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Transcript
Biology, Seventh Edition Solomon • Berg • Martin Chapter 12 Gene Expression Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 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 Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression An “inborn error of metabolism” Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression Mutations affecting biochemical pathways Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression • 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 Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression Nucleotide structure of RNA Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression Overview of transcription and translation Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression • 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 Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression • 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 Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression Transcription Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression Synthesis of mRNA Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression • 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 Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression Three representations of a tRNA molecule Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression Ribosome structure Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression Initiation of translation in bacteria Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression Elongation cycle in translation Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression Termination of translation Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression • 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 Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression Coupled transcription and translation in bacteria Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression • 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 Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression Infection cycle of an RNA tumor virus Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression • 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 Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression • 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 Copyright © 2005 Brooks/Cole — Thomson Learning Biology, Seventh Edition CHAPTER 12 Gene Expression Mutations Copyright © 2005 Brooks/Cole — Thomson Learning