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Protein Synthesis

The Flow of Genetic Information
Plant scientists have worked out an explanation on why there is a difference between the tall and dwarf varieties of pea plants that Mendel could not explain. In this case, Dwarf peas lack the growth hormones called gibberellins that stimulates the normal elongation of stems. These peas fail to make their own gibberellins due to a missing protein, because they do not have a properly functioning gene for that protein.

In 1909, a British physician Archibald Garrod decided to look into a disease called "inborn error of metabolism". With research, Garrod reasoned and hypothesized that a gene dictates the production of a specific enzyme.

Beadle and Edward Tatum began researching with bread mold and mutants. Throughout all their research, their results provided strong support to the one gene-one hypothesis.
  • Their hypothesis states that the function of a gene is to dedicate the production of a specific enzyme.
More and more researchers learned more about proteins.
  • They made minor revisions to the one gene-one hypothesis.
  • Not all protein are enzymes, neither gene products
Many proteins are constructed from two or more different polypeptide chains.
  • each polypeptide is specified by its own gene
Beadle and Tatum's idea has therefore been modified as the one gene-one polypeptide hypothesis.

Basic Principles of Transcription and Translation
Genes provide the instructions for making specific proteins
  • a gene does not build a protein directly
  • Each polypeptide of a protein has monomers arranged in a particular linear order
  • The monomers are the 20 amino acids
Getting from DNA to protein requires two major stages, transcription and translation.

Transcription: the synthesis of RNA under the direction of DNA
  • During transcription, DNA provides a template for assembling RNA nucleotides.
  • Initiation: transcription factors mediate the binding of RNA polymerase to an initiation sequence\
  • Elongation: RNA polymerase unwinds DNA and adds nucleotides to the 3' end
  • Termination: RNA polymerase reaches the terminator sequence
  • The resulting RNA molecule is a transcript of the gene's protein building instructions called messenger RNA (mRNA)
  • It carries genetic message from the DNA to the protein-synthesizing machinery of the cell.

Translation: is the actual synthesis of a polypeptide, which occurs under the direction of mRNA.
  • The cell must translate the base sequence of a polypeptide
  • The sites of translation are ribosomes which are complex particles that facilitate the orderly linking of amino acids into polypeptide chains.
  • Each ribosome has three binding sites for mRNA.
  • P Site: holds tRNA carrying the growing polypeptide chain
  • A SIte: holds the tRNA carrying the next aminio acid to be added to the chain
  • E Site: Discharged tRNAs leave the ribosome
  • The mRNA reads along it's codon by tRNA's anticodons
  • tRNA is a series of 3 nucleotide bases (the triplet code) whereas 'U', for uracil, replaces 'T' in RNA
  • Initiation: brings together mRNA, a tRNA bearing the first amino acid of the polypeptide, and the two subunits of a ribosome
  • Elongation: amino acids are added one by one to the preceeding amino acid
  • Termination: The stop codon reaches the A site
  • Many copies of a polypeptide can be made simultaneously with a single mRNA because several ribosomes called polyribosomes can attach to the mRNA.
  • DNA -> RNA -> Protein

  • Point Mutations: chemical changes in just one base pair of a gene
  • Base-pair substitution: the replacement if one nucleotide and its partner with another pair of nucleotides
  • Missense mutations: the altered codon codes for a different amino acid
  • Nonsense mutations: changes a codon for an amino acid into a stop codon.
  • Mutagens: physical and chemical agents that interact with DNA in ways that cause mutations such as things that we are exposed to in our environment