Genetics Exam 3 Study Guide Genetic Code Transcription Chapter 12 1 Transcription process by which base sequence in DNA is converted into RNA The enzyme responsible is RNA polymerase 2 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 a Read section 12 3 i ii iii in vitro protein synthesizing system along with the ability to produce synthetic mRNAs using polynucleotide phosphorylase an RNA polymerase Nirenberg and matthei added RNA homopolymers to the in vitro translation system to decipher which amino acids were encoded by the first few codons based on which amino acids were incorporated into the polypeptide The deciphering of the genetic code determining the amino acid specified by each triplet was one of the most exciting genetic breakthroughs of the past 50 years Once the necessary experimental techniques became available the genetic code was broken in a rush The first breakthrough was the discovery of how to make synthetic mRNA If the nucleotides of RNA are mixed with a special enzyme polynucleotide phosphorylase a single stranded RNA is formed in the reaction No DNA is needed for this synthesis and so the nucleotides are incorporated at random The ability to synthesize mRNA offered the exciting prospect of creating specific mRNA sequences and then seeing which amino acids they would specify The first synthetic messenger obtained poly U was made by reacting only uracil nucleotides with the RNA synthesizing enzyme producing UUUU In 1961 Marshall Nirenberg and Heinrich Matthaei mixed poly U with the proteinsynthesizing machinery of E coli in vitro and observed the formation of a protein The main excitement centered on the question of the amino acid sequence of this protein It proved to be polyphenylalanine a string of phenylalanine molecules attached to form a polypeptide Thus the triplet UUU must code for phenylalanine iv b 3 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 a Base transition mutations in which a purine is switched with another purine A with G or a pyrimidine is switched with another pyrimidine U with C b Base transversion mutations in which a purine is switched with a pyrimidine or vice versa Base transitions are 5 10 times more common than base transversions 4 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 a Organization of the genetic code i i The genetic code is written in linear form using the ribonucleotide bases that compose mRNA molecules as letters The ribonucleotide sequence is derived from the complementary nucleotide bases in DNA ii iii iv v vi Each word within the mRNA consists of three ribonucleotide letters referred to as a triplet code With several exceptions each group of three ribonucleotides called a codon specifies one amino acid The code is unambiguous each triplet specifies only a single amino acid The code is degenerate that is a given amino acid can be specified by more than one triplet codon This is the case for 18 of the 20 amino acids The code contains one start and three stop signals triplets that initiate and terminate translation No internal punctuation such as a comma is used in the code Thus the code is said to be commaless Once translation of mRNA begins the codons are read one after the other with no breaks between them 1 vii viii 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 With only minor exceptions almost all viruses prokaryotes archea and eukaryotes use a single coding dictionary ix DNA is transcribed to messenger RNA mRNA and the mRNA is translated to proteins on the ribosomes A sequence of three nucleotides on an mRNA molecule is called a codon Most codons specify a particular amino acid to be added to the growing protein chain In addition one codon codes for the amino acid methionine and functions as a start signal Three codons do not code for amino acids but instead function as stop signals 1 Nearly every mRNA gene that codes for a protein begins with the start codon AUG and thus begins with a methionine Nearly every protein coding sequence ends with one of the three stop codons UAA UAG and UGA which do not code for amino acids but signal the end of translation 2 During translation nucleotide base triplets codons in mRNA are read in sequence in the 5 3 direction along the mRNA Amino acids are specified by the string of codons 3 Before mRNA can be translated into an amino acid sequence the mRNA must first be synthesized from DNA through transcription Base pairing in mRNA synthesis follows slightly different rules than in DNA synthesis uracil U replaces thymine T in pairing with adenine A The codons specified by the mRNA are then translated into a string of amino acids i The code is degenerate and ordered protects against point mutations ii Most mutations base transitions affecting the third position do not change the amino acid iii Mutations base transitions affecting the second position iv often result in replacing once amino acid with a similar one Base substitution can be 1 Missense changes amino acid and results in altercation of the translation product 2 Nonsense changes an amino acid codon into a stop codon 3 Silent no amino acid change b Organized to protect 5 Know what the start codon is and what amino acid it codes for a 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 6 Know the requirements of RNA polymerase as an enzyme template no primer substrate direction of synthesis nucleotides used a 5 to 3 synthesis b DNA template c No primer needed d Substrate same as DNA polymerase EXCEPT substrate nucleotides contain ribose rather than deoxyribose form of the sugar e Nucleoside monophosphates are the nucleotides used 7 Know what the promoter is what it does Know that bacterial promoters contain two sequences 10 TATA and the 35 sequence that RNA
View Full Document
Unlocking...