Chapter 15 Genetic Code How Genes Work How does an organism s genotype specify its phenotype Last Class The molecular basis for DNA replication The molecular players involved in DNA replication Replication of telomeres Repairing mistakes and damage in DNA Learning objectives By the end of this class you should be able to Explain a genotype and a phenotype Describe what the cell uses to transmit the DNA code to the protein machinery Explain the central dogma of molecular biology Describe how the nucleotide code in mRNA codes for the amino acid codes of proteins Genetics for diploids Genotype the make up of an organism Set of genes present in an organism Usually represented by letters Domestic cat Hair length gene represented by the letter H Phenotype characteristics of an organism Trait that the genotype codes for Domestic cat Short hair Long hair Phenotypes Genetic Code Hypothesis Watson and Crick later proposed that the sequences of bases in DNA might act as a code Next big question We have DNA We can make proteins DNA has the code for proteins How does the code get to the protein machinery RNA as an Intermediate RNA Polymerase Francois Jacob and Jacques Monod proposed the messenger RNA mRNA hypothesis which states that messenger RNA carries information from to the site of protein synthesis RNA as an Intermediate RNA polymerase transcribes one strand of the DNA making a complementary copy according to the rules of complementary base pairing Next big question mRNA is the intermediate between DNA and the protein machinery What are the rules that specify the relationship between the sequence of nucleotides and the sequence of amino acids in a protein How many bases specify one A A How many bases specify one A A George Gamow predicted that each codon would be three nucleotides long Based on the fact that this was the minimum code length that could specify the 20 different amino acids found in proteins This is redundant with some amino acids being specified by more than one codon How many bases specify one A A By making deletion and insertion in viral DNA Francis Crick and colleagues established that the code is based on three nucleotides for each amino acid They found that the reading frame sequence of codons of a gene could be out of register by mutation but then restored if the total number of deletions or additions were multiples of three Reading Frame The fat cat ate the rat Reading frame shifted The atc ata tet her at Deciphering the genetic code The genetic code is Redundant All but methionine and tryptophan are coded by more than codon Unambiguous One codon never codes for more than one Universal With only a few exceptions all codons specify the same amino acids in all organisms nearly Conservative When several codons specify the same amino acid the first two bases in those codons are almost always identical Central Dogma of Molecular Biology Central Dogma
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