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Continuation of Chatper 11 and CHapter 12 (Summer2011)

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Biological Sciences
Rosada Silva

BIOB11Summer2011 Chapter 11 Translation and Chapter 12 The Nucleus and Control of Gene Expression DNA to RNA to Protein o Clear association between genes and proteins was made one geneone polypeptide hypothesis o Do chromosomes participate in protein productionNO In eukaryotes DNA isnt found outside the nucleus an intermediate messenger is needed o RNA suggested as intermediateit is found in both the nucleous and the cytoplasm and the RNA concentration correlates with protein production o When cells are fractioned and separated by centrifugationa small microsomal fraction contained most new protein synthesismostly RNA associated with ribosomesThe Genetic Code o 20 amino acidsonly 4 unique bases in mRNA o Information stored in gene is present in the form of genetic codebut how o Only 4 nucleic acids in mRNA A C G Uthis means it cant be one base per amino acid or 2 bases per amino acid the smallest possibility is 3 bases per amino acid 4364TRIPLET CODE o Francis Crick and Syndney Brenner 1961 showed bacteriophages with singlebase insertions or deletions in a gene did not produce functional proteincalled frameshift mutations o Mutations of 24 nucleotide insertions and deletions were also nonfuntional o BUT some insertions and deletions of 3 nucleotides were functional o How could a 3 nucleotide be code be readoriginal sequence AGCATCG o Overlapping code AGC GCA CAT ATC TCG o Non overlapping code AGC ATC o If the code were overlapping a mutation in one nucleotide it would change more than one aminoe acid therefore the codes are not read overlappingly o Triplet code codon can specify 64 different amino acidsbut there are only 20 o Some of the amino acids must be coded for by more than one codondegenerate code o Nirenberg and Matthaei 1962 made artificial mRNA and identified the polypeptide producedused enzyme polynucleotide phosphorylase which randomly joins together and RNA nucleotides aroundo They first used artificial mRNA containing only one type of nucleotide o In further experiments artifial RNAs containing 2 or 3 different bases were made and figured out the different sequences possible o Khorana 1965 chemically synthesized RNA of known repeating sequences and saw what amino acids it produced but didnt know which one coeded for which o Nirenberg and Leder used ribosomebound tRNAs and short mRNAs with known codonsthe tRNAs in the mixture was then to find with valine therefore since they knew the code of the tRNA they figured out which was the code for valine o The genetic code is essentially universalsame in almost or organisms on earth rdo The first 2 codon bases for aparticular amino acid are invariant whereas the 3 may varywobble baseit acts as a safeguard spontaneous mutations often dont cause an amino acid changeo Another safeguardamino acids with similar properties cluster o Synonymous mutations no amino acid changeo Nonsynonymous mutationsamino acid change1
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