Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 7

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 7.1 The Griffith Experiment The work of Sutton and Morgan established that genes reside on chromosomes But chromosomes contain proteins and DNA So which one is the hereditary material Several experiments ultimately revealed the nature of the genetic material

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 7.1 The Griffith Experiment In 1728, Frederick Griffith discovered transformation while working on Streptococcus pneumoniae The bacterium exists in two strains S Forms smooth colonies in a culture dish Cells produce a polysaccharide coat and can cause disease R Forms rough colonies in a culture dish Cells do not produce a polysaccharide coat and are therefore harmless

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7.2 The Avery and Hershey-Chase Experiments Two key experiments that demonstrated conclusively that DNA, and not protein, is the hereditary material Oswald Avery and his coworkers Colin MacLeod and Maclyn McCarty published their results in 1744 Alfred Hershey and Martha Chase published their results in 1752

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Avery and his colleagues prepared the same mixture of dead S and live R bacteria as Griffith did They then subjected it to various experiments All of the experiments revealed that the properties of the transforming principle resembled those of DNA 1. Same chemistry and physical properties as DNA 2. Not affected by lipid and protein extraction 3. Not destroyed by protein- or RNA-digesting enzymes 4. Destroyed by DNA-digesting enzymes The Avery Experiments

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Viruses that infect bacteria have a simple structure DNA core surrounded by a protein coat Hershey and Chase used two different radioactive isotopes to label the protein and DNA Incubation of the labeled viruses with host bacteria revealed that only the DNA entered the cell Therefore, DNA is the genetic material The Hershey-Chase Experiment

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7.3 Discovering the Structure of DNA DNA is made up of nucleotides Each nucleotide has a central sugar, a phosphate group and an organic base The bases are of two main types Purines – Large bases Adenine (A) and Guanine (G) Pyrimidines – Small bases Cytosine (C) and Thymine (T)

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 7.3 The four nucleotide subunits that make up DNA 5-C sugar Nitrogenous base

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Erwin Chargaff made key DNA observations that became known as Chargaff’s rule Rosalind Franklin’s X-ray diffraction experiments revealed that DNA had the shape of a coiled spring or helix Purines = PyrimidinesA = T and C = G Rosalind Franklin ( ) Fig. 7.4

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In 1753, James Watson and Francis Crick deduced that DNA was a double helix Fig. 7.4 James Watson (1728- ) Francis Crick ( ) They came to their conclusion using Tinkertoy models and the research of Chargaff and Franklin

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7.4 How the DNA Molecule Replicates The two DNA strands are held together by weak hydrogen bonds between complementary base pairs A and T C and G ATACGCAT If the sequence on one strand isThe other’s sequence must be TATGCGTA Each chain is a complementary mirror image of the other So either can be used as template to reconstruct the other

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display There are 3 possible methods for DNA replication Fig. 7.5 Original DNA molecule is preserved Daughter DNAs contain one old and one new strand Old and new DNA are dispersed in daughter molecules

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 7.6 Thus, DNA replication is semi-conservative

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display How DNA Copies Itself The process of DNA replication can be summarized as such The enzyme helicase first unwinds the double helix The enzyme primase puts down a short piece of RNA termed the primer DNA polymerase reads along each naked single strand adding the complementary nucleotide

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 7.7 How nucleotides are added in DNA replication Template strandNew strand Sugar- phosphate backbone C G T A A T C G A A HO 3’ O O O O O O O O O O 5’ 3’ OH 5’ P P P P P P P P P P T O OH P P P DNA polymerase

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display DNA polymerase can only build a strand of DNA in one direction The leading strand is made continuously from one primer The lagging strand is assembled in segments created from many primers Fig. 7.8

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display RNA primers are removed and replaced with DNA Ligase joins the ends of newly-synthesized DNA Mechanisms exist for DNA proofreading and repair Fig. 7.7

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7.5 Transcription The path of genetic information is often called the central dogma DNARNAProtein A cell uses three kinds of RNA to make proteins Messenger RNA (mRNA) Transfer RNA (tRNA) Ribosomal RNA (rRNA)

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 7.5 Transcription Gene expression is the use of information in DNA to direct the production of proteins It occurs in two stages Fig. 7.7

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 7.5 Transcription The transcriber is RNA polymerase It binds to one DNA strand at a site called the promoter It then moves along the DNA pairing complementary nucleotides It disengages at a stop signal Fig. 7.11

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 7.6 Translation Translation converts the order of the nucleotides of a gene into the order of amino acids in a protein The rules that govern translation are called the genetic code mRNAs are the “blueprint” copies of nuclear genes mRNAs are “read” by a ribosome in three- nucleotide units, termed codons Each three-nucleotide sequence codes for an amino acid or stop signal

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The genetic code is (almost) universal Only a few exceptions have been found Fig. 7.7

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Ribosomes The protein-making factories of cells Fig Sites play key roles in translation They use mRNA to direct the assembly of a protein A ribosome is made up of two subunits Each of which is composed of proteins and rRNA

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Transfer RNA tRNAs bring amino acids to the ribosome They have two business ends Anticodon which is complementary to the codon on mRNA 3’–OH end to which the amino acid attaches Fig Hydrogen bonding causes hairpin loops 3-D shape

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Making the Protein mRNA binds to the small ribosomal subunit The large subunit joins the complex, forming the complete ribosome mRNA threads through the ribosome producing the polypeptide Fig. 7.16

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig How translation works The process continues until a stop codon enters the A site The ribosome complex falls apart and the protein is released

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7.7 Architecture of the Gene Most eukaryotic genes exist in multiple copies Clusters of almost identical sequences called multigene families As few as three and as many as several hundred genes Transposable sequences or transposons are DNA sequences that can move about in the genome They are repeated thousands of times, scattered randomly about the chromosomes

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