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DNA REPLICATION
HOW IS DNA DUPLICATED IN THE SYNTHESIS STAGE?
DISCOVERY OF THE GENETIC
MATERIAL
• Griffith – studied the bacteria that causes
pneumonia in mice; this disease-causing factor
could be passed between two strains of
bacteria
• Avery – using the same type of cells as Griffith,
broke the strains down to the component
macromolecules to determine nucleic acids
(DNA) were responsible for the transformation
• Hershey and Chase – confirmed DNA was the
transforming factor by using radioactive
labeling in viruses
DNA STRUCTURE
• Levene – basic structure of nucleotides
• Five carbon sugar, phosphate group, and nitrogenous
base
• Many nucleotides compose a nucleic acid (DNA or
RNA)
• Sugars
• DNA – deoxyribose (one less oxygen)
• RNA – ribose
• Nitrogenous Bases
• Guanine (G) and adenine (A) – double rings (purine)
• Thymine (T) and cytosine (C) – single rings (pyrimidine)
CHARGAFF’S RULE
• Amount of A = T and amount of C = G which
means…
• Chromosome consists of 51-245 million base pairs
• Only 5 centimeters long...how does it fit?
• Tightly coils around histones
• DNA + histone = nucleosome
WILKINS, FRANKLIN, WATSON, AND
CRICK
• Photo 51 via x-ray diffraction
• Double helix/twisted ladder
• Controversies in science
• DNA Structure
• Two outside strands
containing an alternating
phosphate and sugar
• Cytosine and Guanine paired
together by three hydrogen
bonds
• Thymine and adenine paired
together by two hydrogen
bonds
SO THAT’S THE STRUCTURE, HOW IS IT
REPLICATED?
• Semiconservative replication – one strand serves as
a template (Watson and Crick)
THE PROCESS
• Unwinding – DNA Helicase
• Replication fork
• Hydrogen bonds are broken
• RNA Primase follows behind to add an RNA primer
• Orientation – 3’ and 5’ ends
THE PROCESS (CONT.)
• DNA Polymerase adds the appropriately paired
nucleotides to the template DNA strand
• The template strand is read from the 3’ to the 5’ end
• The new strand is created in the 5’ to the 3’ orientation
antiparallel to the original one
New
Old
LEADING AND LAGGING STRANDS
• Because the template strand is always read from the 3’
to the 5’ end, it is “easier” for DNA polymerase to
replicate one of the template strands (leading strand).
• Okazaki Fragments result in the lagging strand
CONCLUSION
• DNA replication occurs in many places on the
chromosome at once
• Another DNA polymerase comes through and
replaces the RNA Primers with nucleotides
• DNA Ligase links the many fragments of DNA
• Including the Okazaki fragments
• Prokaryotic replication – one origin of replication
(DNA is called a plasmid)
• Transformation – bacteria extracts DNA from an outside
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