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Gene Expression

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  1. Gene Expression

  2. 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

  3. An“inborn errorof metabolism”

  4. Mutationsaffectingbiochemical pathways

  5. Information flow from DNA to protein • Transcription • RNA molecule complementary to the template DNA strand synthesized • Translation • Polypeptide chain specified by messenger RNA (mRNA) is synthesized

  6. Nucleotidestructure of RNA

  7. Overview of transcription and translation

  8. Structure of RNA • 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

  9. Transcription • 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

  10. Transcription

  11. Synthesis of mRNA

  12. Translation • 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

  13. Three representations of a tRNA molecule

  14. Ribosomestructure

  15. Initiation of translation in bacteria

  16. Elongation cycle in translation

  17. Termination of translation

  18. Coupled transcription and translation in bacteria • 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

  19. Coupled transcription and transla-tion in bacteria

  20. Retroviruses • 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

  21. Infection cycle of an RNA tumor virus

  22. Mutations • 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

  23. Some mutations involve larger DNA segments • 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

  24. Mutations

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