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 particlelabeled with 35S
DNA (blue)being injected into bacterium
35S remainsoutside cells
virus particlelabeled with 32P
DNA (blue)being injected into bacterium
35P remainsinside cells
Fig. 9-2, p.139
Hershey and Chase Results
35S remains outside cells
32P 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
newnew old 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 clonedMice, pigs, cattle, cats, etc.
• Most cloning attempts are still unsuccessful
• Many clones have defects
• Clones may vary in their phenotype
More Clones