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CHAPTER 10 Molecular Biology of the Gene
Modules 10.6 – 10.16
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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
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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
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Studies of inherited metabolic disorders first suggested that phenotype is expressed through proteins Studies of the bread mold Neurospora crassa led to the one gene-one polypeptide hypothesis Figure 10.6B
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The “words” of the DNA “language” are triplets of bases called codons
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
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Gene 1 Gene 3 Gene 2 Codon Amino acid
DNA molecule Gene 2 DNA strand TRANSCRIPTION RNA Codon TRANSLATION Polypeptide Amino acid Figure 10.7
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10.8 The genetic code is the Rosetta stone of life
Virtually all organisms share the same genetic code Figure 10.8A
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An exercise in translating the genetic code
Transcribed strand DNA Transcription RNA Start codon Stop codon Translation Polypeptide Figure 10.8B
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10.9 Transcription produces genetic messages in the form of RNA
RNA nucleotide RNA polymerase Direction of transcription Template strand of DNA Newly made RNA Figure 10.9A
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In transcription, the DNA helix unzips
RNA polymerase In transcription, the DNA helix unzips DNA of gene Promoter DNA Terminator DNA Initiation 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 Elongation Area shown in Figure 10.9A Termination Growing RNA Completed RNA RNA polymerase Figure 10.9B
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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 Exon Intron Exon Intron Exon DNA Transcription Addition of cap and tail Cap RNA transcript with cap and tail Introns removed Tail Exons spliced together mRNA Coding sequence NUCLEUS CYTOPLASM Figure 10.10
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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
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Each tRNA molecule has a triplet anticodon on one end and an amino acid attachment site on the other
Figure 10.11B, C
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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
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10.13 An initiation codon marks the start of an mRNA message
Start of genetic message End Figure 10.13A
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mRNA, a specific tRNA, and the ribosome subunits assemble during initiation
Large ribosomal subunit Initiator tRNA P site A site Start codon Small ribosomal subunit mRNA 1 2 Figure 10.13B
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The mRNA moves a codon at a time relative to the ribosome
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
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Amino acid Polypeptide A site P site Anticodon mRNA 1 Codon recognition mRNA movement Stop codon New peptide bond 2 Peptide bond formation 3 Translocation Figure 10.14
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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
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Summary of transcription and translation
DNA Stage mRNA is transcribed from a DNA template. 1 mRNA RNA polymerase Amino acid TRANSLATION Stage Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP. 2 Enzyme tRNA Initiator tRNA Anticodon Stage Initiation of polypeptide synthesis 3 Large ribosomal subunit The mRNA, the first tRNA, and the ribosomal subunits come together. Start Codon Small ribosomal subunit mRNA Figure 10.15
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New peptide bond forming
Growing polypeptide Stage Elongation 4 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 Stage Termination 5 The ribosome recognizes a stop codon. The poly-peptide is terminated and released. Stop Codon Figure (continued)
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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 or by mutagens The change of a single DNA nucleotide causes sickle-cell disease
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Sickle-cell hemoglobin
Normal hemoglobin DNA Mutant hemoglobin DNA mRNA mRNA Normal hemoglobin Sickle-cell hemoglobin Glu Val Figure 10.16A
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Types of mutations NORMAL GENE mRNA Protein Met Lys Phe Gly Ala
BASE SUBSTITUTION Met Lys Phe Ser Ala BASE DELETION Missing Met Lys Leu Ala His Figure 10.16B
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