from gene to protein

• Alkaptonuria sufferers 


  urine darken from yellow to brown after exposure to air
• Alkaptonuria is inherited as an autosomal recessive disease
• Garrod studied alkaptonuria
• Garrod's observations

  • urine in alkaptonuria patients contains large amounts of homogentisic acid
  • alkaptonuria is inherited through an autosomal recessive gene
• Garrod's hypothesis

  • alkaptonuria patients lack the enzyme to break down homogentisic acid
  • lack of the enzyme is due to defective gene
• alkaptonuria is caused by the missing enzyme named

  homogentisic acid oxidase
• Garrod's conclusions

  • defects in genetic material can lead to specific diseases, which can be inherited
  • Mendelian genetic inheritance can be observed in humans
  • lack of enzyme is due to defect in a gene
• Beadle and Tatum's experiment

  • to find out is one gene, one enzyme was true
  • Drosophila as model organism was to complex
  • instead used Neurospora (bread mould)
• one gene, one enzyme was rejected due to

  not all proteins being enzymes, are structural proteins therefore accepted the one gene, one protein
• one gene, one protein was rejected and one gene, one polypeptide was accepted as


  some proteins are made of more than one polypeptide chain
• DNA is found in the nucleus, proteins are made in the cytoplasm
• messenger between DNA and proteins is RNA
• RNA structure

  • ribose sugar
  • bases (adenine, guanine, cytosine and uracil)
  • usually single stranded
• Pulse-chase experiments provided evidence for messenger RNA

  • proved that RNA is made in the nucleus and moves into the cytoplasm
• 2 main steps in gene expression

  • transcription (4 bases to 4 bases)
  • translation (4 bases to 20 amino acids)
• gene expression

  labels

  A) transcription

  B) translation

  C) mRNA

  D) proteins

  
• transcription is the synthesis of a mRNA molecule using one strand of DNA as a template, catalysed by RNA polymerase
• transcription rules 


  G pairs with C

  A pairs with U
• RNA polymerase doesn't require a primer
• 3 stages of transcription

  • initiation
  • elongation
  • termination
• RNA polymerase moves along the DNA template, unwinding double helix and catalysing addition o ribonucleotides to 3' end of the growing molecule
• termination signal for transcription is a termination signal in the newly produced RNA (terminator)
• translation from RNA to protein must have 3 letter code as 1 base would only provide 4 possible amino acids, 2 bases would provide 16 amino acids, 3 bases would provide 64 (this is sufficient for the amino acids to be encoded)
• Crick and Brenner proved that the code is 3 letters (groups of 3 bases are called codons)
• UUU was the first codon to be proved (codes for phenylalanine) using artificial mRNA consisting entirely of uracil
• 61 of the 64 codons code for amino acids
• 3 codons are stop codons, signalling the end of translation, these are

  UAA, UAG and UGA
• the genetic code is specific but redundant
• genetic code is specific because each codon only specify one amino acid
• amino acids can be specified by more than one codon (so is redundant)
• genetic code is (almost) universal meaning

  must have been established very early during evolution
• translation always begin at the start methionine and the correct reading frame continues from there
• replication is the synthesis of DNA
• transcription is the synthesis of the mRNA
• translation is the synthesis of protein using mRNA as template
• adapters are required to link mRNA and amino acids, these adapter are

  transfer RNAs
• tRNA structure

  • ~80 nucleotides in length
  • single stranded (base pairs form within the chain)
  • clover leaf structure (makes L shape molecule)
  • anticodon at one end, base-pairs with the codon
  • amino acid attachment site is the 3' hydroxyl group at the end of the RNA chain
• tRNA

  labels

  A) amino acid attachment site

  B) anticodon

  
• each tRNA is specific for a single amino acid determined by its anticodon
• how does tRNA become attached to the correct amino acid?


  specific attachment carried out by amino-acyl tRNA synthetases, which are activating enzymes