Chapter 14
DNA: The Genetic
Material
Question?
Traits
are inherited on
chromosomes, but what in
the chromosomes is the
genetic material?
Two possibilities:
Protein
DNA
Qualifications
Protein:
very
complex.
high specificity of function.
DNA:
simple.
not
much known about it
(early 1900’s).
For testing:
Name(s)
of experimenters
Outline of the experiment
Result of the experiment and
the importance of the result
Griffith - 1928
Pneumonia
in mice.
Two strains:
S
- pathogenic
R - harmless
Griffith’s Experiment
Result
Something
turned the R cells
into S cells.
Transformation - the
assimilation of external
genetic material by a cell.
Problem
Griffith
used heat.
Heat denatures proteins.
So could proteins be the
genetic material?
DNA - heat stable.
Griffith’s results contrary to
accepted views.
Avery, McCarty and
MacLeod - 1944
Repeated
Griffith’s
experiments, but added
specific fractions of S cells.
Result - only DNA
transformed R cells into S
cells.
Result - not believed.
Hershey- Chase 1952
Genetic
information of a virus
or phage.
Phage - virus that attacks
bacteria and reprograms host
to produce more viruses.
Bacteria with Phages
Phage Components
Two
main chemicals:
Protein
DNA
Which
material is transferred
to the host?
Used Tracers
Protein
- CHONS, can trace
with 35S.
DNA - CHONP, can trace with
32P.
Experiment
Used
phages labeled with
one tracer or the other and
looked to see which tracer
entered the bacteria cells.
Result
DNA
enters the host cell, but
the protein did not.
Therefore:
DNA is the genetic material.
Picture Proof
Chargaff - 1947
Studied
the chemical
composition of DNA.
Found that the nucleotides
were in certain ratios.
Chargaff’s Rule
A
=T
G = C
Example: in humans,
A = 30.9%
T = 29.4%
G = 19.9%
C = 19.8%
Why?
Not
known until Watson and
Crick worked out the
structure of DNA.
Watson and Crick 1953
Used
X-ray crystallography
data (from Rosalind Franklin)
Used model building.
Result - Double Helix Model
of DNA structure.
(One page paper, 1953).
Book & Movies
“The
Double Helix” by James
Watson- His account of the
discovery of the shape of
DNA
Movie – The Double Helix
DNA Composition
Deoxyribose
Sugar (5-C)
Phosphate
Nitrogen
Bases:
Purines
Pyrimidines
DNA Backbone
Polymer
of sugar-phosphate.
2 backbones present.
Nitrogen Bases
Bridge
the backbones
together.
Purine + Pyrimidine = 3 rings.
Constant distance between
the 2 backbones.
Held together by H-bonds.
Chargaff’s Rule
Explained
by double helix
model.
A = T, 3 ring distance.
G = C, 3 ring distance.
Watson and Crick
Published
a second paper
(1954) that speculated on the
way DNA replicates.
Proof of replication given by
others.
Replication
The
process of making more
DNA from DNA.
Problem: when cells
replicate, the genome must
be copied exactly.
How is this done?
Models for DNA
Replication
Conservative
- one old
strand, one new strand.
Semiconservative - each
strand is 1/2 old, 1/2 new.
Dispersive - strands are
mixtures of old and new.
Replication Models
Meselson - Stahl
late 1950’s
Grew
bacteria on two isotopes
of N.
Started on 15N, switched to 14N.
Looked at weight of DNA after
one, then 2 rounds of
replication.
Results
Confirmed
the
Semiconservative Model of
DNA replication.
Replication - Preview
DNA
splits by breaking the
H-bonds between the
backbones.
Then DNA builds the missing
backbone using the bases on
the old backbone as a
template.
Origins of Replication
Specific
sites on the DNA
molecule that starts
replication.
Recognized by a specific
DNA base sequence.
Prokaryotic
Circular
DNA.
1 origin site.
Replication runs in both
directions from the origin
site.
Eukaryotic Cells
Many
origin sites.
Replication bubbles fuse to
form new DNA strands.
DNA Elongation
By
DNA Polymerases.
Adds DNA triphosphate
monomers to the growing
replication strand.
Matches A to T and G to C.
Energy for Replication
From
the triphosphate
monomers.
Loses two phosphates as
each monomer is added.
Problem of
Antiparallel DNA
The
two DNA strands run
antiparallel to each other.
DNA can only elongate in the
5’--> 3’ direction.
Leading Strand
Continuous
replication
toward the replication fork in
the 5’-->3’ direction.
Lagging Strand
Discontinuous
synthesis
away from the replication
fork.
Replicated in short segments
as more template becomes
opened up.
Priming
DNA
Polymerase cannot
initiate DNA synthesis.
Nucleotides can be added
only to an existing chain
called a Primer.
Primer
Make
of RNA.
10 nucleotides long.
Added to DNA by an enzyme
called Primase.
DNA is then added to the
RNA primer.
Priming
A
primer is needed for each
DNA elongation site.
Okazaki Fragments
Short
segments (100-200
bases) that are made on the
lagging strand.
All Okazaki fragments must
be primed.
RNA primer is removed after
DNA is added.
Enzymes
Replaces
RNA primers with
DNA nucleotides.
DNA Ligase - joins all DNA
fragments together.
Other Proteins
in Replication
Helicase
- unwinds the DNA
double helix.
Single-Strand Binding
Proteins - help hold the DNA
strands apart.
Enzyme Summary
Lets
see replication in action!
http://www.mhhe.com/socsci
ence/anthropology/stein2003/
stein.html
DNA Replication
Error Rate
1
in 1 billion base pairs.
About 3 mistakes in our DNA
each time it’s replicated.
Reasons for Accuracy
DNA
Polymerase self-checks
and corrects mismatches.
DNA Repair Enzymes a family of enzymes that
checks and corrects DNA.
DNA Repair
50+
different DNA repair
enzymes known.
Failure to repair may lead to
Cancer or other health
problems.
Example:
Xeroderma
Pigmentosum Genetic condition where a
DNA repair enzyme doesn’t
work.
UV light causes damage,
which can lead to cancer.
Xeroderma Pigmentosum
Cancer
Protected from UV
Thymine Dimers
T-T
binding from side to side
causing a bubble in DNA
backbone.
Often caused by UV light.
Excision Repair
Cuts
out the damaged DNA.
DNA Polymerase fills in the
excised area with new bases.
DNA Ligase seals the
backbone.
Problem - ends of DNA
DNA
Polymerase can only
add nucleuotides in the
5’--->3’ direction.
It can’t complete the ends of
the DNA strand.
Result
DNA
gets shorter and shorter
with each round of
replication.
Telomeres
Repeating
units of TTAGGG
(100- 1000 X) at the end of the
DNA strand (chromosome)
Protects DNA from unwinding
and sticking together.
Telomeres shorten with each
DNA replication.
Telomeres
Telomeres
Serve
as a “clock” to count
how many times DNA has
replicated.
When the telomeres are too
short, the cell dies by
apoptosis.
Implication
Telomeres
are involved with
the aging process.
Limits how many times a cell
line can divide.
Telomerase
Enzyme
that uses RNA to
rebuild telomeres.
Can make cells “immortal”.
Found in cancer cells.
Found in germ cells.
Limited activity in active cells
such as skin cells
Comment
Control
of Telomerase may
stop cancer, or extend the life
span.
NEWS FLASH
The
DNA of Telomeres is
actually used to build
proteins.
These proteins seem to
impede telomerase.
Feedback Loop??
Summary
Know
the Scientists and their
experiments.
Why DNA is an excellent
genetic material.
How DNA replicates.
Problems in replication.