Textbook Notes (368,562)
Canada (161,962)
Biology (87)
BIOL 367 (16)
Reg Storms (15)
Chapter 14

BIOL 367 Chapter 14: biol 367 chapter 14

8 Pages
Unlock Document

BIOL 367
Reg Storms

CHAPTER 14 : RNA PROCESSING I: SPLICING RNA pol cannot distinguish the coding region of the gene from the noncoding regions, so it transcribes everything. Thus, the cell must remove the noncoding RNA from the original transcript, a process called splicing. Eukaryotes also tack a special cap and polyA tail. All these events occur in the nucleus. Genes in Pieces - Introns = noncoding parts - Coding parts = exons Evidence for Split Genes - In R-looping experiments, RNA is hybridized to its DNA template. The DNA template strands are separated to allow a double-stranded hybrid to form between one of these strands and the RNA product. Such a hybrid is a bit more stable than a double stranded DNA. After the hybrid forms, it is examined by electron microscopy - If the hexon gene had no introns, a smooth, linear hybrid would occur where the mRNA lined up with its DNA template. - The intron regions of the DNA will not find counterparts in the mRNA and so will form un- hybridized loops b) showing the three intron loops (labeled A, B, C), the hybrid (heavy red line) and the un-hybridized region of the DNA upstream of the gene (upper left). The fork at the lower right is due to the 3’ end of the mRNA, which cannot hybridize because the 3; end of the gene is not included RNA Splicing - The introns are transcibed, yielding a primary transcript, an overlarge gene producdt that is cut down to size by removing the introns - R-looping was done with double instead of ssingle stranded DNA, the RNA hybridizd to one of the DNA strands, displacing the other 1 Splicing Signals - Almost all introns in a nuclear mRNA precursors begins and end the same way: exon/GU-intron-AG/exon - Sequencing of many genes has revealed the following mammalian consensus sequence: 5’-AG/GUAAGU-intron-YNCURAC-Y NYAG/G-3n - Slashes = exon-intron borders, Y is either pryimidines, Y n a string of about nine pyrimidines, R I either purine, A is a special A in the branchpoint sequence within the intron, and N is any base - Branchpoint sequence near the end of an intron is also requierd for the next exon to be recognized as such Effect of Splicing on Gene expressoin - One reason that splicing may have evolved ot become so prominent in higher eukaryotes is that ist actually facilitates gene expressoin - They stimualte efficient mRNA 3’ end formation, and they make translation more efficient THE MECHANISM OF SPLICING OF NUCLEAR MRNA PRECURSORS A branched intermediate - The intermediate in nuclear mRNA precursor splicing is branched, so it looks like a lariat, or cowboy’s lasso - Two step lariat model of splicing: 1 step is the formation of the lariat- shaped intermediate. This occurs when the 2’-hydroxylgroup of anadenosine nucleotide in the middle of the intron attacks the phosphodiester bond between the first exon and the G at the beginning of the intron (the 5’ splice site), forming the loop of the lariat and simultaneously separating the first exon from the intron 2 step compeltes the splicing process – the 3’ hydroxylgroup left at the end of the 1 exon st attacks the phosphodiester bond linking the intron to the 2 exon (the 3’ splice site). This forms the exon-exon phosphodiester bond and releases the intron, inlariat form, at the same time 2 Fig 14.11: Demonstration of a critical signal within a yeast intron - The wild type gene contained 2 exons (blue & yellow) the intron contained a conserved sequence (red) found in all yeast introns. Yeast cells spliced this gene properly - Mutant #1 – deleted the conserved intron sequence, which destroyed the ability of this gene’s transcript to be spliced - Mutant #2 – had extra, non-intron DNA (pink) inserted into the intron downstream of the conserved intron sequence. The transcript of this gene was aberrantly spliced to the first AG within the insert - Mutant #3 – moved the conserved intron sequence downstream into the second exon. The transcript of this gene was aberrantly spliced to the first AG downstream of the relocated conserved sequence. - These experiments suggested that the conserved sequence is critical for splicing and that it designates a downstream AF as the 3’-splice site - The special intron sequence is so important because it contains the branch point adenine nucleotide: the final A in the sequence UACUAAC Spliceosomes - Spliceosomes contain the pre- mRNA, and also contain many RNAs and proteins - Some of these RNAs and proteins come in the form of small nuclear ribonucleoproteins (snRNPs “snurps”) - Both snRNAs and the protein splicing factors are the agents that recognize these splicing signals 3 Intron splicing, removal of introns from mRNA precursors, is catalyzed by a macromolecular machine, the spliceosome. U1 snRNP recognizes the 5’-splice site first, tand then is replaced by U6 snRNP. U2 snRNP recognizes the branchpoint, and the protein U2AF (U2-associated factor) recognizes the 3’-splice site. U5 snrNP binds to the 5’ and 3’ splice sites after initial recognition by other factors - Base pairing between the splice site and U1 is required for splicing Fig. Alignment of wild-type and mutant 5’-splice sites with wild-type and mutant U1 snRNAs a) 12S splice site mutation. The wild-type and mutant sequences are identified at right. Watson-Crick base pairs between the mRNA precursors and U1 RNA are represented by bertical lines; wobble base pairs, by dots. Mutated bases are represented by red letters. The end of the exon is represented by an organ box b) 13S splice site mutation. All symbols the same except that the end of the exon is represented by a blue box Figure. A Model for interaction between a yeast 5’ splicce site and U6 snRNA - The invariant ACA (nt 47-49_ of yeast U6 base pairs with the UGU (nt 4-6) of the intron 4 Figure. Base pairing between yeast U2 and yeast branchpoint sequences
More Less

Related notes for BIOL 367

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.