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Chapter 10

Chapter 10. 0. Molecular Biology of the Gene. Sabotage Inside Our Cells A saboteur Lies low waiting for the right moment to strike. Viruses are biological saboteurs Hijacking the genetic material of host cells in order to reproduce themselves Viruses provided some of the earliest evidence

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Chapter 10

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  1. Chapter 10 0 Molecular Biology of the Gene

  2. Sabotage Inside Our Cells • A saboteur • Lies low waiting for the right moment to strike

  3. Viruses are biological saboteurs • Hijacking the genetic material of host cells in order to reproduce themselves • Viruses provided some of the earliest evidence • That genes are made of DNA

  4. Head DNA Tail Tailfiber 300,000 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 Figure 10.1A

  5. Radioactive protein Empty protein shell Bacterium Radioactivityin liquid Phage Phage DNA DNA Batch 1Radioactiveprotein Centrifuge Pellet Mix radioactively labeled phages with bacteria. The phages infect the bacterial cells. Measure the radioactivity in the pellet and the liquid. Agitate in a blender to separate phages outside the bacteria from the cells and their contents. Centrifuge the mixture so bacteria form a pelletat the bottom of the testtube. 1 2 3 4 Radioactive DNA Batch 2RadioactiveDNA Centrifuge Radioactivityin pellet Pellet Figure 10.1B • The Hershey-Chase experiment

  6. Phage DNA directs hostcell to make more phageDNA and protein parts.New phages assemble. Phage injects DNA. Phage attachesto bacterial cell. Cell lyses and releases new phages. • Phage reproductive cycle Figure 10.1C

  7. Sugar-phosphate backbone Phosphate group Nitrogenous base A A Sugar Nitrogenous base(A, G, C, or T) Phosphategroup C C DNA nucleotide O H H3C C C N O C C T CH2 H T O P N O O O– Thymine (T) O C C H H H H G G C C H O Sugar(deoxyribose) T T DNA nucleotide DNA polynucleotide • 10.2 DNA and RNA are polymers of nucleotides • DNA is a nucleic acid • Made of long chains of nucleotide monomers Figure 10.2A

  8. H H H H O N N O C H C H H3C C C H N N N C C N C N N C H H C C C C C C C C C C H O H N N N O H N N H N N H H H H H Adenine (A) Guanine (G) Thymine (T) Cytosine (C) Purines Pyrimidines • DNA has four kinds of nitrogenous bases • A, T, C, and G Figure 10.2B

  9. Nitrogenous base (A, G, C, or U) Key Hydrogen atom O Phosphategroup Carbon atom C H Nitrogen atom H N C Oxygen atom O C C Phosphorus atom H O P O CH2 O N Uracil (U) O– O C C H H H H C C OH O Sugar(ribose) • RNA is also a nucleic acid • But has a slightly different sugar • And has U instead of T Figure 10.2C, D

  10. 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

  11. Twist • The structure of DNA • Consists of two polynucleotide strands wrapped around each other in a double helix Figure 10.3C

  12. G C O T A OH P Hydrogen bond –O O A T OH O H2C A T Basepair O CH2 O O C G P O O– –O C G O P O H2C O O C T G A O CH2 C G O O P O O– – O O P O H2C O O G C A T O CH2 O O A T P O – O O– O P A T O O H2C O A T A T CH2 O OH O O– P G C HO O T A Partial chemical structure Ribbon model Computer model • Hydrogen bonds between bases • Hold the strands together • Each base pairs with a complementary partner • A with T, and G with C Figure 10.3D

  13. T A T T A A T A T A G C G G G C C C G C C C G G G C G C C A A T A T A A T T A T T T A A A T Both parental strands serve as templates Two identical daughtermolecules of DNA Parental moleculeof DNA DNA REPLICATION • 10.4 DNA replication depends on specific base pairing • DNA replication • Starts with the separation of DNA strands • Then enzymes use each strand as a template • To assemble new nucleotides into complementary strands Nucleotides Figure 10.4A

  14. G C T A G C G C A T T A C G A T C G G C C G G C C C G G A C A T A T T G A T T G T T A A A A A C T T T A • DNA replication is a complex process • Due in part to the fact that some of the helical DNA molecule must untwist Figure 10.4B

  15. Parental strand Origin of replication Daughter strand Bubble Two daughter DNA molecules • 10.5 DNA replication: A closer look • DNA replication • Begins at specific sites on the double helix Figure 10.5A

  16. 5 end 3 end P HO 5 4 2 3 A T 3 1 1 4 2 5 P P C G P P G C P P A T OH P 3 end 5 end • Each strand of the double helix • Is oriented in the opposite direction Figure 10.5B

  17. DNA polymerase molecule 3 5 Daughter strandsynthesizedcontinuously Parental DNA 5 3 Daughter strandsynthesizedin pieces 3 5 5 3 DNA ligase Overall direction of replication • Using the enzyme DNA polymerase • The cell synthesizes one daughter strand as a continuous piece • The other strand is synthesized as a series of short pieces • Which are then connected by the enzyme DNA ligase Figure 10.5C

  18. 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 its DNA bases • A particular gene, a linear sequence of many nucleotides • Specifies a polypeptide

  19. DNA Transcription RNA Translation Protein Figure 10.6A • The DNA of the gene is transcribed into RNA • Which is translated into the polypeptide

  20. Figure 10.6B • Studies of inherited metabolic disorders in mold • First suggested that phenotype is expressed through proteins

  21. 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

  22. DNA molecule Gene 1 Gene 2 Gene 3 DNA strand A A A C A C G G A A C A Transcription U U U G U G C C U U G U RNA Codon Translation Polypeptide Amino acid Figure 10.7

  23. Second base U C A G U UAU UGU UGC UGA Stop UUU UCU Cys Phe Tyr UUC UAC C UCC Ser U UCA UUA UAA Stop A Leu UCG UAG Stop UGG Trp G U CAU CGU CUU CCU His C CAC CGC CUC CCC C Pro Arg Leu CUA CCA CAA CGA A Gln CAG CGG CUG CCG G Third base First base U ACU AUU AAU AGU Ser Asn ACC AGC AUC AAC Ile C A Thr AUA AGA ACA AAA A Lys Arg Met or start ACC AGG AAG AUG G U GUU GAU GGU GCU Asp C GGC GCC GUC GAC Gly Ala G Val GUA GCA GGA GAA A Glu GUG GCG GGG GAG G Figure 10.8A • 10.8 The genetic code is the Rosetta stone of life • Nearly all organisms • Use exactly the same genetic code UUG

  24. Strand to be transcribed T A C T T C A A A A T C DNA A T G A A G T T T T A G Transcription G U U U A G A U A A G U RNA Startcondon Stopcondon Translation Met Polypeptide Lys Phe Figure 10.8B • An exercise in translating the genetic code

  25. RNA nucleotides RNA polymerase A C C A T T A U T C T G U G A C A U C C A C C A G A T T T A G G Direction of transcription Template Strand of DNA Figure 10.9A Newly made RNA • 10.9 Transcription produces genetic messages in the form of RNA • A close-up view of transcription

  26. 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

  27. RNA polymerase DNA of gene Promoter DNA Terminator DNA Area shown In Figure 10.9A Growing RNA Completed RNA RNA polymerase Figure 10.9B • Transcription of a gene 1 Initiation 2 Elongation 3 Termination

  28. 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 • 10.10 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

  29. 10.11 Transfer RNA molecules serve as interpreters during translation • Translation • Takes place in the cytoplasm

  30. 0 Amino acid attachment site Hydrogen bond RNA polynucleotide chain Anticodon Figure 10.11A • A ribosome attaches to the mRNA • And translates its message into a specific polypeptide aided by transfer RNAs (tRNAs)

  31. Amino acid attachment site Anticodon Figure 10.11B, C • 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

  32. tRNAmolecules Growingpolypeptide Largesubunit mRNA Small subunit Figure 10.12A • 10.12 Ribosomes build polypeptides • A ribosome consists of two subunits • Each made up of proteins and a kind of RNA called ribosomal RNA

  33. The subunits of a ribosome • Hold the tRNA and mRNA close together during translation tRNA-binding sites Largesubunit Next amino acid to be added to polypeptide Growing polypeptide tRNA mRNA-binding site mRNA Smallsubunit Codons Figure 10.12B, C

  34. Start of genetic message End Figure 10.13A • 10.13 An initiation codon marks the start of an mRNA message

  35. Large ribosomalsubunit Met Met Initiator tRNA P site A site U C U A C A A U G AUG Startcodon Small ribosomalsubunit mRNA 1 2 Figure 10.13B • mRNA, a specific tRNA, and the ribosome subunits • Assemble during initiation

  36. 10.14 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

  37. 1 Codon recognition 2 Peptide bondformation Translocation 3 • Each addition of an amino acid • Occurs in a three-step elongation process Aminoacid Polypeptide P site A site Anticodon mRNA Codons mRNAmovement Stopcodon New Peptidebond Figure 10.14

  38. 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

  39. Elongation continues • Until a stop codon reaches the ribosome’s A site, terminating translation

  40. 10.15 Review: The flow of genetic information in the cell is DNARNAprotein • The sequence of codons in DNA, via the sequence of codons • Spells out the primary structure of a polypeptide

  41. 1     mRNA is transcribed from a DNA template. 2      Each amino acidattaches to its propertRNA with the help of aspecific enzyme and ATP. 3       Initiation ofpolypeptide synthesis The mRNA, the first tRNA, and the ribosomal subunits come together. 4 Elongation A succession of tRNAsadd their amino acids to the polypeptide chain as the mRNA is moved through the ribosome, one codon at a time. 5 Termination The ribosome recognizes a stop codon. The poly-peptide is terminated and released. • Summary of transcription and translation DNA Transcription mRNA RNApolymerase Amino acid Translation Enzyme ATP tRNA Anticodon Largeribosomal subunit InitiatortRNA Start Codon Smallribosomal subunit mRNA New peptidebond forming Growingpolypeptide Codons mRNA Polypeptide Figure 10.15 Stop codon

  42. Normal hemoglobin DNA Mutant hemoglobin DNA C A T T T C mRNA mRNA G A A G U A Normal hemoglobin Sickle-cell hemoglobin Glu Val Figure 10.16A • 10.16 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

  43. Normal gene U G C U U C A G A A U G A G G mRNA Met Lys Gly Protein Phe Ala Base substitution A A G A U G C A U G A G U U C Lys Met Phe Ser Ala Missing U Base deletion G G C G A C A U A U G A G U U Figure 10.16B Lys Ala His Met Leu • Substituting, inserting, or deleting nucleotides alters a gene • With varying effects on the organism

  44. MICROBIAL GENETICS • 10.17 Viral DNA may become part of the host chromosome • Viruses • Can be regarded as genes packaged in protein

  45. When phage DNA enters a lytic cycle inside a bacterium • It is replicated, transcribed, and translated • The new viral DNA and protein molecules • Then assemble into new phages, which burst from the host cell

  46. In the lysogenic cycle • Phage DNA inserts into the host chromosome and is passed on to generations of daughter cells • Much later • It may initiate phage production

  47. Phage Attachesto cell Bacterialchromosome Phage DNA Cell lyses,releasing phages Phage injects DNA Many celldivisions Lytic cycle Lysogenic cycle Phages assemble Lysogenic bacterium reproducesnormally, replicating the prophageat each cell division Phage DNAcircularizes Prophage OR Phage DNA inserts into the bacterialchromosome by recombination New phage DNA andproteins are synthesized Figure 10.17 • Phage reproductive cycles 1 7 2 4 3 5 6

  48. Membranousenvelope RNA Protein coat Figure 10.18A Glycoprotein spike CONNECTION • 10.18 Many viruses cause disease in animals • Many viruses cause disease • When they invade animal or plant cells • Many, such as flu viruses • Have RNA, rather than DNA, as their genetic material

  49. 1 Entry 2 Uncoating RNA synthesisby viral enzyme 3 RNA synthesis(other strand) 4 Proteinsynthesis 5 6 Assembly • Some animal viruses • Steal a bit of host cell membrane as a protective envelope • Can remain latent in the host’s body for long periods Glycoprotein spike VIRUS Protein coat Viral RNA(genome) Envelope Plasma membraneof host cell Viral RNA(genome) mRNA Template New viralgenome Newviral proteins Exit 7 Figure 10.18B

  50. RNA Protein Figure 10.19 CONNECTION • 10.19 Plant viruses are serious agricultural pests • Most plant viruses • Have RNA genomes • Enter their hosts via wounds in the plant’s outer layers

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