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Rapid Pace of Disease Gene
Discovery
Total disease genes identified- 1,112 (as of yr 2000)
Susceptibility genes
DNA Structure and Function
Chapter 10
1. The search for genetic material lead to DNA
History
• 1940’s- T.H. Morgan’s group showed that genes are
located on chromosomes
• But…. Are _________ or DNA the genetic
material??
• Most believed it to be protein
• However, this was not consistent with experiments with
microorganisms, like bacteria and viruses.
• 1928- Frederick _______ - He studied Streptococcus
pneumoniae, a bacterium that causes pneumonia in mammals.
• One strain, the R strain, was harmless.
• The other strain, the S strain, was ___________.
• Experiment- Griffith mixed _____________ S strain with live R
strain bacteria and injected this into a mouse.
• Result- Mouse died, but the S strain was recovered from the
mouse’s blood.
S
R
Killed S R + Killed S
• Griffith called this
phenomenon,
______________-a
change in genotype
and phenotype due to
the assimilation of a
foreign substance
(now known to be
DNA) by a cell.
Injection
Dies
Lives
Lives
Fig. 16.1
Dies
Live S
found
• What was the transforming substance in Grifith’s
experiments??
• Finally in 1944, Oswald_______, Maclyn McCarty and Colin
MacLeod announced that the transforming substance was _____ .
• Still, many biologists were skeptical.
• In part, this reflected a belief that the genes of bacteria could not be
similar in composition and function to those of more complex
organisms.
• In 1952, Alfred Hershey and Martha Chase showed that DNA
was the genetic material of the phage T2
• Phage T2 is a ______ that infects bacteria
What is a virus??
• Viruses consist of a DNA (sometimes ____ ) enclosed by a
protective coat of protein.
• To replicate, a virus infects a host cell and takes over the
cell’s metabolic _________________ .
• The T2 phage, consisting almost entirely of
DNA and protein, attacks Escherichia coli
(E. coli), a common _______________
bacteria of mammals.
• This phage can quickly turn an E. coli cell
into a ___________ factory that releases
phages when the cell ruptures.
Fig. 16.2a
• The Hershey and Chase Experiment-Designed
to determine the source of genetic material in
the phage
• Procedure: Two parallel experiments
#1 Grow T2 phage in the presence of _____________sulfur,
marking the ___________ but not DNA
•Recall that sulfur is a component of some_____________.
ATP
#2 Grow T2 phage in the presence of
_____________ phosphorus , marking the
_______ but not proteins
•Recall that __________is a component of DNA
•Allow each batch to infect separate E. coli cultures.
•Shortly after infection, use blender to “shake loose” any
remaining phage components
Fig. 5.29
• Now
centrifuge –
bacteria pellet
and phage do
not.
• Tested the
pellet and
____________
of the
separate
experiments
for the
presence of
radioactivity.
virus particle
labeled with 35S
Hershey-Chase experiment (cont.)
virus particle
labeled with 32P
bacterial cell (cutaway view)
label
outside cell
label inside cell
Hershey-Chase experiment (cont.)
• Results of Hershey-Chase experiment• If used radioactive sulfur (proteins labeled)- most of
the radioactivity was in the______________, not in
the pellet.
• If used radioactive phosphorous(DNA labeled)most of the ______________ was in the pellet with
the bacteria.
•Conclusion- injected _______ of the
phage provides the _________
information for new viruses.
•
Circumstantial evidence that DNA is the genetic
material in eukaryotes
1. Cells double the amount of DNA in a cell prior to
________________ and then distribute the DNA
equally to each daughter
2. Diploid sets of chromosomes have twice as much DNA
as the __________ sets in gametes of the same
organism.
• 1947- Erwin Chargaff had developed a series of
rules based on a survey of DNA composition in
organisms.
• Known that DNA was a polymer of nucleotides
consisting of a nitrogenous base, deoxyribose, and a
phosphate group.
• Known that there were four bases -adenine (A), thymine
(T), guanine (G), or cytosine (C).
• Chargaff noted that the four bases are found in
characteristic, but not necessarily equal, ratios
• He also found a peculiar regularity in the ratios of
nucleotide bases which are known as Chargaff’s rules.
Chargaff’s rules• 1. The number of adenines was approximately equal
to the number of thymines (%T = %A).
• 2. The number of guanines was approximately equal
to the number of cytosines (%G = %C).
• Human DNA is 30.9% adenine, 29.4% thymine, 19.9%
guanine and 19.8% cytosine.
Nucleotide Bases
ADENINE
(A)
phosphate
group
GUANINE
(G)
deoxyribose
THYMINE
(T)
CYTOSINE
(C)
Watson and Crick discovered the double
helix by building models to conform to Xray data
• By the beginnings of the 1950’s, the race was on to
move from the structure of a single DNA strand to
the three-dimensional structure of DNA.
• Among the scientists working on the problem were Linus
______, in California, and Maurice Wilkins and Rosalind
________, in London.
The basic structure of DNA was
known by 1952
• The ________group
of one nucleotide is
attached to the
T
Bases
_____
of the next nucleotide
in line.
• The result is a
“__________ ” of
alternating
phosphates and
sugars, from which
the bases project.
A
C
G
Fig. 16.3
The double helix- a _________ homology
Pairs of nitrogen bases, one
from each strand, form
rungs.
The
ladder
forms a
twist
every
____
bases
Fig. 16.5
The _______________
chains of each strand
are like the side ropes
of a rope ladder.
• In addition, Watson and Crick determined that
chemical side groups off the nitrogen bases would
form hydrogen bonds, connecting the two strands.
• Adenine could
form two ________ bonds
only with thymine
• Guanine could form ______
hydrogen bonds only with
cytosine.
• This finding _________
Chargaff’s rules.
Fig. 16.6
• The linear sequence of the four bases can be varied
in countless ways.
• Each gene has a unique order of nitrogen bases.
• In April 1953, Watson and Crick published a
succinct, one-page paper in Nature reporting their
double helix model of DNA.
The Nobel Prize in Chemistry was awarded in later years
Base pairing is at the heart of DNA
replication
Base pairing enables existing DNA strands to serve as
templates for new complimentary strands
• In a second paper, Watson and Crick published their
_____________for how DNA replicates.
When a cell copies a DNA molecule, each strand serves as a _________
for ordering nucleotides into a new ________________ strand.
Fig. 16.7
Doublestranded
DNA
1.Separate
strands
2.Add
complemen
tary bases
3.Connect
bases
DNA
Replication
• Each parent strand
remains ________
• Every DNA
molecule is half
“____” and half
“_____”
new
old
old
new
DNA replication requires a large number
of enzymes and other proteins
A. DNA replication is fast and efficient
Time to copy the entire genome and divide into daughter cells:
• Bacterium E. coli (5 million base pairs- )- _______________
• Human cells- (6 billion base pairs)- ___________
Accuracy
• Less than one error per __________nucleotides.
Machinery
• More than a dozen enzymes and other proteins required
B. Where does replication start? –
Answer- The ______________________
Bacteria- one site in DNA recognized by specific proteins
Origin of replication (cont.)
• Eukaryotes- ______________ of origin sites
per chromosome.
15.3
15.2
15.1
14
13.3
13.1
12
11
11.1
11.2
12
13.1
13.2
13.3
14
• At each origin site, the DNA strands separate forming a
replication “______ ” with ______________ at each
end.
15
21
22
23.1
23.3
31.1
31.2
31.3
32
33.1
33.3
34
35.1
• The replication bubbles _______ as the DNA is
replicated and eventually ______ .
Fig. 16.10
• DNA polymerases add the ____________
(nitrogen base, deoxyribose, and a triphosphate
tail).
• Rate of elongation
• ____nucleotides/second in bacteria
• ___per second in human cells. The raw
nucleotides
One
are nucleoside triphosphates.
phosphate
Two phosphates are
removed upon addition of
nucleotide
Fig. 16.11
connects
nucleotides
Summary of DNA replication
• At the replication fork
• the leading stand is copied ______________ from a
single primer
• the lagging strand is copied in ____________ using
many primers.
Fig. 16.16
Enzymes proofread DNA during its replication
and repair damage in existing DNA
• Nucleotide pairing errors occur at a rate of one error
per __________ base pairs.
• Yet the final error rate is only one per billion nucleotides.
How do we account for this discrepency??
Answer- ______________
a. ______________ proofreads each new nucleotide against
the template nucleotide as soon as it is added. If incorrect, it
corrects it.
b. Each cell continually monitors and repairs its genetic
material, with over ______ repair enzymes identified in
humans.