilen beş asal ucunda, diğeri ise molekülün kuyruk adı verilen üç asal ucunda.
Beş asal başlık, pre-mRNA'nın ilk nükleotidine bir trifosfat bağı ile bağlanan, modifiye edilmiş bir guanin nükleotidi olan tek bir 7-metilguanozinden oluşur.
Pre-mRNA transkriptinin üç asal ucuna doğru, genellikle A, A, U, A, A, A olan spesifik bir nükleotid kümesine poliadenilasyon sinyali denir. Üç asal uç işleme makinesini işe alır ve üç asal bölünme bölgesinde transkripti kesmek için bir endoribonükleazı yönlendirir.
Başka bir enzim olan poliadenilat polimeraz veya PAP, daha sonra transkriptin üç asal ucuna 200 kadar uzun bir adenin nükleotid dizisi ekler.
Beş asal başlık ve üç asal poli-A kuyruğu, transkriptin uçlarını eksonükleazlar tarafından bozulmaya karşı korur. Üç asal kuyruk ayrıca mRNA transkriptinin çekirdeği terk etmeye hazır olduğunu taşıyan moleküllere sinyal verir. Çekirdeğin dışında, beş asal kapak, ribozomun transkripte bağlanmasına yardımcı olur, böylece translasyona başlayabilir.
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Q1: What is the 5' cap and why is it added to pre-mRNA?
The 5' cap is a 7-methylguanosine, a modified guanine nucleotide attached to the first nucleotide of pre-mRNA via a triphosphate linkage. It protects the transcript from degradation by exonucleases and helps the ribosome attach during translation. The cap also helps cells distinguish mRNA from other RNA types.
Q2: How does the poly-A tail protect mRNA and signal its readiness for translation?
Polyadenylate polymerase adds approximately 200 adenine nucleotides to the 3' end of pre-mRNA, forming the poly-A tail. This tail protects mRNA from degradation by exonucleases in the cytoplasm and signals to transport molecules that the transcript is ready to exit the nucleus for translation.
Q3: What is a polyadenylation signal and how does it direct 3' end processing?
A polyadenylation signal is a specific sequence of nucleotides, typically A, A, U, A, A, A, located toward the 3' end of pre-mRNA. It recruits the 3' end processing machinery and directs an endoribonuclease to cut the transcript at the 3' cleavage site, allowing polyadenylate polymerase to add the poly-A tail.
Q4: What is the difference between exons and introns in pre-mRNA?
Exons are RNA sequences that remain in the mature transcript and are expressed as proteins. Introns are portions removed during splicing by the spliceosome. Interestingly, a single RNA segment can function as an exon in one cell type and an intron in another, enabling alternative rna splicing to produce multiple proteins from one gene.
Q5: When does the 5' cap get added during transcription?
The 5' cap is added early during transcription, once approximately 20-40 ribonucleotides have been joined together by RNA polymerase. A group of enzymes replaces the 5' phosphate with modified 7-methyl guanosine, which helps the cell recognize mRNA and prepares it for subsequent translation.
Q6: How do the 5' cap and 3' poly-A tail work together to protect mRNA?
Both the 5' cap and 3' poly-A tail protect mRNA ends from degradation by exonucleases. The 5' cap marks the transcript as mRNA and facilitates ribosome binding, while the 3' tail signals nuclear export and protects the transcript in the cytoplasm, ensuring the mRNA remains stable for translation.
Q7: What happens to pre-mRNA after the 5' cap and poly-A tail are added?
After both modifications are complete, the mature mRNA exits the nucleus for translation. The 5' cap and 3' poly-A tail signal to transport molecules that the transcript is ready to leave the nucleus. Outside the nucleus, the 5' cap helps the ribosome attach to initiate protein synthesis.