DNA Structure and Function Chapter 9

Miescher Discovered DNA 1868 Johann Miescher investigated chemical composition of nucleus Isolated organic acid high in phosphorus He called it nuclein We call it DNA (deoxyribonucleic acid)

Griffith Discovers Transformation 1928 Attempting to develop a vaccine Isolated two strains of Streptococcus pneumoniae –Rough strain was harmless –Smooth strain was pathogenic

Griffith's experiment Griffith Discovers Transformation

Transformation Harmless R cells were transformed by material from dead S cells Descendents of transformed cells were also pathogenic

What Is the Transforming Material? Avery found protein-digesting enzymes did not change results – extracts still transformed bacteria But treated with DNA-digesting enzymes – extracts lost transforming ability Concluded that DNA, not protein, transforms bacteria

Bacteriophages Viruses that infect bacteria Consist of protein and DNA Inject their hereditary material into bacteria

Bacteriophages

Hershey-Chase experiments

Subunits of DNA Watson-Crick Model

Hershey and Chase’s Experiments Created labeled bacteriophages –Radioactive sulfur –Radioactive phosphorus Allowed labeled viruses to infect bacteria Where were the radioactive labels after infection?

virus particle labeled with 35 S DNA (blue) being injected into bacterium 35 S remains outside cells virus particle labeled with 32 P DNA (blue) being injected into bacterium 35 P remains inside cells Fig. 9-2, p.139

Hershey and Chase Results 35 S remains outside cells 32 P remains inside cells

Structure of DNA 2nm diameter overall 0.34 nm between each pair of bases 3.4 nm length of each full twist of helix In 1953, Watson and Crick showed that DNA is a double helix

DNA close up Watson-Crick Model

Watson and Crick

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

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

Nucleotide Bases phosphate group deoxyribose ADENINE (A) THYMINE (T) CYTOSINE (C) GUANINE (G)

Composition of DNA 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 A=T and G=C

DNA Structure Allows It to Duplicate Two nucleotide strands held together by hydrogen bonds Hydrogen bonds between two strands are easily broken Each single strand serves as template for new strand

Rosalind Franklin’s Work Expert in x-ray crystallography Used technique to examine DNA fibers Concluded that DNA was some sort of helix

2-nanometer diameter overall 0.34-nanometer distance between each pair of bases 3.4-nanometer length of each full twist of the double helix In all respects shown here, the Watson–Crick model for DNA structure is consistent with the known biochemical and x-ray diffraction data. The pattern of base pairing (A only with T, and G only with C) is consistent with the known composition of DNA (A = T, and G = C). Fig. 9-6, p.141 DNA Models

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

Base Pairing during Replication Each old strand is template for new complementary strand

DNA replication details Base Pairing during Replication

Enzymes in Replication Enzymes unwind the two strands and complementary base pairs unzip DNA polymerase attaches new complementary nucleotides DNA ligase fills in gaps Enzymes wind two strands together

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

Clones Nuclear transfer from adult cell Structural and functional problems –Most adult DNA inactive Potential benefits –Replacement organs –Endangered animals

Cloning Making a genetically identical copy of an individual Researchers have been creating clones for decades Clones can be created by embryo splitting (artificial twinning)

How Dolly was created Cloning

Impacts, Issues Video Goodbye Dolly

More Clones Numerous species been cloned Mice, pigs, cattle, cats, etc. Most cloning attempts are still unsuccessful Many clones have defects Clones may vary in their phenotype

More Clones