Genetics - Lecture 10

• The genetic code is written in linear form, using the ribonucleotide bases that compose mRNA molecules as “letters”
• Genetic code


  Written in linear form, using the ribonucleotide bases that compose mRNA molecules as "letters"
• Sequence of RNA


  Derived from the complementary bases in the DNA
• mRNA


  Triplet codons specify one amino acid
• Genetic code


  Contains "start" and "stop" signals, certain codons (nonsense codons) that are necessary to initiate and to terminate translation
• Characteristics of the genetic code

  • Unambiguous
  • Degenerate
  • Commaless
  • Nonoverlapping
  • Nearly universal (both prokaryotes and eukaryotes)
• mRNA


  Serves as an intermediate in transferring genetic info from DNA to proteins
• Triplet code Provides 64 (- 3 stop codons) to specify the 20 amino acids
• The triplet nature of the code was revealed by frameshift mutations
• Genetic code Reads 3 nucleotides at a time in a continuous linear manner, the code is non-overlapping and commaless
• Genetic code


  Degenerate, which means that some amino acids are specified by more than one code
• Nonsense codons Don't specify an amino acid
• Studies by Nirenberg, Matthaei, and others led to deciphering of the code
• Triplet binding assay - Ribosomes bind to a single codon of 3 nucleotides, and the complementary amino acid-charged tRNA will be able to bind
• AUG is the start codon. Every single mRNA will have AUG in the beginning. No matter prokaryote or eukaryote
• Long RNAs with di-, tri-, and tetranucleotide repeats were used for in vitro translation to determine more codon assignments
• The coding dictionary reveals several interesting patterns among the 64 codons
• Genetic code is Degenerate, with many amino acids specified by more than 1 codon. Only tryptophan and methionine are encoded by a single codon
• Stop codons


  UAA, UGA, UAG
• Wobble hypothesis Predicts that the initial 2 ribonucleotides of triplet codes are often more critical than the 3rd
• The genetic code shows order in that chemically similar amino acids often share 1 or 2 middle bases in the triplets encoding them
• The initial amino acid incorporated in all proteins is a modified form of methionine- N-formylmethionine (fmet)
• AUG is the only codon to encode for methionine
• When AUG appears internally in mRNA, an unformylated methionine is inserted into the protein
• 3 codons (UAG, UAA, and UGA) serve as termination codons and don't code for any amino acid
• The genetic code has been confirmed in studies of Phage MS2
• MS2 is a bacteriophage with only 3 (now actually 4; genes to make- lysis protein, coat protein, maturation protein, and replace protein) genes on a 3500-base RNA genome
• Sequencing of the gene products confirmed the genetic code
• These genes specify a coat protein, an RNA-directed replicase, and a maturation protein
• mtDNA revealed some exceptions to the universal genetic code. UGA is NOT a stop codon here.
• Different initiation points create overlapping genes
• In some viruses, overlapping genes (open reading frame ORF) have been identified in which initiation at different AUG positions out of frame with one another leads to distinct polypeptides
• Transcription synthesizes RNA on a DNA template
• RNA is synthesized on a DNA template by the process of transcription
• The genetic information stored in DNA in transferred to RNA, which serves as the intermediate molecule between DNA and proteins
• Studies with bacteria and phages provided evidence for the existence of mRNA
• Analysis of RNA produced immediately after bacteriophage infection of E. coli shows that the base composition of the newly synthesized RNA resembles that of the phage DNA and not that of the bacterial host. This suggests that RNA synthesis may be a preliminary step in protein synthesis.
• RNA polymerase directs the synthesis of RNA using a DNA template. No primer is required for initiation, and the enzyme uses ribonucleotides instead of deoxyribonucleotides.
• RNA polymerase from E. coli contains the subunits, a, B, B’, w, and o
• Active form of RNA polymerase contains a2, B, B’, and o with a molecular weight of 500,000 Da