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Gene Expression. Early evidence indicating most genes specify the structure of proteins Garrod’s work on inborn errors of metabolism in early 1900s Beadle and Tatum’s work with Neurospora mutants in the 1940s. An “inborn error of metabolism”. Mutations affecting biochemical pathways.

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An of proteins“inborn errorof metabolism”


Mutations of proteinsaffectingbiochemical

pathways


  • Information flow from DNA to protein of proteins

    • Transcription

      • RNA molecule complementary to the template DNA strand synthesized

    • Translation

      • Polypeptide chain specified by messenger RNA (mRNA) is synthesized


Nucleotide of proteinsstructure of

RNA



  • Structure of RNA of proteins

    • RNA

      • Formed from nucleotide subunits

      • Each nucleotide subunit contains ribose, a base, and three phosphates

      • Like DNA, RNA subunits are covalently joined by a 5΄—3΄ linkage to form alternating sugar-phosphate backbone


  • Transcription of proteins

    • Incoming nucleotides with three phosphates pair with complementary bases on the DNA strand

    • RNA polymerase cleaves two phosphates from each nucleotide and covalently links the remaining phosphate to the 3΄end of the RNA chain


Transcription of proteins


Synthesis of mRNA of proteins


  • Translation of proteins

    • Polypeptide chain specified by the mRNA is synthesized

      • Initiation

      • Repeating cycles of elongation

      • Termination

    • Each sequence of three nucleotide bases in the mRNA constitutes a codon

    • Requires tRNAs and cell machinery, including ribosomes



Ribosome of proteinsstructure





  • Coupled transcription and translation in bacteria of proteins

    • Unlike eukaryotic cells, in bacterial cells translation and transcription coupled

    • Translation of the bacterial mRNA molecule usually begins before the 3´ end of the transcript is completed


Coupled of proteinstranscription and transla-tion in bacteria


  • Retroviruses of proteins

    • Flow of genetic information is reversed by reverse transcriptase

      • Enzyme associated with retroviruses

      • Retroviruses synthesize DNA from an RNA template

      • Example of retrovirus is HIV-1, AIDS virus



  • Mutations of proteins

    • Base substitution mutations

      • Often result from errors in base pairing during replication

      • Missense mutations

      • Nonsense mutations

    • Frameshift mutations

      • One or two nucleotide pairs are inserted into or deleted from the molecule


  • Some mutations involve larger DNA segments of proteins

    • Due to change in chromosome structure

    • Wide range of effects as they involve many genes

      • DNA sequences that “jump” into the middle of a gene

      • Known as transposons or transposable genetic elements

      • Most are retrotransposons


Mutations of proteins


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