RNA Processing & Protein Synthesis Lecture 11

RNA Processing & Protein Synthesis Sections 6.3, 8.1 RNA Processing Learning Objectives Describe the functions of each of the eukaryotic RNA processing events 5’ cap polyA tail mRNA splicing Describe the mechanism and purpose of alternative splicing. Explain how m...

RNA Processing & Protein Synthesis Sections 6.3, 8.1 RNA Processing Learning Objectives Describe the functions of each of the eukaryotic RNA processing events 5’ cap polyA tail mRNA splicing Describe the mechanism and purpose of alternative splicing. Explain how mRNA degradation can be regulated by the environment. RNA Processing Key Concepts Eukaryotic pre-mRNAs are modified by the addition of a 5’ 7–methylguanosine cap and a 3’ poly-A tail. In addition, introns are removed by splicing through the action of the large ribozyme complex called the spliceosome. snRNAs in the spliceosome recognize splice sites in the pre-mRNA and catalyze the splicing reaction. Exons can be joined in various combinations through alternative splicing, which provides an important mechanism for tissue-specific control of gene expression. mRNA sequences can be modified by RNA editing (deamination of Cytosine or Adenosine) mRNAs in eukaryotic cells are degraded at different rates which contributes to the mechanisms available for control of gene expression. RNA Processing occurs in eukaryotes Eukaryotic mRNAs are processed in the __________________ before being exported to the _____________________ RNA processing begins while the RNA molecule is being transcribed The tail of RNA polymerase II is ___________________________, which allows processing proteins to assemble. Addition of the 5’ cap 5’ cap: Functions: Addition of the 3’ polyA tail 3’ Poly A Tail: Functions: Splicing of pre-mRNA removes _________, keeps _________ Function: Carried out by: ________________ composed of ____________ RNAs (snRNAs) complexed with proteins to form __________________ __________________ (snRNPs) The spliceosome is made of snRNAs complexed into snRNPs _____________ splicing allows for different combinations of exons CASE STUDY: Spinal Muscular Atrophy Mutations that disrupt critical splice sites can lead to disease. Example: Spinal Muscular Atrophy – Neurodegenerative disease; loss of motor neurons – Most common monogenic cause of death in infants – Caused by a mutation that disrupts proper mRNA splicing of SMN – Ultimately results in a truncated (shortened), non-functional SMN protein CASE STUDY: Spinal Muscular Atrophy  CASE STUDY: Spinal Muscular Atrophy Discuss: The SMN2 coding region is often duplicated in the genome. If there are 3+ copies of the SMN2 gene, patients have a mild disease phenotype. Why do you think this is true?  CASE STUDY True or False: When SMN2 is processed, exon 7 is effectively treated like an intron 80% of the time. a) True b) False Join.nearpod.com pin =  CASE STUDY Which of these could explain why SMN2 is processed differently than SMN1? a) The C in SMN1 is likely important for the 5’ splice site of exon 7 b) The C is SMN1 is likely important for the 3’ splice site of the intron between exons 6 and 7 c) The T in SMN2 removes a lariat branchpoint from the intron Join.nearpod.com pin =  CASE STUDY Which of the following could potentially be an effective drug treatment for SMA? a) Antisplicing morpholino, which blocks the splice site between exons 7 and 8 b) Nusinersen, a short segment of nucleotides that blocks removal of exon 7 c) ETS, an activator protein that binds the enhancer of the SMN1 gene Join.nearpod.com pin = Promising SMA treatments based on understanding of underlying basic biology Nusinersen, a short Risdiplam is another segment of nucleotides drug that modifies that blocks the splicing splicing of this transcript site to force inclusion of exon 7 in SMN2 Introducing a functional copy of SMN1 to the genome! mRNA degradation Prokaryotes: mRNA is rapidly degraded (half-life ~2-3 minutes) Eukaryotes: mRNA stability variable (minutes to hours) WHY the difference?  NOTE CHECK Mammalian mRNAs have different lifetimes, ranging from less than 30 minutes to approximately 20 hours. This is a form of regulation of gene expression. One would expect the shorter-lived mRNAs to code for: a. regulatory proteins. b. structural proteins. c. proteins involved in basic metabolism. NOTE CHECK d. general transcription factors Join.nearpod.com pin = The final product: a mature mRNA molecule  NOTE CHECK You are examining a eukaryotic cell line and determine that it is producing abnormally long mRNA molecules. Which of the following defects could explain this phenotype? a) The enzyme that adds the 5’ cap is not functioning b) The enzyme that adds the poly-A tail is missing c) One of the snRNPs involved in splicing is operating at very low efficiency NOTE CHECK Join.nearpod.com pin = Protein Synthesis Sections8.1 Protein Synthesis Learning Objectives Explain the role of tRNAs in translation Describe the structure and function of ribosomes Contrast the initiation of translation in bacterial and eukaryotic cells Outline the events of initiation, elongation, and termination of translation Summarize the mechanisms that regulate translation of specific mRNAs and global translation rates Protein Synthesis Key Concepts tRNAs serve as adaptors that align amino acids on the mRNA template. Peptide bond formation is catalyzed by rRNA in the ribosome. Translation initiation is different in prokaryotes and eukaryotes due to the lack of a nucleus in prokaryotes. In both cases, the ribosomal small subunit and initiator tRNA first interact with the mRNA before the large ribosomal subunit joins the complex. Translation begins at a start codon and ends at a stop codon. These are specific sequences found in the mRNA. Translation of mRNAs can be regulated by repressor proteins and miRNAs Global translational activity can be regulated by modification of translation initiation factors. Translation is literally switching languages! *Can you predict the anticodon sequence on the tRNA corresponding to a given Image source: Khan Academy codon? Attachment of amino acids to tRNAs 20 amino acids to be specified 64 codons 61 code for amino acids 3 code for stop codons 40 tRNAs Some tRNAs recognize more than one codon! HOW? Nonstandard codon–anticodon base pairing: “_____________” Which base position has the most relaxed pairing standards? Ribosomes are made of structural _________ and catalytic _____ Overview of translation: three key stages Translation initiation signals differ in eukaryotes and prokaryotes Prokaryotic: Eukaryotic: Prokaryotic mRNAs can be ___________________  NOTE CHECK Which of the following is NOT a functional implication of the difference between prokaryotic and eukaryotic RNA processing and ribosome recognition strategies? a) Prokaryotic translation can begin while genes are still in the process of being transcribed b) Prokaryotic cells could not translate eukaryotic transcripts, and vice versa c) Multiple genes/proteins in a related process can controlled under a single eukaryotic promoter NOTE CHECK Join.nearpod.com pin =