Download Bio1100Ch16W

CHAPTER 16
THE MOLECULE BASIS OF
INHERITANCE
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. 16.2
Dies
Live S
found
• What was the transforming substance in Griffith’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
Fig. 16.3
• 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. 16.3
• 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. 5.26
Hershey-Chase experiment (cont.)
• Now centrifuge – bacteria pellet and phage do not.
• Tested the pellet and ________________ of the
separate experiments for the presence of radioactivity.
Fig. 16.4
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.
•
_________________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 __________ had developed a series of
rules based on a survey of DNA _______________
in organisms.
• Known that DNA was a polymer of nucleotides
consisting of a _____________ base, deoxyribose, and a
phosphate group.
• Known that there were four _________ -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• 1. The number of adenines was approximately equal
to the number of ____________ (%T = %A).
• 2. The number of guanines was approximately equal
to the number of ___________ (%G = %C).
• _________ DNA is 30.9% adenine, 29.4% thymine,
19.9% guanine and 19.8% cytosine.
2. 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 _______
of the next nucleotide
in line.
• The result is a
“_____________ ” of
alternating
phosphates and
sugars, from which
the bases project.
Fig. 16.3
T
Bases
A
C
G
Fig. 16.5
How was the structure of DNA resolved??
• Answer: By ________ crystallography – (done by Maurice
Wilkins and Rosalind Franklin)……….
• In this technique, X-rays are diffracted as they passed through
aligned fibers of purified DNA.
• The diffraction pattern can be used to deduce the three-dimensional
shape of molecules.
• ….AND by _________
• James Watson learned
from their research
that DNA was _________
in shape and he deduced
the width of the helix
and the spacing of bases.
Franklin
Fig. 16.6
• Watson and his colleague Francis Crick began to
work on a model of DNA with two strands,
the________________ .
• Using molecular models made of wire, they first
tried to place the sugar-phosphate chains on
the________ .
• However, this did not ______ the X-ray
measurements and other information on the
chemistry of DNA.
• The key _______________ came when Watson put
the sugar-phosphate chain on the outside and the
nitrogen bases on the inside of the double helix.
The double helix- a _________ homology
Pairs of nitrogen bases, one
from each strand, form
rungs.
The
ladder
forms a
twist
every
_____
bases
Fig. 16.7
The _______________
chains of each strand
are like the side ropes
of a rope ladder.
• Pairing like nucleotides did not fit the uniform
diameter indicated by the X-ray data.
•A __________________pair would be too
wide and a pyrimidine-pyrimidine pairing
would be too short.
•Only a ____________________
pairing would produce the _____
diameter indicated by the X-ray data.
• 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. 16.8
• 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 came in later years
3. 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. 16.9
Doublestranded
DNA
1.Separate
strands
2.Add
complemen
tary bases
3.Connect
bases
But how was the DNA copied??
Three models proposed
• #1-Watson and Crick’s model -___________________
replication (i.e. each of the daughter strands will have one
old strand and one newly made strand.
• #2- The _______________model- (an all new copy of DNA is
made)
• #3- The _______________model- daughter stands contain
both old and new DNA
Parent DNA
1st replication
2nd replication
Fig. 16.10
Conservative
SemiConservative
Dispersive
Which model is correct??
• Late 1950s - Matthew Meselson and Franklin_____
provided evidence for the _______________model.
Experiment
1. Label bacterial DNA with a heavy ________ of
Nitrogen (15N), then __________ in 14N medium
2. _______replicated strands by density in a centrifuge.
Prediction:
20 min
40 min
Fig. 16.11
Which model is correct??
Results
After 1st replication- only _______(15N-14N) DNA detected
Thus, conservative model is____________
After 2nd replication- both light and hybrid (15N-14N)
DNA detected
Thus, the ________________ model is incorrect
Why? Because no light DNA should be present here
Thus, semiconservative model is correct.
4. 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- )- 40 minutes
• Human cells- (6 billion base pairs)- 6 hours
___________
• Less than one error per billion 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.
• The replication bubbles _____________as the DNA is
replicated and eventually _________ .
Fig. 16.12
15
21
22
23.1
23.3
31.1
31.2
31.3
32
33.1
33.3
34
35.1
• DNA polymerases add the _________ (nitrogen
base, deoxyribose, and a triphosphate tail).
• Rate of elongation
• 500 nucleotides/second in bacteria
• 50 per second in human cells. The raw nucleotides
are nucleoside triphosphates.
Two phosphates are
removed upon addition of
nucleotide
Fig ure not in text
One
_________
connects
nucleotides
• The strands in the double helix are______________.
• The sugar phosphate____________run in opposite
directions.
5’ end
“ _________ “ terms derived
from the carbon number in
the ribose sugar ring
3’ end
• Each DNA strand has a 3’
end with a free _________
group attached to
deoxyribose and a _______
with a free __________
group attached to
deoxyribose.
• The 5’ -> 3’ direction of
one strand runs counter to
the 3’ -> 5’ direction of
the other strand.
Fig. 16.13
• At the replication fork, one parental
strand (3’-> 5’ into the fork),
the_________________, can be used
by polymerases as a template for a
continuous complimentary strand.
3’
Problem- DNA polymerases can only add
nucleotides to the free _______ of a growing
DNA strand.
•But how is the other stand copied??
SolutionThe other parental strand (5’->3’ into the fork),
the__________________ , is copied away
from the fork in short segments (___________
fragments)
Fig. 16.14
How are Okazaki fragments generated??
1. The enzyme _________ adds
10 nucleotides of RNA to DNA
template
• This RNA sequence is
called a__________
2. DNA polymerase then starts at
the ______ end of the primer
3. The primer is then replaced by
DNA nucleotides and joined to
the previous Okazaki fragment
by_____________________ .
Note- Okazaki fragments are only
__________ nucleotides long, thus
___________ are used during
replication
Fig. 16.15
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
5. 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.
• In_______________ , special enzymes fix
incorrectly paired nucleotides.
• A hereditary defect in
one of these enzymes
is associated with a
form of colon cancer.
• In ___________________
repair, a nuclease cuts
out a segment of a
damaged strand.
• The gap is filled in by
_______________ and
ligase.
Fig. 16.17
6. The ends of DNA molecules are
replicated by a special mechanism
Problem: The usual
replication machinery
provides no way to
____________ the 5’ ends
of daughter DNA strands.
• Repeated rounds of
replication produce
___________________
DNA molecules.
Solution: An enzyme
called________________
Fig. 16.18
Fluorescent “dots”
Fig. 16.19a are telomeres
Background
• The ends of eukaryotic chromosomal DNA
molecules, the_____________ , have special
nucleotide sequences.
• In human telomeres, this sequence is
typically TTAGGG, repeated between
_______________________ times.
• Telomeres protect genes from being eroded
through multiple rounds of DNA replication.
How?
•Telomerase uses a short molecule of
_______ as a template to extend the 3’
end of the telomere.
Fig. 16.18
A few interesting facts about telomerase
• Telomerase is not present in most cells of
____________ organisms.
• Thus, the DNA of dividing somatic cells and cultured cells
does tend to become ____________ and may limit the
_______________ of cells.
• Telomerase is present in
• ______________ cells, ensuring that zygotes have long
telomeres.
• ______________ somatic cells.
• This overcomes the progressive shortening that would
eventually lead to self-destruction of the cancer.
Big question- Is telomere shortening responsible for the life
span of a human??