Chapter 13 Gene Structure and Expression.docx

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Biological Sciences
Effiette Sauer

Chapter 13 Gene Structure and Expression 13.1a Gene Specify either Protein or RNA Products  Proteins are encoded by genes made of DNA  Beadle and Tatum Experiment o Worked with neurospora, to show the direct relationship between genes and enzymes o Hypothesis: one gene one enzyme theory o Wild-type neurospora form of mold found in nature able to survive on minimal medium o Treated wild-type with radiation causing mutation, so that the mutated mold could not grow on MM, but could grow on it when vitamins or minerals were added to it o The assemble of arginine comes from an assembly line process with different enzymes o Since it is sequence process if the first enzyme can’t produce the first product then the final doesn’t get produced o Proved that many proteins consist of more than one subunit. Each of these subunits is separate molecule called a polypeptide made by a separate gene o Beadle and Tatum hypothesized the one gene one polypeptide rule 13.1b Pathway from Gene to Poly Peptide  Pathway involves transcription and translation  Transcription: the mechanism by which the information encoded in DNA is made into a complementary RNA copy  Translation: the use of information encoded in the RNA to assemble amino acids into a polypeptide  Central Dogma: DNA – RNA – Protein  Transcription o RNA polymerase creates RNA since that is complementary to the DNA sequence of the given gene o Template strand is read by RNA polymerase o mRNA: RNA transcribed from a gene encoding a polypeptide  Translation o mRNA associates with a ribosome, a particle on which amino acids are linked into a polypeptide chain  In prokaryotic cells transcription and translation of a gene can occur is simultaneously  Eukaryotes require a process of mRNA being transported outside of the nucleus 13.1c Genetic Code  T is equal to U  Breaking the Genetic Code o Genetic Code: The nucleotide information that specifies the amino acid of polypeptide o Combination of 3 nucleotides provide the capacity to code 20 amino acids o Codon: three letter code o The template strand for a give gene is always red 3’-5’  Features of the Code o By convention codons are written 5’-3’, substituting T for U o Of the 64 codons AUG is for methionine the start codon o UAA UGA UAG are stop codons o Degeneracy: the many synonyms in a nucleic acid code o Commaless: the words of the nucleic acid code are sequential with no indicators of the start of end of a codon o Reading frame: the start codon and every three from there makes a codon for one amino acid o The code in universal, indicating that it has been preserved through evolution 13.2 Transciption DNA-Directed RNA Synthesis  For a given gene only one of the two DNA strands act as template for synthesis  Only a small part of DNA molecule serves as a gene  RNA polymerase catalyzes the assemble of the RNA strand  Produces a single RNA strand 13.2a Transcription Proceeds in Three Steps  Consists of two main parts the promoter region and transcriptional unit o Promoter: a section upstream, TATA Box o Transcriptional Unit: the section of a gene that is copied into an RNA molecule  Initiation: the molecular machinery for transcription assembles at the promoter region and begins synthesizing an RNA copy of the gene  Elongation: RNA polymerase moves along the gene xtending the RNA chain  Termination: the transcription ends and RNA polymerase releases the mRNA  Gene organization is the same, the promoter region only differs  In eukaryotes RNA polymerase II cannot bind to the DNA it binds after transcription factors have bound to it, prokaryotes RNA polymerase binds directly to DNA  Prokaryotes: contain terminators, DNA sequences, signal the end of transcription, o Method 1: base pairing creates a hairpin affect on mRNA o Method 2: protein bonds to termination sequence 13.3 mRNA processing in eukaryotes  UTR untranslated RNA provide sites for the 3’ poly A tail and 5’ Guanine Cap 13.3a Precursor mRNA modification  Modification to mRNA ends
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