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Download Topic 10 (From Genotype to Phenotype)
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THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN • 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 its DNA bases • A particular gene, a linear sequence of many nucleotides – Specifies a polypeptide Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings • The DNA of the gene is transcribed into RNA – Which is translated into the polypeptide DNA Transcription RNA Translation Protein Figure 10.6A Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 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 © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings DNA molecule Gene 1 Gene 2 Gene 3 DNA strand A A A C C G G C A A A A Transcription RNA U U U G G C C G U U U U Codon Translation Polypeptide Figure 10.7 Amino acid Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings The genetic code is the Rosetta stone of life • Nearly all organisms – Use exactly the same genetic code Second base U U C UUU Phe UUC UCU UCC UUA UCA UUG Leu CUU C CUC First base CUA CUG A Leu UGA Stop A UGG Trp CCU CAU His CAC CGU CGC CAA CAG Gln CGA CGG AAU Asn AAC U AGU Ser AGC C AAA AGA A AGG Arg G U GGU C GGC Gly GGA A CCC CCA CCG ACA ACC GUA Pro Thr AAG GAU GCU GCC Val GUG Figure 10.8A Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings UAC UGU Cys U UGC C UAA Stop AUA GUC G Ser Tyr UAG Stop ACU ACC GUU UAU G UCG AUU AUC Ile Met or AUG start A GCA GCG GAC Ala Lys Asp GAA GAG Glu GGG G U C Arg A G G RNA codon table, with corresponding amino acids Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings • An exercise in translating the genetic code Strand to be transcribed T A C T T C A A A A T C A T G A A G T T T T A G U A G DNA Transcription A U G A A G U U U RNA Start condon Stop condon Translation Figure 10.8B Polypeptide Met Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Lys Phe Transcription produces genetic messages in the form of RNA • A close-up view of transcription RNA nucleotides RNA polymerase T C C A A U T T A C C A T A G G T Direction of transcription Figure 10.9A Newly made RNA Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings A Template Strand of DNA • In the nucleus, the DNA helix unzips – And RNA nucleotides line up along one strand of the DNA, following the base pairing rules • As the single-stranded messenger RNA (mRNA) peels away from the gene – The DNA strands rejoin Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings • Transcription of a gene RNA polymerase DNA of gene Promoter DNA Terminator DNA 1 Initiation 2 Elongation 3 Termination Completed RNA Figure 10.9B Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Area shown In Figure 10.9A Growing RNA RNA polymerase Eukaryotic RNA is processed before leaving the nucleus • Noncoding segments called introns are spliced out – And a cap and a tail are added to the ends 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 Figure 10.10 Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Transfer RNA molecules serve as interpreters during translation • Translation – Takes place in the cytoplasm Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings • A ribosome attaches to the mRNA – And translates its message into a specific polypeptide aided by transfer RNAs (tRNAs) Amino acid attachment site Hydrogen bond RNA polynucleotide chain Figure 10.11A Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Anticodon • Each tRNA molecule – Is a folded molecule bearing a base triplet called an anticodon on one end • A specific amino acid – Is attached to the other end Amino acid attachment site Figure 10.11B, C Anticodon Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Ribosomes build polypeptides • A ribosome consists of two subunits – Each made up of proteins and a kind of RNA called ribosomal RNA tRNA molecules Growing polypeptide Large subunit mRNA Figure 10.12A Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Small subunit • The subunits of a ribosome – Hold the tRNA and mRNA close together during translation tRNA-binding sites Large subunit Next amino acid to be added to polypeptide Growing polypeptide tRNA mRNAbinding site mRNA Small subunit Codons Figure 10.12B, C Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings An initiation codon marks the start of an mRNA message Start of genetic message End Figure 10.13A Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings • mRNA, a specific tRNA, and the ribosome subunits – Assemble during initiation Met Met Large ribosomal subunit Initiator tRNA P site U A C A U G U A C A U G Start codon 1 mRNA A site Small ribosomal subunit Figure 10.13B Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 2 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation • Once initiation is complete – Amino acids are added one by one to the first amino acid Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings • Each addition of an amino acid – Occurs in a three-step elongation process Amino acid Polypeptide P site A site Anticodon mRNA Codons 1 Codon recognition mRNA movement Stop codon 2 Peptide bond formation New Peptide bond Figure 10.14 Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 3 Translocation • The mRNA moves a codon at a time – And a tRNA with a complementary anticodon pairs with each codon, adding its amino acid to the peptide chain Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings • Elongation continues – Until a stop codon reaches the ribosome’s A site, terminating translation Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Review: The flow of genetic information in the cell is DNARNAprotein • The sequence of codons in DNA, via the sequence of codons – Spells out the primary structure of a polypeptide Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings • Summary of transcription and translation DNA Transcription 1 mRNA is transcribed from a DNA template. mRNA RNA polymerase Amino acid Translation 2 Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP. Enzyme ATP tRNA Anticodon Large ribosomal subunit Initiator tRNA Start Codon mRNA 3 Initiation of polypeptide synthesis The mRNA, the first tRNA, and the ribosomal subunits come together. Small ribosomal subunit New peptide bond forming Growing polypeptide 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. Codons mRNA Polypeptide 5 Termination The ribosome recognizes a stop codon. The poly-peptide is terminated and released. Figure 10.15 Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Stop codon Mutations can change the meaning of genes • Mutations are changes in the DNA base sequence – Caused by errors in DNA replication or recombination, or by mutagens Normal hemoglobin DNA C T T mRNA A T G U A C mRNA G Figure 10.16A Mutant hemoglobin DNA A A Normal hemoglobin Glu Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Sickle-cell hemoglobin Val • Substituting, inserting, or deleting nucleotides alters a gene – With varying effects on the organism Normal gene A U G A A G U U U G G C G C A mRNA Met Protein Lys Phe Gly Ala Base substitution A U G A A G U U U A G C G C A Met Lys Phe Ser Ala U Missing Base deletion A U G A A G U U G G C G C A U Figure 10.16B Met Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Lys Leu Ala His