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14.6: pre-mRNA Processing
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pre-mRNA Processing
 
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14.6: pre-mRNA Processing

14.6: pre-mRNA Processing

In eukaryotic cells, transcripts made by RNA polymerase are modified and processed before exiting the nucleus. Unprocessed RNA is called precursor mRNA or pre-mRNA, to distinguish it from mature mRNA.

Once about 20-40 ribonucleotides have been joined together by RNA polymerase, a group of enzymes adds a “cap” to the 5’ end of the growing transcript. In this process, a 5’ phosphate is replaced by modified guanosine that has a methyl group attached to it. This 5’ cap helps the cell distinguish mRNA from other types of RNA in the cell and plays a role in subsequent translation.

During or shortly after transcription, a large complex called the spliceosome cuts out various parts of the pre-mRNA transcript, rejoining the remaining sequences. RNA sequences that remain in the transcript are called “exons” (expressed sequences) while portions removed are called “introns”. Interestingly, a single RNA segment can be an exon in one cell type and an intron in another. Similarly, a single cell can contain multiple variants of a gene transcript that has been alternatively spliced, enabling the production of multiple proteins from a single gene.

When transcription is completed, an enzyme adds approximately 30-200 adenine nucleotides to the 3’ end of the pre-mRNA molecule. This poly-A tail protects the mRNA from degradation in the cytoplasm. The mature mRNA then exits the nucleus for translation.

In eukaryotic cells, transcripts made by RNA polymerase are modified and processed before exiting the nucleus. Unprocessed RNA is called precursor mRNA or pre-mRNA, to distinguish it from mature mRNA.

Once about 20-40 ribonucleotides have been joined together by RNA polymerase, a group of enzymes adds a “cap” to the 5’ end of the growing transcript. In this process, a 5’ phosphate is replaced by modified guanosine that has a methyl group attached to it. This 5’ cap helps the cell distinguish mRNA from other types of RNA in the cell and plays a role in subsequent translation.

During or shortly after transcription, a large complex called the spliceosome cuts out various parts of the pre-mRNA transcript, rejoining the remaining sequences. RNA sequences that remain in the transcript are called “exons” (expressed sequences) while portions removed are called “introns”. Interestingly, a single RNA segment can be an exon in one cell type and an intron in another. Similarly, a single cell can contain multiple variants of a gene transcript that has been alternatively spliced, enabling the production of multiple proteins from a single gene.

When transcription is completed, an enzyme adds approximately 30-200 adenine nucleotides to the 3’ end of the pre-mRNA molecule. This poly-A tail protects the mRNA from degradation in the cytoplasm. The mature mRNA then exits the nucleus for translation.


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