From Gene to RNA Kanokporn Boonsirichai Molecular Biology of the Cell Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter.
Figure 7-10 Essential Cell Biology (© Garland Science 2010) Orientation matters in transcription
Question to consider Could RNA polymerase used for transcription be used as the polymerase (primase) that makes RNA primer required for replication? Why or why not?
RNA Processing Kanokporn Boonsirichai Molecular Biology of the Cell Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter.
Figure 7-17 Essential Cell Biology (© Garland Science 2010) Gene organization Bacteria Uninterrupted coding sequence Genes are grouped together in an “operon”. Eukaryotes Coding sequence is interrupted by “introns”. Coding sequence is broken into pieces of “exons”. gene A gene Bgene C promoter
mRNA Processing Pre-mRNA Mature mRNA
Eukaryotic mRNA Processing Capping Addition of 7-methylguanosine at the 5’ end Splicing Removal of introns 3’cleavage Cleavage of the transcript at the 3’end of the gene Polyadenylation Addition of adenine ribonucleotides at the 3’ end
1. Addition of 5’ cap
2. Pre-mRNA Splicing: removal of introns Splicing signals are located in the transcript itself. R = purine (A or G) Y = pyrimidine (C or T) N = any one of the 4 bases
2’-OH is utilized to form a lariat structure Branch point
Spliceosome A complex of snRNPs (small ribonucleoproteins)
Interactions among snRNPs are mediated by their snRNAs
How is Fidelity Achieved? snRNAs are associated with snRNPs. Several components of the spliceosome are carried on the phosphorylated tail of RNA polymerase(to keep track of introns and exons). The “exon definition hypothesis” Exon size is somewhat uniform (averaging about 150 nucleotide pairs). Exon-intron boundaries are marked with spliceosome components
Other Splicing Systems Transplicing => attachment of two different RNAs
What might be the importance of RNA splicing?
Alternative splicing allows for the production of multiple related proteins from a single transcript.
Processing of the 3’ end Cleavage factors Poly-A polymerase
Cleavage signals Poly-A addition
Mature mRNAs are exported from the nucleus
Questions to consider What do you think might be the reasons for the eukaryotes to have their mRNA processed?