March3%20posting

March3%20posting - RNA processing: mRNA in Eukaryotes...

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RNA processing: mRNA in Eukaryotes Eukaryotic mRNA must be processed before translation, as it is still being polymerized. This includes chemical modification. After modification, the mRNA can be transported out of the nucleus. Modifications: 5 cap 3 poly(A) tail splicing Intron Intron Intron Exon Exon Modifications occur simultaneously.
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Methylated guanine 5 to 5 linkage (not an error!) Not encoded in DNA Capping enzyme Functions: Recognition by ribosome Protection 5 cap
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3 poly(A) tail No terminator sequence Imprecise termination AAUAAA polyadenylation site Endonuclease complex cleaves 10-30 nucleotides downstream of the polyadenylation site Poly(A) polymerase adds 100+ of A’s Function: mRNA stability
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THEFAT CAT ATETHE BIG RAT introns
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Splicing Most eukaryotic genes are interrupted by introns Introns removed after transcription Exons spliced together Exons contain the coding information of the protein Intron Intron Intron Exon Exon Which are the exons on the figure below?
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Demonstrating introns: re-annealing DNA and mRNA
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Spliceosome forms (complex of snRNPs and proteins) at exon- intron “splicing signals” on the pre-mRNA The splicing proteins cleave 5 junction, then 3 Join exons together Intron released and degraded in the nucleus Splicing:a closer look ( Wikipeadia )snRNPs: small nuclear ribonucleoproteins , are particles that combine with pre-mRNA and various proteins to form spliceosomes (a type of large molecular complex). SnRNPs "recognize" the places along a strand of pre-mRNA and are essential in the removal of introns . Exon 2 Exon 1
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Splicing Mutations in the DNA can affect splicing of the RNA and can cause genetic disease or produce genetic susceptibility to disease: cystic fibrosis - affects exocrine (mucus) glands In one of the forms of the disease, there is a mutation that changes the splicing of the gene for a particular membrane protein CFTR (which is a channel for chloride). The result is that the cells no longer have the CFTR. Position of error at splicing junction?
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Not to belabour the point…. Remember this slide? And this picture from the PRS question? exon intron Does the scale of the size of the introns and the exons seem reasonable and consistent with the pie-chart drawing?
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And another thing…. Have you ever heard of introns being described as “junk DNA”? Just because one doesn’t know the function of it, doesn’t validate labeling it as “junk”. And if you recall that regulatory sequences - like enhancers - can be located within introns, does it even make sense to refer to introns as “junk DNA”?
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rRNA transcription RNAP I: specific for making ribosomes (see below) Hundreds of copies of rRNA genes Transcription of rRNA genes takes place in a part of the nucleus: the nucleolus. All of rRNA genes are found there.
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tRNA transcription RNAP III Transcribes genes for tRNA and other small RNAs.
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March3%20posting - RNA processing: mRNA in Eukaryotes...

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