Genetic Code Transcription Chapter 12 Transcription process by which base sequence in DNA is converted into RNA The enzyme responsible is RNA polymerase Understand how the genetic code was deciphered using synthetic mRNAs and in vitro translation Be able to determine possible codon assignments in experiments involving repeating copolymers as the mRNA In vitro means to be cell free When deciphering the genetic code using synthetic mRNAs and in vitro translation there are a few essential factors for the synthesis to happen The in vitro mixture must contain ribosomes tRNAs amino acids and other molecules essential to translation One or more of the amino acids must be radioactive in order to follow protein synthesis The mRNA is then added to serve as the template that will be translated Possible codon assignments in experiments involving repeating copolymers as the mRNA include mRNA UUUUUU AAAAAA CCCCCC GGGGGG Amino Acid Abbreviation phenylalanine phe lysine lys proline pro not an adequate template Know what base transition and base transversions are Know that adenine and guanine are purines and that cytosine and thymine are pyrimidines Know that base substitution mutations are often transitions than transversions Base Transitions Mutations in which a purine is switched with another purine A with G or a pyrimidine is switched with another pyrimidine U with C Base Transversions Mutations in which a purine is switched with a pyrimidine or vice versa Base transition mutations are 5 10 times more common than base transversions Know the organization of the genetic code and its important features degenerate universal and understand how the genetic code is organized to protect against the effects of base substitution type point mutations that are transitions at the 3rd and 2nd codon positions The genetic code was made into a coding dictionary based off the first second and third position of each triplet codon Francis Crick illustrated it here When looking at the code one can notice there is a pattern of degeneracy and that it is ordered In a set of codons specifying the same amino acid the first two letters are the same with only the third differing The code being degenerate and ordered protects against point mutations This genetic code is universal with a few exceptions Base substitution mutations Missense Mutation Changes the amino acid Nonsense Mutation Changes an amino acid codon into a stop codon Silent Mutation No amino acid change Know what the start codon is and what amino acid it codes for The only start codon there is AUG which codes for methionine met Know the requirements of RNA polymerase as an enzyme template no primer substrate direction of synthesis nucleotides used RNA polymerase is similar to DNA polymerase except The substrate nucleotides contain ribose instead of deoxyribose form of the sugar DNA template used No primer is required to initiate synthesis Synthesis direction is 5 to 3 Know what the promoter is what it does Know that bacterial promoters contain two sequences 10 TATA and the 35 sequence that RNA polymerase binds to initiate transcription Promoter An upstream regulatory region of a gene to which RNA polymerase binds prior to the initiation of transcription The 10 TATA is rich in adenine and thymine hence the letters used and is located 10 nucleotides upstream from the site of initial transcription The 10 region is known as the Pribnow box The 35 sequence is TTGACA and is located 35 nucleotides upstream Know what the three steps in the processing of eukaryotic mRNAs and what their functions are 1 Capping Occurs at the 5 end where a 7 methylguanosine 7 mG It is added before synthesis of the initial transcript is complete It protects the 5 end of these molecule from nuclease attack It is added after the 3 end of the initial transcript is cleaved enzymatically at a position some 10 to 35 ribonucleotides from a highly conserved AAUAAA sequence 2 Addition of poly A tail a The 5 cap and poly A tail are necessary for mRNA stability and transport out of the nucleus 3 Removal of introns splicing sequences within the genes Exons are the coding sequences In this step he introns are spliced cut out and the exons join to create the mature mRNA Introns are intervening noncoding Be able to describe in general terms how splicing of mRNAs is done by the spliceosome Know which parts of the intron are important in RNA splicing Spliceosome a article of enzymes that cuts at the intron exon borders and joins the exons The spliceosome recognizes the sequence at the intron exon borders and branch point pulls the exons together and promotes rearrangement of the phosphodiester bonds Basically the intron is cut out and the exons are joined Be able to draw the structure sequence organization of typical monocistronic prokaryotic and eukaryotic gene showing the location of the promoter the transcription start and termination sites and the translation start and stop codons location of introns and exons location of the polyadenylation signal Translation and Proteins Chapter 13 Know the major components of the translation apparatus mRNA tRNAs 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 In the translation apparatus the mRNA Sends the message by presenting a triplet codon that calls for a specfic amino acid tRNA Brings the amino acids to the ribosomes by adapting specific codons in the mRNA to their correct amino acid Ribosomes The site of translation of mRNA codons into the amino acid sequence Differences in prokaryotic and eukaryotic ribosomes 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 Before translation can proceed the tRNA molecules must be chemically linked to their respective amino acids through the process known as tRNA charging The enzymes that carry out this reaction are called aminoacyl tRNA synthetases There are 20 different aminoacyl tRNA synthetases each correlating with one amino acid The enzyme recognizes the amino acid and all the tRNAs for it 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
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