11.3 Rna transcription  (Page 3/8)

Eukaryotic genes that encode polypeptides are composed of coding sequences called exon s ( ex -on signifies that they are ex pressed) and intervening sequences called intron s ( int -ron denotes their int ervening role). Transcribed RNA sequences corresponding to introns do not encode regions of the functional polypeptide and are removed from the pre-mRNA during processing. It is essential that all of the intron-encoded RNA sequences are completely and precisely removed from a pre-mRNA before protein synthesis so that the exon-encoded RNA sequences are properly joined together to code for a functional polypeptide. If the process errs by even a single nucleotide, the sequences of the rejoined exons would be shifted, and the resulting polypeptide would be nonfunctional. The process of removing intron-encoded RNA sequences and reconnecting those encoded by exons is called RNA splicing and is facilitated by the action of a spliceosome containing small nuclear ribonucleo proteins (snRNPs). Intron-encoded RNA sequences are removed from the pre-mRNA while it is still in the nucleus. Although they are not translated, introns appear to have various functions, including gene regulation and mRNA transport. On completion of these modifications, the mature transcript , the mRNA that encodes a polypeptide, is transported out of the nucleus, destined for the cytoplasm for translation. Introns can be spliced out differently, resulting in various exons being included or excluded from the final mRNA product. This process is known as alternative splicing . The advantage of alternative splicing is that different types of mRNA transcripts can be generated, all derived from the same DNA sequence. In recent years, it has been shown that some archaea also have the ability to splice their pre-mRNA.

Key concepts and summary

• During transcription , the information encoded in DNA is used to make RNA.
• RNA polymerase synthesizes RNA, using the antisense strand of the DNA as template by adding complementary RNA nucleotides to the 3’ end of the growing strand.
• RNA polymerase binds to DNA at a sequence called a promoter during the initiation of transcription .
• Genes encoding proteins of related functions are frequently transcribed under the control of a single promoter in prokaryotes, resulting in the formation of a polycistronic mRNA molecule that encodes multiple polypeptides.
• Unlike DNA polymerase, RNA polymerase does not require a 3’-OH group to add nucleotides, so a primer is not needed during initiation.
• Termination of transcription in bacteria occurs when the RNA polymerase encounters specific DNA sequences that lead to stalling of the polymerase. This results in release of RNA polymerase from the DNA template strand, freeing the RNA transcript .
• Eukaryotes have three different RNA polymerases. Eukaryotes also have monocistronic mRNA, each encoding only a single polypeptide.
• Eukaryotic primary transcripts are processed in several ways, including the addition of a 5’ cap and a 3′- poly-A tail , as well as splicing , to generate a mature mRNA molecule that can be transported out of the nucleus and that is protected from degradation.

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