Lecture 13 Bio325 Fall 07

Lecture 13 Bio325 Fall 07 - Processing of mRNA's in...

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1 Processing of mRNA’s in eukaryotic cells: 1. Capping • 5 cap: a guanine nucleotide is added to the 5’ end of the mRNA with a 5’-5’ linkage and 3 phosphate groups; the guanine is methylated at N-7, and the adjacent riboses are methylated • only mRNA molecules are capped (not tRNA, rRNA, etc) • capping protects the RNA from degradation by nucleases functions of capping: • the cap also serves as a translation signal Fig. 8.13 2. Polyadenylation Fig. 8.14 • the 3’ end of the mRNA is cleaved off and 30 to 250 nucleotides of poly(A) is added by poly-A- polymerase functions of the poly(A) tail: • the poly(A) tail can facilitate nuclear export and translation; it also helps to prevent degradation of the mRNA
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2 in eukaryotic cells, the open reading frame of a gene is often split up into many separate parts. Instead of one continuous gene, it is spread out along the chromosome with non-translated regions in-between : the non-translated regions are called introns , short for intervening sequences; the translated regions are called exons , short for expressed sequences some genes have hundreds of exons, spread out over millions of base-pairs Fig. 8.15 • in eukaryotic cells with introns, the mRNA is transcribed from genomic DNA with all the intron sequences present. This version of the mRNA is called the pre-mRNA • the introns must then be spliced out of the pre-mRNA to form the mature mRNA Fig. 17.9
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3 ? How are introns removed from the pre-mRNA? • there are specific sequences present at the beginning and the end of each intron • the splicing sequences are recognized by specific protein-RNA complexes called snRNPs (small nuclear ribonucleoproteins); these also form a larger complex of proteins called the spliceosome Fig. 8.16 • the RNA and proteins in the spliceosome recognize the splice sequences in the mRNA, cut out the intron sequence, and connect the ends of the exons • the RNA in snRNPs plays a critical
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This note was uploaded on 06/19/2008 for the course BIO 325 taught by Professor Saxena during the Fall '08 term at University of Texas.

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Lecture 13 Bio325 Fall 07 - Processing of mRNA's in...

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