Transcript 10b
THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN 10.6 The DNA genotype is expressed as proteins, which provide the molecular basis for phenotypic traits • The information constituting an organism’s genotype is carried in its sequence of bases Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • A specific gene specifies a polypeptide – The DNA is transcribed into RNA, which is translated into the polypeptide DNA TRANSCRIPTION DNA TRANSLATION Protein Figure 10.6A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Phenotype is because of proteins (polypeptides) made from a particular genotype . • Studies of the bread mold Neurospora crassa led to the one gene-one polypeptide hypothesis Figure 10.6B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 10.7 Genetic information written in codons is translated into amino acid sequences • The “words” of the DNA “language” are triplets of bases called codons – The codons in a gene specify the amino acid sequence of a polypeptide Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Gene 1 Gene 3 DNA molecule Gene 2 DNA strand TRANSCRIPTION RNA Codon TRANSLATION Polypeptide Figure 10.7 Amino acid Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 10.8 The genetic code is the Rosetta stone of life • Virtually all organisms share the same genetic code Figure 10.8A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • An exercise in translating the genetic code Transcribed strand DNA Transcription RNA Start codon Polypeptide Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Translation Stop codon Figure 10.8B 10.9 Transcription produces genetic messages in the form of RNA RNA polymerase RNA nucleotide Direction of transcription Template strand of DNA Figure 10.9A Newly made RNA Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings RNA polymerase • In transcription, the DNA helix unzips – RNA nucleotides line up along one strand of the DNA following the base-pairing rules – The single-stranded messenger RNA peels away and the DNA strands rejoin DNA of gene Promoter DNA Initiation Elongation Terminator DNA Area shown in Figure 10.9A Termination Growing RNA Completed RNA Figure 10.9B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings RNA polymerase 10.10 Eukaryotic RNA is processed before leaving the nucleus • Noncoding segments called introns are spliced out • A cap and a tail are added to the ends • After being shipped out of the nucleus, the mRNA associates with a ribosome, tRNA to make the polypeptide Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Exon Intron Exon Intron Exon DNA Cap RNA transcript with cap and tail Transcription Addition of cap and tail Introns removed Tail Exons spliced together mRNA Coding sequence NUCLEUS CYTOPLASM 10.11 Transfer RNA molecules serve as interpreters during translation • In the cytoplasm, a ribosome attaches to the mRNA and translates its message into a polypeptide • The process is aided by transfer RNAs Amino acid attachment site Hydrogen bond RNA polynucleotide chain Anticodon Figure 10.11A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Each tRNA molecule has a triplet anticodon on one end and an amino acid attachment site on the other Amino acid attachment site Anticodon Figure 10.11B, C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 10.12 Ribosomes build polypeptides Next amino acid to be added to polypeptide Growing polypeptide tRNA molecules P site A site Growing polypeptide Large subunit tRNA P A mRNA mRNA binding site Codons mRNA Small subunit Figure 10.12A-C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 10.13 An initiation codon marks the start of an mRNA message Start of genetic message End Figure 10.13A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • mRNA, a specific tRNA, and the ribosome subunits assemble during initiation Large ribosomal subunit Initiator tRNA P site A site Start codon mRNA Small ribosomal subunit 1 Figure 10.13B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 2 10.14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation • The mRNA moves a codon at a time relative to the ribosome – A tRNA pairs with each codon, adding an amino acid to the growing polypeptide Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Amino acid Polypeptide A site P site Anticodon mRNA 1 Codon recognition mRNA movement Stop codon New peptide bond 3 Translocation Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 2 Peptide bond formation Figure 10.14 10.15 Review: The flow of genetic information in the cell is DNARNAprotein • The sequence of codons in DNA spells out the primary structure of a polypeptide – Polypeptides form proteins that cells and organisms use Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Summary of transcription and translation TRANSCRIPTION DNA mRNA RNA polymerase Stage 1 mRNA is transcribed from a DNA template. Amino acid TRANSLATION Enzyme Stage 2 Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP. tRNA Initiator tRNA mRNA Figure 10.15 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Anticodon Large ribosomal subunit Start Codon Small ribosomal subunit Stage 3 Initiation of polypeptide synthesis The mRNA, the first tRNA, and the ribosomal subunits come together. New peptide bond forming Growing polypeptide Codons Stage 4 Elongation A succession of tRNAs add their amino acids to the polypeptide chain as the mRNA is moved through the ribosome, one codon at a time. mRNA Polypeptide Stop Codon Figure 10.15 (continued) Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Stage 5 Termination The ribosome recognizes a stop codon. The polypeptide is terminated and released. 10.16 Mutations can change the meaning of genes • Mutations are changes in the DNA base sequence – These are caused by errors in DNA replication by mutagens – Example The change of a single DNA nucleotide causes sickle-cell disease Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Normal hemoglobin DNA mRNA Mutant hemoglobin DNA mRNA Normal hemoglobin Sickle-cell hemoglobin Glu Val Figure 10.16A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Types of mutations NORMAL GENE mRNA Protein Met Lys Phe Gly Ala Lys Phe Ser Ala BASE SUBSTITUTION Met Missing BASE DELETION Met Lys Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Leu Ala His Figure 10.16B