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Saboteurs Inside Our Cells • The invasion and damage of cells by the herpesvirus can be compared to the actions of a saboteur intent on taking over a factory – The herpesvirus hijacks the host cell’s molecules and organelles to produce new copies of the virus Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Viruses provided some of the earliest evidence that genes are made of DNA • Molecular biology studies how DNA serves as the molecular basis of heredity Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 1 THE STRUCTURE OF THE GENETIC MATERIAL 10.1 Experiments showed that DNA is the genetic material • The Hershey-Chase experiment showed that certain viruses reprogram host cells to produce more viruses by injecting their DNA Head DNA Tail Tail fiber Figure 10.1A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The Hershey-Chase Experiment 1 Mix radioactively labeled phages with bacteria. The phages infect the bacterial cells. Phage 2 Agitate in a blender to separate phages outside the bacteria from the cells and their contents. Radioactive protein Bacterium DNA 3 Centrifuge the mixture so bacteria form a pellet at the bottom of the test tube. 4 Empty protein shell Measure the radioactivity in the pellet and liquid. Radioactivity in liquid Phage DNA Batch 1 Radioactive protein Centrifuge Pellet Batch 2 Radioactive DNA Radioactive DNA Figure 10.1B Centrifuge Pellet Radioactivity in pellet Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 2 • Phage reproductive cycle Phage attaches to bacterial cell. Phage injects DNA. Phage DNA directs host cell to make more phage DNA and protein parts. New phages assemble. Cell lyses and releases new phages. Figure 10.1C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 10.2 DNA and RNA are polymers of nucleotides • DNA is a nucleic acid, made of long chains of nucleotides Phosphate group Nitrogenous base Sugar Phosphate group Nitrogenous base (A, G, C, or T) Nucleotide Thymine (T) Sugar (deoxyribose) DNA nucleotide Polynucleotide Sugar-phosphate backbone Figure 10.2A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 3 • DNA has four kinds of bases, A, T, C, and G Thymine (T) Cytosine (C) Adenine (A) Pyrimidines Guanine (G) Purines Figure 10.2B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • RNA is also a nucleic acid – RNA has a slightly different sugar – RNA has U instead of T Nitrogenous base (A, G, C, or U) Phosphate group Uracil (U) Sugar (ribose) Figure 10.2C, D Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 4 10.3 DNA is a double-stranded helix • James Watson and Francis Crick worked out the three-dimensional structure of DNA, based on work by Rosalind Franklin Figure 10.3A, B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • The structure of DNA consists of two polynucleotide strands wrapped around each other in a double helix 1 chocolate coat, Blind (PRA) Figure 10.3C Twist Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 5 • Hydrogen bonds between bases hold the strands together – Each base pairs with a complementary partner – A pairs with T – G pairs with C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Three representations of DNA Hydrogen bond Ribbon model Partial chemical structure Computer model Figure 10.3D Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 6 DNA REPLICATION 10.4 DNA replication depends on specific base pairing • In DNA replication, the strands separate – Enzymes use each strand as a template to assemble the new strands A A Nucleotides Parental molecule of DNA Both parental strands serve as templates Two identical daughter molecules of DNA Figure 10.4A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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 7 • A specific gene specifies a polypeptide – The DNA is transcribed into RNA, which is translated into the polypeptide DNA TRANSCRIPTION RNA TRANSLATION Protein Figure 10.6A 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 8 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 9 • An exercise in translating the genetic code Transcribed strand DNA Transcription RNA Start codon Translation Polypeptide Stop codon Figure 10.8B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 10.9 Transcription produces genetic messages in the form of RNA RNA polymerase RNA nucleotide Direction of transcription Figure 10.9A Newly made RNA Template strand of DNA Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 10 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 Terminator DNA Initiation Elongation Area shown in Figure 10.9A Termination Growing RNA Completed RNA RNA polymerase Figure 10.9B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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 11 • 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 mRNA binding site A mRNA Codons mRNA Small subunit Figure 10.12A-C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 12 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 2 Figure 10.13B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 13 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 Figure 10.14 3 2 Peptide bond formation Translocation Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 14 10.15 Review: The flow of genetic information in the cell is DNA→RNA→protein • 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 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. Figure 10.15 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 15 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 Stage 5 Termination The ribosome recognizes a stop codon. The polypeptide is terminated and released. Figure 10.15 (continued) Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 16 Normal hemoglobin DNA Mutant hemoglobin DNA mRNA 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 Leu Ala His Figure 10.16B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 17 10.21 The AIDS virus makes DNA on an RNA template • HIV is a retrovirus Glycoprotein Envelope Protein coat RNA (two identical strands) Reverse transcriptase Figure 10.21A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 10.22 Virus research and molecular genetics are intertwined • Virus studies help establish molecular genetics • Molecular genetics helps us understand viruses – such as HIV, seen here attacking a white blood cell Figure 10.22 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 18