The mRNA of prokaryotes can be used without any modification to direct protein synthesis, but posttranscription processing of mRNA is needed in eukaryotes. The DNA sequences that code for mRNA (exons) are frequently interrupted by intervening DNA sequences (introns). When a protein-coding gene is first transcribed, the hnRNA contains both exons and introns. Before the transcript exits the nucleus, its introns are removed and its exons are spliced to form mature mRNA (Figure 2-4). The hnRNA that is synthesized by RNA polymerase has a 7-methyl-guanosine "cap" added at the 5' end. The cap appears to facilitate the initiation of translation and to help stabilize the mRNA. In addition, most eukaryotic mRNA has a chain of 40-200 adenine nucleotides attached to the 3' end of the mRNA. The poly (A) tail is not transcribed from DNA; rather, it is added after transcription by the nuclear enzyme poly (A) polymerase. The poly (A) tail may help stabilize the mRNA and facilitate mRNA exit from the nucleus. After the mRNA enters the cytoplasm, the poly (A) tail is gradually shortened.
FIGURE 2-4. Transcription and RNA splicing.
The horizontal black line between exons indicates an intron. The region before the first exon is the 5' regulatory region of the gene, such as a TATA box. There also are c/s-regulatory elements in introns and downstream of the last exon. The heterogeneous nuclear RNA (hnRNA), containing both exons and introns, is spliced to form mRNA. mRNAs are then exported from the nucleus to the cytoplasm, where they will direct the synthesis of distinct proteins.
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