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Molecular Biology of the Gene DNA Lesson Part 1 Dr. Wilson Muse Schoolcraft College 10.1 Experiments showed that DNA is the genetic material – Frederick Griffith discovered that a “transforming factor” could be transferred into a bacterial cell – Disease-causing bacteria were killed by heat – Harmless bacteria were incubated with heat-killed bacteria – Some harmless cells were converted to diseasecausing bacteria, a process called transformation – The disease-causing characteristic was inherited by descendants of the transformed cells Copyright © 2009 Pearson Education, Inc. 10.1 Experiments showed that DNA is the genetic material – Alfred Hershey and Martha Chase used bacteriophages to show that DNA is the genetic material – Bacteriophages are viruses that infect bacterial cells – Phages were labeled with radioactive sulfur to detect proteins or radioactive phosphorus to detect DNA – Bacteria were infected with either type of labeled phage to determine which substance was injected into cells and which remained outside Copyright © 2009 Pearson Education, Inc. 10.1 Experiments showed that DNA is the genetic material – The sulfur-labeled protein stayed with the phages outside the bacterial cell, while the phosphoruslabeled DNA was detected inside cells – Cells with phosphorus-labeled DNA produced new bacteriophages with radioactivity in DNA but not in protein Animation: Hershey-Chase Experiment Animation: Phage T2 Reproductive Cycle Copyright © 2009 Pearson Education, Inc. Head DNA Tail Tail fiber Head DNA Tail Tail fiber Radioactive protein Phage Bacterium Empty protein shell Radioactivity in liquid Phage DNA DNA Batch 1 Radioactive protein Centrifuge Pellet 2 Agitate in a blender to 1 Mix radioactively labeled phages with bacteria. The phages infect the bacterial cells. Batch 2 Radioactive DNA separate phages outside the bacteria from the cells and their contents. 3 Centrifuge the mixture so bacteria form a pellet at the bottom of the test tube. 4 Measure the radioactivity in the pellet and the liquid. Radioactive DNA Centrifuge Pellet Radioactivity in pellet Phage Radioactive protein Bacterium Phage DNA DNA Batch 1 Radioactive protein 2 Agitate in a blender to 1 Mix radioactively labeled phages with bacteria. The phages infect the bacterial cells. Batch 2 Radioactive DNA Empty protein shell separate phages outside the bacteria from the cells and their contents. Radioactive DNA Empty protein shell Radioactivity in liquid Phage DNA Centrifuge Pellet 3 Centrifuge the mixture so bacteria form a pellet at the bottom of the test tube. 4 Measure the radioactivity in the pellet and the liquid. Centrifuge Pellet Radioactivity in pellet 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. DNA • DNA is often called the blueprint of life. • In simple terms, DNA contains the instructions for making proteins within the cell. Nucleotides O O -P O O O O -P O O One deoxyribose together with its phosphate and base make a nucleotide. O O -P O O Phosphate Nitrogenous base O C C C O Deoxyribose 10.2 DNA and RNA are polymers of nucleotides – The monomer unit of DNA and RNA is the nucleotide, containing – Nitrogenous base – 5-carbon sugar – Phosphate group Copyright © 2009 Pearson Education, Inc. – DNA and RNA are polymers called polynucleotides – A sugar-phosphate backbone is formed by covalent bonding between the phosphate of one nucleotide and the sugar of the next nucleotide – Nitrogenous bases extend from the sugarphosphate backbone Animation: DNA and RNA Structure Copyright © 2009 Pearson Education, Inc. Sugar-phosphate backbone Phosphate group Nitrogenous base Sugar Nitrogenous base (A, G, C, or T) DNA nucleotide Phosphate group Thymine (T) Sugar (deoxyribose) DNA nucleotide DNA polynucleotide Nitrogenous base (A, G, C, or T) Phosphate group Thymine (T) Sugar (deoxyribose) Thymine (T) Cytosine (C) Pyrimidines Adenine (A) Guanine (G) Purines Nitrogenous base (A, G, C, or U) Phosphate group Uracil (U) Sugar (ribose) Hydrogen Bonds, cont. • When making hydrogen bonds, cytosine always pairs up with guanine, • And adenine always pairs up with thymine. • (Adenine and thymine are shown here.) O N O C C C C N C C CC N N N C N O • The bases attract each other because of hydrogen bonds. • Hydrogen bonds are weak but there are millions and millions of them in a single molecule of DNA. • (The bonds between cytosine and guanine are shown here.) C N Hydrogen Bonds Twist Hydrogen bond Base pair Ribbon model Partial chemical structure Computer model Base pair Ribbon model Hydrogen bond Partial chemical structure 10.4 DNA replication depends on specific base pairing – DNA replication follows a semiconservative model – The two DNA strands separate – Each strand is used as a pattern to produce a complementary strand, using specific base pairing – Each new DNA helix has one old strand with one new strand Animation: DNA Replication Overview Copyright © 2009 Pearson Education, Inc. Two Stranded DNA • Remember, DNA has two strands that fit together something like a zipper. • The teeth are the nitrogenous bases but why do they stick together? DNA by the numbers • Each cell has about 2 m of DNA. • The average human has 75 trillion cells. • The average human has enough DNA to go from the earth to the sun more than 400 times. • DNA has a diameter of only 0.000000002 m. The earth is 150 billion m or 93 million miles from the sun. DNA REPLICATION Makin’ copies ........ Copyright © 2009 Pearson Education, Inc. 10.4 DNA replication depends on specific base pairing – DNA replication follows a semiconservative model – The two DNA strands separate – Each strand is used as a pattern to produce a complementary strand, using specific base pairing – Each new DNA helix has one old strand with one new strand Animation: DNA Replication Overview Copyright © 2009 Pearson Education, Inc. Parental molecule of DNA Nucleotides Parental molecule of DNA Both parental strands serve as templates Nucleotides Parental molecule of DNA Both parental strands serve as templates Two identical daughter molecules of DNA 10.5 DNA replication proceeds in two directions at many sites simultaneously – DNA replication begins at the origins of replication – DNA unwinds at the origin to produce a “bubble” – Replication proceeds in both directions from the origin – Replication ends when products from the bubbles merge with each other – DNA replication occurs in the 5’ 3’ direction – Replication is continuous on the 3’ 5’ template – Replication is discontinuous on the 5’ template, forming short segments Copyright © 2009 Pearson Education, Inc. 3’ 10.5 DNA replication proceeds in two directions at many sites simultaneously – Proteins involved in DNA replication – DNA polymerase adds nucleotides to a growing chain – DNA ligase joins small fragments into a continuous chain Animation: Origins of Replication Animation: Leading Strand Animation: Lagging Strand Animation: DNA Replication Review Copyright © 2009 Pearson Education, Inc. Origin of replication Parental strand Daughter strand Bubble Two daughter DNA molecules 5 end P 5 4 3 2 1 P 3 end 2 3 1 4 5 P P P P P P 3 end 5 end DNA polymerase molecule 5 3 3 5 Daughter strand synthesized continuously Parental DNA 3 5 5 3 DNA ligase Overall direction of replication Daughter strand synthesized in pieces THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN Copyright © 2009 Pearson Education, Inc. 10.6 The DNA genotype is expressed as proteins, which provide the molecular basis for phenotypic traits – A gene is a sequence of DNA that directs the synthesis of a specific protein – DNA is transcribed into RNA – RNA is translated into protein – The presence and action of proteins determine the phenotype of an organism Copyright © 2009 Pearson Education, Inc. 10.6 The DNA genotype is expressed as proteins, which provide the molecular basis for – Demonstratingphenotypic the connectionstraits between genes and proteins – The one gene–one enzyme hypothesis was based on studies of inherited metabolic diseases – The one gene–one protein hypothesis expands the relationship to proteins other than enzymes – The one gene–one polypeptide hypothesis recognizes that some proteins are composed of multiple polypeptides Copyright © 2009 Pearson Education, Inc. DNA Nucleus Cytoplasm DNA Transcription RNA Nucleus Cytoplasm DNA Transcription RNA Nucleus Cytoplasm Translation Protein