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Chapter 14
Vocabulary:
Purine
Pyrimidine
Chargaff’s rule
Semiconservative replication
DNA polymerase I
DNA polymerase II
DNA polymerase III
Helicase
Primase
Gyrase
Ligase
Single-strand binding protein
Outline
14.1 The Nature of the Genetic Material
Chromosomes are composed of DNA and protein
Griffith’s Transformation experiments
R strain is harmless
S strain is pathogenic
Mice + R = healthy
Mice + S = death (blood samples had live bacteria in it)
Mice + heat killed S = healthy
Mice + live R + heat killed S = death (blood samples had live S bacteria in it)
Explanation: S had been killed, but its genetic information (the component that
made it lethal) was still intact.
Avery, MacLeod, McCarty extension of transformation experiments
Transformation still occurs after treatment with protease
Transformation still occurs after treatment with lipase
Transformation still occurs after treatment with RNase
Transformation ceases after treatment with DNase
Explanation: DNA carries the genetics of the bacteria that allowed for
transformation; not proteins
Hershey Chase Phage experiments
Label P of bacteriophage DNA
Label S of bacteriophage proteins
Allow phage to infect bacteria
Labeled P found in cells; labeled S found in supernatant
Explanation: DNA carries the genetic code of the virus; not proteins
14.2 DNA Structure
Polymer of nucleotides
3 components of a nucleotide:
5 carbon sugar (DNA uses deoxyribose; RNA uses ribose)
Phosphate group
Nitrogenous base
Purines – adenine and guanine
Pyrimidines - cytosine, thymine in DNA and uracil in RNA
Chargaff’s Rule: A=T, C≡G
Rosalind Franklin: x-ray diffraction studies indicate DNA is helical
Watson and Crick’s Double helix:
Covalent phosphodiester bonds between nucleotides on outside
Hydrogen bonds between bases (complementary base pairing) on inside
Anti-parallel (5’ 3’ and 3’ 5’)
14.3 Basic Characteristics of DNA Replication
Semi-conservative
Messelson-Stahl proof of semiconservative replication
Bacteria grown in heavy N isotope (incorporates into DNA)
Bacteria allowed to divide in light N isotope
Cells harvested in intervals
Immediate: all DNA heavy
Round one: all DNA medium density (between light and heavy)
Round two: half DNA medium density; half DNA light density
Explanation: initially all DNA was heavy, after first replication DNA is medium
because it has a heavy and a light strand, after second replication half DNA is medium
(heavy and light strand) and half DNA is light (2 light strands). Semi-conservative
model!
I don’t mention the incorrect possibilities for replication (why give them a reason to
second guess themselves on the exam!)
For replication DNA helix must be opened
Both strands can serve as a template
Complementary base pairing determines sequence of new strand
DNA polymerase adds new nucleotides 5’  3’
14.4 Prokaryotic Replication
Single circular chromosome
Begins at A=T rich region (less Hydrogen bonds) and proceeds bi-directionally
Components of the Replisome:
Helicase – uses ATP to open helix
SS binding proteins – keep single strand of DNA from re-annealing
DNA gyrase - prevent supercoiling of DNA by relieving torque (force that causes
twisting)
Primase – adds ribonucleotides to start DNA strand
DNA polymerase III – uses energy in tri-phosphate nucleotides to add nucleotides
to growing strand of DNA using complementary base pairing
DNA polymerase I – replaces RNA primer and fills gaps
Ligase – seals nicks in DNA backbone
Leading strand can be synthesized continuously because DNA pol III can add free
nucleotides in a 5’  3’ direction.
Lagging strand is synthesized in Okazaki fragments so that DNA pol III can add in 5’ 
3’ on the anti-parallel strand.
14.5 Eukaryotic Replication
Multiple linear chromosomes
Multiple origins of replication
Primase uses RNA and DNA polymerases
Telomeres are replicated using telomerase.
Uses an internal RNA as a template and NOT the DNA
Activity decreases as we age
Link between aging and telomerase activity (or telomere length)
Is telomerase the fountain of youth or a highway to cancer!?!
14.6 DNA Repair
DNA polymerase II involved in repair
Sources of mutation:
Spontaneous (errors that escape proofreading during replication)
Radiation (x-rays and UV)
Environmental chemicals
Food and water contaminants
Natural sources
Specific repair looks for one specific form of damage and corrects it. (ex. Photo-repair)
Non-specific repair recognizes some form of damage, removes that region of DNA and
re-synthesizes the DNA using the undamaged strand
The history of the discovery of DNA is so dramatic that I spend (probably too much) time
on it. See supplemental notes! I mention names of researchers involved, but I only expect
them to remember Rosalind Franklin, Maurice Wilkins, James Watson and Francis Crick.
The structure of DNA is very important for understanding replication. The basic process
of replication, including enzymes, is fundamental and students should be very familiar
with it by the end of the chapter. I am not concerned with nuances between prokaryotic
and eukaryotic replication as much as an overall understanding of the process. Videos
help students visualize this process! I generally show a video first, then lecture about
replication, then show the video again. They should be familiar with mutation as a fact of
life, a source of evolution and a cell’s ability to correct them.