PCB3063 General Genetics Exam 3 Study Guide Fall 2014 Chapter 12 Genetic Code Transcription Transcription Process by which base sequence in DNA is converted into RNA The enzyme responsible is RNA polymerase q 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 RNA homopolymers containing only one type of nucleotide were synthesized Each mRNA was tested to determine which amino acids were incorporated into the newly synthesized proteins UUU poly phenylalanine AAA poly lysine Specific triplet codon assignments were possible because homopolymers were used q 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 1 PCB3063 General Genetics Exam 3 Study Guide Fall 2014 Base transition mutations are more 5 10 times more common than base transversions Purines Adenine Guanine Pyrimidines Cytosine Thymine q 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 Characteristics of the Genetic Code Genetic code is written in linear form using ribonucleotide bases that compose o Ribonucleotide sequence is derived from the complementary nucleotide mRNA molecules as letters bases in DNA Each word within the mRNA consists of three ribonucleotide letters triplet code o Each group of three ribonucleotides codon specifies an amino acid The code is unambiguous each triplet specifies only ONE amino acid The code is degenerate a given amino acid can be specified by more than one triplet codon o Applies to 18 of the 20 amino acids The code contains one start and three stop signals triplets that initiate terminate translation The code is commaless NO internal punctuation is used in the code o Once translation of mRNA begins the codons are read one after the other with no breaks between them THE code is nonoverlapping once translation commences any single ribonucleotide at a specific location within the mRNA is part of only one triplet The code is nearly universal almost all viruses prokaryotes archaea eukaryotes use a single dictionary o With only minor exceptions The genetic code is degenerate ordered Degenerate One amino acid can be specified by more than one triplet Unambiguous A single triplet specifies only ONE amino acid Ordered Genetic Code Chemically similar amino acids often share one or two middle bases in the different triplets encoding them o Ex U or C often present in the second position of triplets that specify hydrophobic amino acids Protects against point mutations Most mutations base transitions affecting the 3rd position do NOT change the amino acid Mutations base transition affecting the 2nd position often result in displacing one amino acid with a similar one 2 PCB3063 General Genetics Exam 3 Study Guide Fall 2014 q Know what the start codon is and what amino acid it codes for Start Initiation Codon The nucleotide triplet AUG that in an mRNA molecule codes for incorporation of the amino acid methionine as the first amino acid in a polypeptide chain q Know the requirements of RNA polymerase as an enzyme template no primer substrate direction of synthesis nucleotides used RNA polymerase is the enzyme that directs RNA synthesis 5 to 3 synthesis DNA template NO primer required to initiate synthesis Recognizes binds to promoter sequence Initial base a nucleotide triphosphate NTP o NTPs serve as substrates for the enzyme Catalyzes the polymerization of nucleoside monophosphates NMPs or nucleotides into a polynucleotide chain NMP n o Nucleotides linked during synthesis by 3 to 5 phosphodiester bonds o Energy created from cleaving the triphosphate precursor into monophosphate form drives the reaction Inorganic pyrophosphates are produced q 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 q Know what the three steps in the processing of eukaryotic mRNAs and what their functions are Transcription in eukaryotes is more complex Transcription translation are separated spatially Three different RNA polymerases Eukaryotic promoters large complex RNA is processed after transcription RNA Processing 1 Capping 2 Addition of polyA 3 Removal of introns splicing 5 cap polyA tail necessary for mRNA stability transport out of the nucleus 3 PCB3063 General Genetics Exam 3 Study Guide Fall 2014 a Introns are cut out i Introns Intervening noncoding sequences within the gene b Exons are joined i Exons Coding sequences q Be able to describe in general terms how splicing of mRNAs is done by the spliceosome Introns major component of the nuclear derived pre mRNA transcripts of eukaryotes Spliceosome Huge molecular complex particle of enzymes that cuts at the intron exon borders joins exons Recognizes the sequence at the intron exon borders branch point Pulls the exons together promotes rearrangement of the phosphodiester bonds 4 PCB3063 General Genetics Exam 3 Study Guide Fall 2014 Intron is cut out Exons are joined q Know which parts of the intron are important in RNA splicing The branch point 5 splice site GU 3 splice AG site are important in RNA splicing 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 5 PCB3063 General Genetics Exam 3 Study Guide Fall 2014 Chapter 13 Translation Proteins q 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
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