Exam 4 Review Chapter 13 Translation and Proteins Know the major components of the translation apparatus mRNA tRNA and ribosomes know what their roles are in translation and be able to describe the important features of their molecular structures composition size structure important active sites The mRNA carries the information or message in its codons for specific amino acid tRNAs are responsible for bringing the amino acids to the ribosome in accordance to the code Carries an anticodon that corresponds to the codon on the mRNA strand 75 90 nucleotides Anticodons can attract each other and force it to form a loop upon itself Ribosomes complexes of proteins and RNAs About 10 000 in a bacterial cell more in eukaryotic Made of 2 subunits a large and a small Each subunit is made of rRNA and several proteins The Eukaryotic is much bigger than that of a Prokaryotic Euk 80S and Prok 70S Know what the tRNA charging reaction is what it does Know the name of the enzymes that carry out this reaction and what their specificity is they recognize and join the correct tRNA with the correct amino acid tRNA charging is the activation process that causes tRNA molecules to be linked to their Synthetase directs the linkage between tRNA and its respective amino acid each enzyme different enough to carry a specific amino acid for each codon Amino acid is converted to an activated form with an ATP molecule A covalent linkage is then formed between the 5 phosphate group of ATP and the respective amino acids Steps o o carboxyl end of ATP o o Forms a complex with the synthetase specific for each tRNA Next the amino acid is transferred to the appropriate tRNA and can then participate directly in protein synthesis Be able to name the three steps in protein synthesis in addition to tRNA charging chain initiation chain elongation chain termination and explain what happens in each In particular you should understand what happens in the A site and the P site of the ribosome Initiation small subunit binds to the mRNA at position of the start codon Involves the small ribosome subunit an mRNA molecule a specific charged tRNA GTP Mg2 and at least 3 proteinaceous Ifs that enhance the affinity for binding Small subunit binds to several initiation factors then binds to mRNA Small subunit binds to the AUG triplet sets reading frame P SITE Large ribosomal subunit combines GTP acts as the energy for the release of initiation factors Elongation Two sites are formed for tRNA elongation P and A sites Charged tRNA binds to the P site AUG The lengthening of the polypeptide chain by one amino acid at a time is called elongation New amino acids attached to tRNA enter the A site Enzymatic reaction occurs within the large subunit of the ribosome catalyzing the formation of the polypeptide chain Simultaneously the tRNA occupying the P site is broken This catalytic activity is caused by 23S rRNA of the large subunit aka ribozyme The uncharged tRNA previously in the P site enters the E site and causes the entire complex to shift over a site led by protein elongation factors A new 1 2 tRNA can now enter th A site HINT P site contains the polypeptide chain which is being built while the A site contains the amino acid 3 Termination is signaled by a stop codon such as UAG UAA or UGA do not code for an amino acid This leaves the A site empty and cues the GTP dependent release gfactors to cleave the polypeptide chain from the terminal tRNA and release it Ribosome then breaks into its separate subunits again Understand how the correct start codon is identified during the initiation of translation in prokaryotes in prokaryotes and in eukaryotes Know the importance of the Shine Dalgarno sequence what it does where it is located what organisms have it and the Kozak sequence Before the start AUG there is a sequence AGGAGG that precedes the initial AUG This sequence AGGAGG consisting of only purines is called the Shine Dalgarno sequence Located 7 10 bases before the start codon Found in bacteria met is different from tRNAmet and can be recognized primarily It can only enter the P site In Eukaryotes also have different tRNAmet for initiation and internal AUG Eukaryotes require a 5 guanosine cap for initiation tRNAi directly Small subunits binds to cap and slides down to the first AUG Kozak Sequence usually surrounds the first AUG ACCAUGG Helps facilitate initial binding of mRNA Understand the concept of reading frame and be able to identify all three reading frames in a piece of RNA See above for setting the reading frame Know the experiments done with the fungus Neurospora in which the auxotrophic mutants were isolated and characterized and from which it was concluded that each gene contains the information for one polypeptide Be able to explain how Beadle and Tatum s one gene one protein concept has had to be modified in view of modern knowledge Neurospora wild type can produce everything necessary for growth on its own on minimal media Beadle and Tatum bombarded it with xrays to induce mutations The mutants were unable to grow on minimal media but could grow on complete medium They isolated it amongst different minimal medias and found that it required amino acid for growth They isolated it across several amino acids and found that it needed tyrosine to grow Many other mutants were unable to make other metabolites o Because they could find a mutant that lacked a different enzymatically controlled reaction they created the one gene specifies one enzyme became one gene one protein With our current knowledge we know that rRNA and tRNA also play an important role in the process so it can be controlled by multiple factors Be able to solve problems in which the order of enzymes in a biochemical pathway is determined using auxotrophic mutants grown on minimal medium plus intermediates in the pathway DO HOMEWORK PROBLEMS Understand what genetic complementation means and how genetic crosses are used to identify alleles Complementation is when two different mutants are crossed to produce the wild type phenotype sucha as arg 2 3 which code for different enzymes so 2 steps from ornithine to citrulline in cycle above Know the pathway for the metabolism of phenylalanine and tyrosine and be able to identify where the pathway is blocked in the disorders phenylketonuria albinism and alkaptonuria and be able to explain why the genetic block results in the associated phenotype Be able to explain using specific examples why some alleles are recessive some
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