Current LectureMCB 250 1st Edition Lecture 16Outline of Current Lecture 1. Ribosome2. Genetic code3. Wobble4. Codon5. TranslationCurrent Lecture1. Ribosome- Machine that does protein synthesis- Prokaryotic ribosome = 70s composed of 2 subunits( 30s and 50s)- Ribosome is a RNA based machine: it is a ribozyme2. Genetic code- Relates the sequences in the bases in the mRNA to protein sequence. There are 20 amino acids and only 4 bases.- Triplet code- The code is degenerate: most amino acids are specified by more than 1 codon- The code is highly evolved to minimize the consequence of errors in replication/making RNA- Reading frame: String of bases, triplet code is read 3 at a time. - Real open reading frame doesn’t have any stop codon for a while, but almost always starts with AUG (or ATG)3. Wobble- E. Coli only has 46 different tRNA’s that are capable of reading 61 different codons- Some tRNA’s recognize multiple codons for the same amino acids- The first and 2nd bases of the codon for the “Watson-crick base pair”- 3rd space = wobble. There is variability base pairing with the 3rd codon.- There are modified bases in the tRNA. Most of the modifications are designed to help the tRNA to fold up or interact with the ribosome appropriately. Mostly codon-anticodonitself is modified.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- Post transcriptionally the A is deaniminated to give an I- Anticodon is UCU 5’ to 3’!!!- The 3rd base of the codon is base pairing with the first base of the anticodon- 3rd base of the anticodon has a bit of wiggle room/wobble. It can shift up a little and get a base pair with G or U- Limited number of tRNA’s, many of them can recognize more than 1 codon. 4. Codon Usage- 6 leucine codons. They aren’t all used equally.- Multiple tRNA genes that encode for the same tRNA. 46 tRNA’s 80 genes. Makes sense due to codon usage5. Genetic Code:- The genetic code is Universal. All organisms use the same codon table, with some exceptions- Mitochondria are degenerate gram negative bacteria and are stripped down to nothing. Strip down translation machinery. They only encode 5 proteinso Separate machinery. UGA specifies Trp instead of stop codon- In viruses (HIV) makes one transfer, one protein, starts at AUG and synthesizes the protein. Signals in there that causes the ribosome to mess up and switch frames- Translate new protein that keeps going the entire length of the mRNA. Makes half lengthand entire length of virus. The proteins are protolytically cleaved to make various proteins of the virus. Major drugs to fight HIV = protease inhibitors because keeps the polyproteins from being cleaved. The fame shift that occurs at some frequency is critical to the life cycle of HIV. - Sometimes termination codon is skipped and can make a longer proton. There are many changes that can change the code happens late in the evolution. 6. Translation- Initiation: prokaryotic and eukaryotic translation differs fundamentally- There are ribosome binding site (shine dalgarno sequence) that base pairs with the 16s of the small ribosomal subunit- Need initiation codon (aug) and factors and initiator tRNA. - How does it position start codon? Upstream of the start codon, there is a sequence called shine dalgarno/ ribosomal binding site. It can base pair with a region of the
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