CP BIOLOGY
DNA NOTES
DNA Research
 Griffith
o studied bacteria that caused pneumonia in mice which killed them
o results of injecting bacteria into mice:
 pneumonia-causing bacteria injected  mice died
 harmless bacteria strain injected  mice lived
 heat-killed pneumonia-causing bacteria injected  mice lived
 mix of heat-killed pneumonia-causing bacteria and harmless
bacteria injected  mice died
o concluded that harmless bacteria must have been transformed by
something in the heat-killed pneumonia-causing bacteria – some
genetic factor that could be passed from one bacterium to another
 Avery
o repeated Griffith’s experiments but used a bacterial extract treated
with enzymes that could destroy molecules
o found that destroying proteins, lipids, carbohydrates, and RNA did
not stop transformation – mice still died
o when DNA destroyed – transformation did not occur and the mice
lived
o concluded that DNA must store and transmit genetic information
from one generation to the next
 Hershey & Chase
o studied bacteriophages – viruses that infect and kill bacteria, work by
inserting DNA into bacteria, made up of a protein coat with DNA
inside it
o marked viruses with radioactive isotopes
 marked protein with radioactive sulfur (no sulfur in DNA)
 marked DNA with radioactive phosphorus (none in proteins)
o infected bacteria and found only radioactive phosphorus in them
o concluded that genetic material in bacteriophage was DNA – not
protein
CP BIOLOGY, DNA NOTES, page 2
Structure of DNA
 DNA is a long molecule made of nucleotides
 each nucleotide has three parts:
o 5-carbon sugar called deoxyribose
o phosphate group
o nitrogenous base
 four nitrogenous bases found in DNA
o two purines – have two rings in base – adenine and guanine
o two pyrimidines – have one ring in base – thymine and cytosine
 backbone of DNA is made up of phosphate groups and sugars
 Chargaff’s rules: amount of A = amount of T in sample of DNA, while amount
of G = amount of C in sample of DNA
 Franklin – studied shape of DNA using X-ray diffraction, found an X-shaped
pattern
 Watson and Crick were building 3-D models of DNA using wire and
cardboard
o determined that DNA was a double helix with the two strands
wrapped around each other after seeing Franklin’s X-ray diffraction
results
o developed principle of base pairing – A always bonds with T, G always
bonds with C (bond using hydrogen bonds)
o double helix looks like a twisted ladder
CP BIOLOGY, DNA NOTES, page 3
DNA
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and Chromosomes
DNA is a very long molecule, makes up chromosomes
in a single human cell – the DNA would stretch more than one meter
eukaryotic chromosomes contain DNA and protein tightly packed together =
chromatin
DNA wraps tightly around proteins called histones forming beadlike
structures called nucleosomes
nucleosomes pack together and form thick fibers (systems of coils and
loops)
normally fibers dispersed in nucleus
during mitosis fibers draw together and pack tightly into chromosome
nucleosomes rearrange to open regions of DNA to “read” genes
DNA Replication
 each strand of helix can be separated and used to reconstruct the other
strand by base pairing
 during replication – DNA molecule separates and produces two new
complementary strands through base pairing – each strand is a template
(model) for new strand
 prokaryotes – starts replicating at a single point
 eukaryotes – replication starts at many points and moves in both directions
until the entire chromosome is completely replicated
 each new copy has one original strand and one new strand
 several enzymes are involved in the process – unzip DNA, add nucleotides,
etc.
 DNA polymerase
o polymerizes nucleotides into DNA
o proofreads each new DNA strand