Gene Expression

Copyright © 2005 Brooks/Cole — Thomson Learning

Biology, Seventh Edition Solomon • Berg • Martin

Chapter 12

Gene ExpressionGene Expression


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



An“inborn errorof metabolism”



Mutationsaffectingbiochemicalpathways



• 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



Nucleotidestructure ofRNA



Overview of transcription and translation



• 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



• 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



Transcription



Synthesis of mRNA



• 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



Three representations of a tRNA molecule



Ribosomestructure



Initiation of translation in bacteria



Elongation cycle in translation



Termination of translation



• 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



Coupled transcription and transla-tion in bacteria



• 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



Infection cycle of an RNA tumor virus



• 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



• 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



Mutations

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Gene Expression