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Processing the RNA Transcript

Before the pre-mRNA leaves the nucleus to be translated, three additional events occur (see Figure 6.2): 
  • A 5’ cap is added. This is a special nucleotide which is added to the 5’ end of the message. It contains a methyl group and a phosphate group. The function of the cap is to help regulate translation.
  • A poly A tail is attached to the 3’ end. This long string of 100-200 adenine nucleotides helps to regulate the degradation of the transcript after it leaves the nucleus.
  • The transcript is spliced. In eukaryotic genes, the coding regions of the DNA (exons) are interspersed with noncoding regions (introns). After the gene is transcribed, the introns are cut, or spliced, from the pre-mRNA so they never are translated. Why does the DNA contain these sequences if they ultimately do not code for the final protein product? It turns out that the presence of introns in the pre-mRNA is necessary for transport of the message out of the nucleus (the splicing machinery appears to be coupled with transport). In addition, alternative splicing can occur, which results in some exons being removed from the message. Thus, different messages can be made and, ultimately, different proteins can be coded for by the same gene. It is also believed introns play a role in evolution, allowing different regions of genes (i.e. the exons) to be rearranged, leading to new proteins.

Transcription and translation


Once the fully processed mRNA (now called the mature mRNA) has been transported out of the nucleus and into the cytoplasm, it can be translated into a protein.

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