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Chapter 17

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Syracuse University
BIO 121
Jason Wiles

BIO CHAPTER 17 CONCEPT 17.1 • DNAcontrols metabolism by directing cells to make specific enzymes and other proteins, via the process of gene expression. Beadle and Tatum’s studies of mutant strains of Neurospora led to one gene-one polypeptide hypothesis. Genes code for polypeptide chains of specify RNA molecules. • Transcription is the synthesis of RNAcomplementary to a temple strand of DNA, providing a nucleotide-to-nucleotide transfer of information. Translation is the synthesis of a polypeptide whose amino acid sequence is specified by the nucleotide sequence in mRNA; this information transfer thus involves a change of language, from that of nucleotides to that of amino acids. • Genetic information is encoded as a sequence of nonoverlapping nucleotide triplets, or codons.A codon in messenger RNA(mRNA) either is translated into an amino acid (61 of the 64 codons) or serves as a stop signal (3 codons). Codons must be read in the correct reading frame. • DNAto protein o Information encoded in DNA o Transcription- synthesizes messanger RNA(mRNA)  Complementary to template DNAstrand  Specifies amino acid sequences of polypeptide chains • DNAsynthesis o Always proceeds in 5’to 3’direction o Leading strand- synthesized continuously o Lagging strand  Synthesized discontinuously  Forms short Okazaki fragments  DNAprimase synthesizes RNAprimers  DNAligase links Okazaki fragments • Genetic code is redundant- some amino acids have more that one codon  Is virtually universal –suggests all organisms have a common ancestor; few minor exceptions to standard code in all organisms • Ribosomes- bring together all machinery for translation o Couple tRNAs to mRNA codons o Catalyze peptide bonds between amino acids CONCEPT 17.2 • RNAsynthesis is catalyzed by RNApolymerase, which links together RNAnucleotides complementary to a DNAtemplate strand. This process follows the same base-pairing rules as DNAreplication, except that in RNA, uracil substitutes for thymine. • The three stages of transcription are initiation, elongation, and termination.A promoter, often including a TATAbox in eukaryotes, establishes where RNAsynthesis is initiated. Transcription factors help eukaryotic RNApolymerase recognize promoter sequences, forming a transcription initiation complex. The mechanisms of termination are different in bacteria and eukaryotes. • In eukaryotes- Introns and Exons o Introns- noncoding regions (interrupt exons); moved from original pre-mRNA o Exons- coding regions; spliced to produce continue polypeptide coding sequence CONEPT 17.3 • Before leaving the nucleus, eukaryotic mRNAmolecules undergo RNAprocessing, which includes RNAsplicing, the addition of a modified nucleotide 5’cap to the 3’end, and the addition of a poly-Atail to the 3’end. • Most eukaryotic genes are split into segments: They have introns interspersed among the exons (the regions included in the mRNA). In RNAsplicing, intr
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