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Solomon IM Ch 13

Solomon IM Ch 13 - Chapter13:GeneExpression 13...

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Chapter 13: Gene Expression 123 13 Gene Expression Lecture Outline I. Most genes carry information for making proteins. A. In 1908, Archibald Garrod proposed that genes are responsible for enzymes while studying a rare genetic disease, alkaptonuria. 1. Garrod published a book: Inborn Errors of Metabolism. 2. Sumner, in 1926, purified a different enzyme and found it to be a protein (Nobel Prize in 1946). B. In the 1940s, George Beadle and Edward Tatum worked on Neurospora (a mold). 1. One mutant gene affected one single step in a metabolic pathway. 2. The “one gene, one enzyme” hypothesis. a) It’s now known that some genes encode proteins that are not enzymes; based on the work of Pauling and others. b) It’s also known that one gene is responsible for one polypeptide chain, and that some proteins have more than one chain. c) Beadle and Tatum used the term wild type , meaning the organism with the normal phenotype; it does not imply dominant or recessive, just the normal. 3. The “one gene, one polypeptide” hypothesis. a) Due to the fact that not all proteins are enzymes and some enzymes are multi subunit, “one gene, one polypeptide” hypothesis was proposed. II. DNA is transcribed to form RNA; RNA is translated to form a polypeptide. A. RNA (ribonucleic acid) is used as an intermediary between DNA and proteins. 1. RNA is a single stranded, uncoiled nucleotide polymer (although some areas may be folded). 2. The sugar in RNA is ribose, and uracil substitutes for thymine. a) Uracil is a pyrimidine and forms two bonds with adenine. B. A copy of the DNA is made in the form of messenger RNA (mRNA) in transcription. C. Translation involves mRNA, and tRNA (transfer RNA) and rRNA (ribosomal RNA) coordinate to produce proteins. 1. Codons consist of sequences of 3 nucleotide sequences of mRNA. a) Codons are read in sequences of 3; this is called the triplet code. b) Codons are written in a 5 ʹ to 3 ʹ fashion. c) Each codon codes for 1 amino acid. d) 4 nucleotides can combine in 4 3 combinations in triplet codon—more than enough to code for the 20 naturally occurring amino acids. 2. Each tRNA molecule has a sequence of 3 nucleotides—the anticodon. a) Anticodons base pair with the codon in a complementary way via hydrogen bonds.
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Instructor’s Manual for Solomon, Berg, and Martin’s Biology, 9 th Edition 124 b) Codons and complementary anticodons are antiparallel when they make hydrogen bonds. Anticodons are written in a 3 ʹ to 5 ʹ fashion. 3. Ribosomes are composed of proteins and rRNA. D. Biologists cracked the genetic code in the 1960s. E. In 1961, Crick and colleagues determined that the code was read in a nonoverlapping sequence of bases, forming a 3 base reading frame. 1. 1 nucleotide code for 1 amino acid? No, 4 bases, 20 amino acids. 2. 2 nucleotides code for 1 amino acid? No, not enough combinations.
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Solomon IM Ch 13 - Chapter13:GeneExpression 13...

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