Download DNA: Structure, Function, and Replication

DNA: Structure, Function, and Replication
Scientists and Experiments
Year Scientists
1928
Frederick
Griffith
Experiment
● Studied Streptococcus pneumoniae
● Used 2 strains:
- Virulent – S-strain (Smooth – cannot be attacked by organisms defenses
because it is protected by a capsule)  will cause death
- Harmless or Non-virulent – R-strain (Rough – no capsule for protection,
so will be killed by organisms defenses)  will NOT cause death in
organism
● Boiled Virulent (S-strain) and then injected mice  mice survived
● Boiled Virulent (S-strain) and mixed it with living Harmless bacteria (Rstrain), then injected this into the mice  mice died  live virulent bacteria
were extracted from the mice
● Evidence showed that the harmless living cells (R-strain) were transformed
into the virulent cells (S-strain); some chemical component of the dead cells
caused this change
● Transformation  change in the genotype or phenotype of a cell due to
assimilation of outside DNA
General Findings
● One of the first to
discover the genetic role
of DNA
● Discovered transformation
Question: What was the transforming agent???
1944
Oswald
Avery
(with
McCarty
and
MacLeod)
● Worked with a similar experiment to Griffith
● He purified various chemicals from the heat-killed virulent bacteria; then he
tried to transform live harmless bacteria from the purified chemicals
● The only substance that worked and allowed transformation was DNA
● His findings and conclusion were met with lots of skepticism
● He proved the
transforming agent was
DNA
● Phage  viruses that infect bacteria
● T2 phage  infects E. coli; made of protein and DNA; can turn an E. coli cell
into a T2-producing factory
● Hershey and Chase wanted to know which component, protein or DNA, were
responsible for this “reprogramming” of the host cell
1952
1947
Early
1950’s
● Showed that DNA was the
genetic material of the
phage T2 by tagging both
● Experiment: Grew two types of T2 phage 
DNA and proteins with
- Batch 1 – grown in presence of radioactive sulfur, marking the proteins
Alfred
radioactive isotopes
- Batch 2 – grown in presence of radioactive phosphorus, marking the DNA
Hershey ● Allowed both batches of T2 to infect separate E. coli cultures
and Martha ● Put each infected E. coli batch in a blender and spun it; then centrifuged it out; ● Showed strong evidence
that nucleic acids, not
Chase
this separated out the heavier bacteria (pellet) and lighter phages/empty
proteins, are the hereditary
protein shell (supernatant)
material
● Tested both the pellet and supernatant for presence of radioactivity
● Findings:
- Radio-labeled proteins mostly in supernatant (phages)
- Radio-labeled DNA mostly in the pellet (bacteria)
 Therefore the injected DNA (not the proteins) provides the genetic
information that makes the infected cells produce new viral DNA and proteins
Erwin
Chargaff
Rosalind
Franklin
and
Maurice
Wilkins
● Knew DNA was composed of nitrogen base (A, T, C, G), sugar, and a
phosphate group (nucleotide)
● Analyzed DNA composition of many species of organisms and came up with
Chargaff’s rules:
- adenine and thymine are present in roughly equal amounts
- cytosine and guanine are present in roughly equal amounts
- Therefore, A = T; C = G
● In various species, the four different nitrogen bases are not present in equal
amounts, but are in characteristic ratios
● Took X-ray diffraction images of DNA using the X-ray crystallography
technique
● The diffraction images were used to deduce the three dimensional shape of
DNA
● This information was used by Watson and Crick when making their models
● Equal amounts of A with T
and C with G (Chargaff’s
rules)
● All species have the same
RATIO of bases
● Took X-ray images of
DNA, and was able to
deduce the three
dimensional structure of
the molecule
● Used Franklin’s X-ray images to make a model of DNA
James
● Figured out that the sugar-phosphate backbone is on the outside, and the
Mid
nitrogen base pairings are on the inside
Watson
1950’s and Francis ● Watson & Crick’s model explained Chargaff’s rules (A=T; C=G)
● Based on their model, they came up with a mechanism for replication
Crick
● Semiconservative Model  During replication, each new daughter strand has
one new strand and one old strand
● Experimented to see which model, the conservative, semiconservative, or
dispersive model were accurate for DNA replication (p.294)
- Conservative Model  the parental DNA remains intact and the new
“daughter strand” is composed of all new DNA
- Semiconservative Model  after replication, each of the 2 new molecules
are composed of one parental strand and one new daughter strand
- Dispersive Model  the replicated DNA consists of pieces of parental
Matthew
DNA and pieces of new DNA in each strand
Late
Meselson ● Cultured several generations of E. coli with a heavy isotope of nitrogen (N15),
1950’s
and
which was eventually incorporated into the DNA of the bacteria
Franklin
● The E. coli was then transferred into a new medium containing N14, a lighter
Stahl
form of nitrogen; therefore, any new DNA made would have the N14, not the
heavier N15
● DNA was then extracted from the Bacteria and centrifuged to separate it out
by the different densities
- In the first replication, showed a band of hybrid densities. This supported
the semiconservative and dispersive models, and ruled out the
conservative model
- The second replication produced a 2 bands of densities: one for the
originally replicated DNA (old and new strand – first replication) and one
for the newly replicated DNA (2 new strands – second replication); this
supported the semiconservative model, and ruled out the dispersive model
● Discovered the structure
of DNA
● Hypothesized mechanism
for DNA replication
● Semiconservative model
of replication
● Proved that Watson &
Crick’s semiconservative
model of replication was
correct