DNA Structure and Function Chapter 13

Miescher Discovered DNA Johann Miescher investigated the chemical composition of the nucleus Johann Miescher investigated the chemical composition of the nucleus Isolated an organic acid that was high in phosphorus Isolated an organic acid that was high in phosphorus He called it nuclein He called it nuclein We call it DNA (deoxyribonucleic acid) We call it DNA (deoxyribonucleic acid)

Mystery of the Hereditary Material Originally believed to be an unknown class of proteins Originally believed to be an unknown class of proteins Thinking was Thinking was Heritable traits are diverse Heritable traits are diverse Molecules encoding traits must be diverse Molecules encoding traits must be diverse Proteins are made of 20 amino acids and are structurally diverse Proteins are made of 20 amino acids and are structurally diverse

Structure of the Hereditary Material Experiments in the 1950s showed that DNA is the hereditary material Experiments in the 1950s showed that DNA is the hereditary material Scientists raced to determine the structure of DNA Scientists raced to determine the structure of DNA Watson and Crick proposed that DNA is a double helix Watson and Crick proposed that DNA is a double helix Figure 13.2 Page 217

Griffith Discovers Transformation Attempting to develop a vaccine Attempting to develop a vaccine Isolated two strains of Streptococcus pneumoniae Isolated two strains of Streptococcus pneumoniae Rough strain was harmless Rough strain was harmless Smooth strain was pathogenic Smooth strain was pathogenic

Griffith Discovers Transformation 1. Mice injected with live cells of harmless strain R. 2. Mice injected with live cells of killer strain S. 3. Mice injected with heat-killed S cells. 4. Mice injected with live R cells plus heat- killed S cells. Mice die. Live S cells in their blood. Mice live. No live R cells in their blood. Mice die. Live S cells in their blood. Mice live. No live S cells in their blood. Figure 13.3 Page 218

Transformation What happened in the fourth experiment? What happened in the fourth experiment? The harmless R cells had been transformed by material from the dead S cells The harmless R cells had been transformed by material from the dead S cells Descendents of the transformed cells were also pathogenic Descendents of the transformed cells were also pathogenic

Oswald & Avery What is the transforming material? What is the transforming material? Cell extracts treated with protein-digesting enzymes could still transform bacteria Cell extracts treated with protein-digesting enzymes could still transform bacteria Cell extracts treated with DNA-digesting enzymes lost their transforming ability Cell extracts treated with DNA-digesting enzymes lost their transforming ability Concluded that DNA, not protein, transforms bacteria Concluded that DNA, not protein, transforms bacteria

Bacteriophages Viruses that infect bacteria Viruses that infect bacteria Consist of protein and DNA Consist of protein and DNA Inject their hereditary material into bacteria Inject their hereditary material into bacteria cytoplasm bacterial cell wall plasma membrane Figure 13.4b Page 219

Hershey & Chase’s Experiments Created labeled bacteriophages Created labeled bacteriophages Radioactive sulfur Radioactive sulfur Radioactive phosphorus Radioactive phosphorus Allowed labeled viruses to infect bacteria Allowed labeled viruses to infect bacteria Asked: Where are the radioactive labels after infection? Asked: Where are the radioactive labels after infection?

virus particle labeled with 35 S virus particle labeled with 32 P bacterial cell (cutaway view) label outside cell label inside cell Hershey and Chase Results Figure 13.5 Page 219

Structure of Nucleotides in DNA Each nucleotide consists of Each nucleotide consists of Deoxyribose (5-carbon sugar) Deoxyribose (5-carbon sugar) Phosphate group Phosphate group A nitrogen-containing base A nitrogen-containing base Four bases Four bases Adenine, Guanine, Thymine, Cytosine Adenine, Guanine, Thymine, Cytosine

Nucleotide Bases phosphate group deoxyribose ADENINE (A) THYMINE (T) CYTOSINE (C) GUANINE (G) Figure 13.6 Page 220

Composition of DNA Chargaff showed: Chargaff showed: Amount of adenine relative to guanine differs among species Amount of adenine relative to guanine differs among species Amount of adenine always equals amount of thymine and amount of guanine always equals amount of cytosine Amount of adenine always equals amount of thymine and amount of guanine always equals amount of cytosine A=T and G=C

Rosalind Franklin’s Work Was an expert in X-ray crystallography Was an expert in X-ray crystallography Used this technique to examine DNA fibers Used this technique to examine DNA fibers Concluded that DNA was some sort of helix Concluded that DNA was some sort of helix

Watson-Crick Model DNA consists of two nucleotide strands DNA consists of two nucleotide strands Strands run in opposite directions Strands run in opposite directions Strands are held together by hydrogen bonds between bases Strands are held together by hydrogen bonds between bases A binds with T and C with G A binds with T and C with G Molecule is a double helix Molecule is a double helix

Watson-Crick Model Figure 13.7 Page 221

DNA Structure Helps Explain How It Duplicates DNA is two nucleotide strands held together by hydrogen bonds DNA is two nucleotide strands held together by hydrogen bonds Hydrogen bonds between two strands are easily broken Hydrogen bonds between two strands are easily broken Each single strand then serves as template for new strand Each single strand then serves as template for new strand

DNA Replication new old Each parent strand remains intact Each parent strand remains intact Every DNA molecule is half “old” and half “new” Every DNA molecule is half “old” and half “new” Figure 13.9 Page 222

Base Pairing during Replication Each old strand serves as the template for complementary new strand Figure Page 223

Enzymes in Replication Enzymes unwind the two strands Enzymes unwind the two strands DNA polymerase attaches complementary nucleotides DNA polymerase attaches complementary nucleotides DNA ligase fills in gaps DNA ligase fills in gaps Enzymes wind two strands together Enzymes wind two strands together

Continuous and Discontinuous Assembly Strands can only be assembled in the 5’ to 3’ direction Figure Page 223

DNA Repair Mistakes can occur during replication Mistakes can occur during replication DNA polymerase can read correct sequence from complementary strand and, together with DNA ligase, can repair mistakes in incorrect strand DNA polymerase can read correct sequence from complementary strand and, together with DNA ligase, can repair mistakes in incorrect strand

Cloning Making a genetically identical copy of an individual Making a genetically identical copy of an individual Researchers have been creating clones for decades Researchers have been creating clones for decades These clones were created by embryo splitting These clones were created by embryo splitting

Showed that differentiated cells could be used to create clones Showed that differentiated cells could be used to create clones Sheep udder cell was combined with enucleated egg cell Sheep udder cell was combined with enucleated egg cell Dolly is genetically identical to the sheep that donated the udder cell Dolly is genetically identical to the sheep that donated the udder cell Dolly: Cloned from an Adult Cell

More Clones Mice Mice Cows Cows Pigs Pigs Goats Goats Guar (endangered species) Guar (endangered species)