Overview The Flow of Genetic Information The information content of DNA is in the form of specific sequences of nucleotides The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins Gene expression the process by which DNA directs protein synthesis includes two stages transcription and translation RNA is the intermediate between genes and the proteins for which they code The central dogma is the concept that cells are governed by a cellular chain of command DNA RNA protein Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Basic Principles of Transcription and Translation Transcription is the synthesis of RNA under the direction of DNA Transcription produces messenger RNA mRNA In prokaryotes mRNA produced by transcription is immediately translated without more processing Eukaryotic RNA transcripts are modified through RNA processing to yield finished mRNA A primary transcript is the initial RNA transcript from any gene Translation is the synthesis of a polypeptide which occurs under the direction of mRNA Ribosomes are the sites of translation Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 17 3 DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide a Bacterial cell Nuclear envelope In a eukaryotic cell the nuclear envelope separates transcription from translation DNA TRANSCRIPTION Pre mRNA RNA PROCESSING mRNA TRANSLATION Ribosome Polypeptide b Eukaryotic cell How are the instructions for assembling amino acids into proteins encoded into DNA Genetic Code There are 20 amino acids but there are only four nucleotide bases in DNA The flow of information from gene to protein is based on a triplet code a series of nonoverlapping three nucleotide words known as a CODON These triplets are the smallest units of uniform length that can code for all the amino acids Example AGT at a particular position on a DNA strand results in the placement of the amino acid serine at the corresponding position of the polypeptide to be produced Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings During transcription one of the two DNA strands called the template strand provides a template for ordering the sequence of nucleotides in an RNA transcript During translation the mRNA base triplets called codons are read in the 5 to 3 direction Each codon specifies the amino acid to be placed at the corresponding position along a polypeptide Codons along an mRNA molecule are read by translation machinery in the 5 to 3 direction Each codon specifies the addition of one of 20 amino acids Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 17 4 DNA molecule Gene 2 Gene 1 Gene 3 DNA template strand TRANSCRIPTION mRNA Codon TRANSLATION Protein Amino acid Cracking the Code All 64 codons were deciphered by the mid 1960s Of the 64 triplets 61 code for amino acids 3 triplets are stop signals to end translation The genetic code is redundant but not ambiguous no codon specifies more than one amino acid Codons must be read in the correct reading frame correct groupings in order for the specified polypeptide to be produced Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Third mRNA base 3 end of codon First mRNA base 5 end of codon Fig 17 5 Second mRNA base The genetic code is nearly universal shared by the simplest bacteria to the most complex animals Genes can be transcribed and translated after being transplanted from one species to another a Tobacco plant expressing a firefly gene b Pig expressing a jellyfish gene Concept 17 2 Transcription is the DNA directed synthesis of RNA RNA synthesis is catalyzed by RNA polymerase which pries the DNA strands apart and hooks together the RNA nucleotides RNA synthesis follows the same base pairing rules as DNA except uracil substitutes for thymine The DNA sequence where RNA polymerase attaches is called the promoter in bacteria the sequence signaling the end of transcription is called the terminator The stretch of DNA that is transcribed is called a transcription unit Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Synthesis of an RNA Transcript The three stages of transcription 1 Initiation 2 Elongation 3 Termination Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 17 7 Promoter Transcription unit 5 3 Start point RNA polymerase 3 5 DNA 1 Initiation 5 3 3 5 Unwound DNA RNA transcript 3 Rewound DNA 3 end 3 5 5 5 3 Termination 3 5 5 3 5 RNA nucleotides 5 3 RNA transcript RNA polymerase Template strand of DNA 2 Elongation 5 3 Nontemplate strand of DNA Elongation Completed RNA transcript 3 Direction of transcription downstream Newly made RNA Template strand of DNA RNA Polymerase Binding and Initiation of Transcription A promoter called a TATA box is crucial in forming the initiation complex in eukaryotes Transcription factors mediate the binding of RNA polymerase II and the initiation of transcription The completed assembly of transcription factors and RNA polymerase II bound to a promoter is called a transcription initiation complex Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 17 8 A eukaryotic promoter includes a TATA box 1 Promoter Template 5 3 TATA box 3 5 Start point Template DNA strand Several transcription factors must bind to the DNA before RNA polymerase II can do so 2 Transcription factors 5 3 3 5 Additional transcription factors bind to the DNA along with RNA polymerase II forming the transcription initiation complex 3 RNA polymerase II Transcription factors 5 3 3 5 5 RNA transcript Transcription initiation complex Elongation of the RNA Strand As RNA polymerase moves along the DNA it untwists the double helix 10 to 20 bases at a time Transcription progresses at a rate of 40 nucleotides per second in eukaryotes A gene can be transcribed simultaneously by several RNA polymerases Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Termination of Transcription The mechanisms of termination are different in bacteria and eukaryotes In bacteria the polymerase stops transcription at the end of the terminator In eukaryotes the polymerase continues transcription after the pre mRNA is cleaved from the growing RNA chain the polymerase eventually falls off the DNA Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Concept 17 3 Eukaryotic cells modify RNA after