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Procesamiento de pre-ARNm: modificación de los extremos del pre-ARNm
En las células eucariotas, las transcripciones producidas por la ARN polimerasa se modifican y procesan antes de salir del núcleo. El ARN no procesado…
En las células eucariotas, el ARNm recién transcrito se denomina ARNm precursor o pre-ARNm. Cada pre-ARNm recibe dos modificaciones importantes. Uno en el extremo de cinco primos de la molécula, llamado tapa, y otro en el extremo de tres primos de la molécula, llamado cola.
La tapa de cinco primos está compuesta por una sola 7-metilguanosina, un nucleótido de guanina modificado, que se une al primer nucleótido del pre-ARNm mediante un enlace trifosfato.
Un conjunto específico de nucleótidos hacia el extremo tres primo de la transcripción de pre-ARNm, generalmente A, A, U, A, A, A, A, se denomina señal de poliadenilación. Recluta la maquinaria de procesamiento de los tres puntos primos y dirige una endorribonucleasa para cortar la transcripción en el sitio de escisión de los tres primos.
Otra enzima, la poliadenilato polimerasa, o PAP, añade entonces una larga cadena de nucleótidos de adenina, hasta 200, al extremo de tres primos de la transcripción.
La tapa de cinco primos y la cola de poli-A de tres primos protegen los extremos de la transcripción de la degradación por exonucleasas. La cola de tres primos también indica a las moléculas transportadoras que la transcripción de ARNm está lista para abandonar el núcleo. Fuera del núcleo, la tapa de cinco primos ayuda al ribosoma a unirse a la transcripción para que pueda comenzar la traducción.
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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.