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DNA AND IT’S ROLE IN HEREDITY
Lesson overview and objectives
- DNA/RNA structural properties
–  What are DNA and RNA made of
–  What are the structural differences between DNA and RNA
–  What is the structure of DNA and how was it determined
-  DNA replication
-  How is DNA replicated
-  What mechanisms are in place to deal with errors?
-  DNA as “the” transporter of information (heredity)
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DNA: BASIC FEATURES
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H
C
Glyceraldehyde is the
smallest monosaccharide
OH
C
2
and exists only as the
and
RNAstraight-chain
are based
form.on sugars
C OH
1
H
DNAH
3
H
Glyceraldehyde
Five-carbon sugars (pentoses)
5
4
C
5
H2OH
O
C
3
C1
H
C
2
OH
H2OH
O
OH
H
H
C
4
H
C
H
H
C
3
OH
Ribose
OH
H
C
H
C
2
OH
Ribose and deoxyribose
each have five carbons,
but very different chemical
properties and biological
roles.
C1
H
Deoxyribose
Six-carbon sugars (hexoses)
6
H
4
4
C
C
6
H2OH
5
O
H
H
OH
1 4 C
CCBS, Department of SystemsCBiology
HO
OH
C
3
HO
C
2
OH
H
C
C
H2OH
5
6
O
H2OH
O
H
H
C
OH
H
C
C
3
C
2
1
OH
5
C
H
OH
H
OH
C
C
C2
C
H2OH
1
The sugar is bound to a base to form a
nucleoside
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The nucleoside binds to phosphate groups to
form nucleotides
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Both RNA and DNA form polymers
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Both RNA and DNA can have complex 3D
structures
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DNA STRUCTURE
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A picture that changed a lot of things
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Rosalind Franklin and the helix
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Chargaff Rule
In 1950 Erwin Chargaff found in the DNA from
many different species:
Amount of A = amount of T
Amount of C = amount of G
Or, the abundance of purines = the abundance
of pyrimidines
.
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Chargaff Rule
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What Is the Structure of DNA?
- Franklin’s X-ray crystallography convinced them
the molecule was helical.
- Other evidence suggested there were two
polynucleotide chains that ran in opposite
directions—antiparallel.
- In 1953, Watson and Crick established the
general structure of DNA.
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Figure 13.8 DNA Is a Double Helix (Part 1)
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Figure 13.8 DNA Is a Double Helix (Part 2)
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What Is the Structure of DNA?
Four key features of DNA structure:
• It is a double-stranded helix of uniform diameter
• It is right-handed
• It is antiparallel
• Outer edges of nitrogenous bases are exposed in
the major and minor grooves
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What Is the Structure of DNA?
Complementary base pairing:
• Adenine (A) pairs with thymine (T) by two
hydrogen bonds
• Cytosine (C) pairs with guanine (G) by three
hydrogen bonds
• Every base pair consists of one purine and one
pyrimidine
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DNA REPLICATION
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How Is DNA Replicated?
Two steps in DNA replication:
• The double helix is unwound, making two
template strands
• New nucleotides are added to the new strand at
the 3′ end and joined by phosphodiester
linkages. Sequence is determined by
complementary base pairing
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Figure 13.12 Each New DNA Strand Grows
from Its 5´ End to Its 3´ End (Part 1)
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Figure 13.12 Each New DNA Strand Grows
from Its 5´ End to Its 3´ End (Part 2)
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Figure 13.14 DNA Polymerase Binds to the
Template Strand (Part 2)
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How Is DNA Replicated? The machinery
DNA polymerases work very fast:
• They are processive: Catalyze many
polymerizations each time they bind to DNA
• Newly replicated strand is stabilized by a
sliding DNA clamp (a protein)
• The sliding DNA clamp was recognized in
dividing cells—called the proliferating cell
nuclear antigen (PCNA).
• PCNA also helps to orient the polymerase for
substrate binding, binds other proteins, and is
involved in other processes.
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Figure 13.18 A Sliding DNA Clamp Increases
the Efficiency of DNA Polymerization
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How Is DNA Replicated? The Lagging strand
• This works great and easy on the “leading
strand”. In this direction, DNA Pol III can work
on a 5’->3’ direction following the forward
movement of the replication fork.
• Now, what happens with the other strand, the
“lagging strand”? Here, the “natural” direction of
DNA synthesis would be 3’->5’… unfortunately,
DNA Pol III can only work 5’->3’.
• Therefore, replication on lagging strand has to
be done in another way
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Direction of synthesis
3’
5’
5’
3’
Direction of overall DNA replication
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Direction of synthesis
3’
5’
5’
3’
Direction of overall DNA replication
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CBS, Department of Systems Biology
Direction of synthesis
3’
5’
5’
3’
Direction of overall DNA replication
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CBS, Department of Systems Biology
Direction of synthesis
3’
5’
5’
3’
Direction of overall DNA replication
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How is DNA replicated: the players
• DNA Helicase: unwinds the DNA molecule
• ssDNA-binding proteins: stabilize the open
molecules
• Primase: synthesizes an RNA primer onto the
exposed strands to start DNA replication
• PCNA (proliferating cell nuclear antigen): stabilises,
directs and guides the replication machinery
• DNA Polymerase III: extends the growing DNA
strand in a 5’->3’ strand
• DNA Polymerase I: replaces RNA primer with DNA
• DNA Ligase: closes the gap between growing
molecules
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How Is DNA Replicated? Recap
• Switch to animation! J
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How Is DNA Replicated? Origin of replication
- Small, circular chromosomes have a single
origin of replication.
- As DNA moves through the replication complex,
two interlocking circular chromosomes are
formed.
- DNA topoisomerase separates the two
chromosomes.
- Large linear chromosomes have many hundreds
of origins of replication.
- Replication complexes bind to the sites at the
same time and catalyze simultaneous
replication.
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How Is DNA Replicated? Telomeres
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How Is DNA Replicated? Telomeres
Eukaryote chromosomes have repetitive
sequences at the ends called telomeres.
These repeats are protective and prolong cell
division, especially in rapidly-dividing cells, like
bone marrow.
Telomerase contains an RNA sequence—acts as
template for telomeric DNA sequences.
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How Is DNA Copied?
Three possible replication patterns:
• Semiconservative: Parent serves as a template
and new molecules have one old and one new
strand
• Conservative: Original helix only serves as a
template
• Dispersive: Parent fragments serve as
templates, assembling old and new parts into
molecules
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How Is DNA copied?
- Meselson and Stahl showed that
semiconservative replication was the correct
model.
- They used density labeling to distinguish parent
DNA strands from new DNA strands.
- DNA was labeled with 15N, making it more
dense.
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Figure 13.11 The Meselson–Stahl
Experiment (Part 1)
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Figure 13.11 The Meselson–Stahl
Experiment (Part 2)
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13.3 How Is DNA copied?
- Results of their experiment explained by the
semi-conservative model:
- If conservative, the first generation of
individuals would have been both high and low
density, but not intermediate.
- If dispersive, density in the first generation
would be half, but this density would not appear
in subsequent generations.
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How Are Errors in DNA Repaired?
DNA polymerases make mistakes in replication,
and DNA can be damaged in living cells.
Cells have three repair mechanisms:
• Proofreading
• Mismatch repair
• Excision repair
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DNA IN HEREDITY
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What Is the Evidence that the Gene Is DNA?
By the 1920s, it was known that chromosomes
consisted of DNA and proteins.
A new dye stained DNA and provided
circumstantial evidence that DNA was the genetic
material:
• It was in the right place
• It varied among species
• It was present in the right amount
Frederick Griffith, working with two strains of
Streptococcus pneumoniae determined that a
transforming principle from dead cells of one
strain produced a heritable change in the other
strain.
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Figure 13.1 Genetic Transformation
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What Is the Evidence that the Gene Is DNA?
To identify the transforming principle:
Oswald Avery treated samples to destroy
different molecules; if DNA was destroyed, the
transforming activity was lost.
There was no loss of activity with destruction of
proteins, carbohydrates, or lipids.
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Figure 13.2 Genetic Transformation by DNA
(Part 1)
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Figure 13.2 Genetic Transformation by DNA
(Part 2)
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What Is the Evidence that the Gene Is DNA?
Hershey-Chase experiment:
• Used bacteriophage T2 virus to determine
whether DNA, or protein, is the genetic material
• Bacteriophage proteins were labeled with 35S;
the DNA was labeled with 32P
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Figure 13.3 Bacteriophage T2: Reproduction
Cycle
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Figure 13.4 The Hershey–Chase Experiment
(Part 1)
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What Is the Evidence that the Gene Is DNA?
- Genetic transformation of eukaryotic cells,
transfection, can be demonstrated.
- A genetic marker—a gene that confers an
observable phenotype—is used.
- Any cell can be transfected, even an egg cell—
result is a transgenic organism.
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Figure 13.5 Transfection in Eukaryotic Cells
(Part 1)
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DNA: FINAL POINTS
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SUMMARY
•  DNA is a double-stranded helix, with the two strands connected by
hydrogen bonds. A bases are always paired with Ts, and Cs are always
paired with Gs, which is consistent with and accounts for Chargaff's rule.
•  Most DNA double helices are right-handed; that is, if you were to hold
your right hand out, with your thumb pointed up and your fingers curled
around your thumb, your thumb would represent the axis of the helix and
your fingers would represent the sugar-phosphate backbone. Only one
type of DNA, called Z-DNA, is left-handed.
•  The DNA double helix is anti-parallel, which means that the 5' end of one
strand is paired with the 3' end of its complementary strand (and vice
versa). As shown in Figure 4, nucleotides are linked to each other by
their phosphate groups, which bind the 3' end of one sugar to the 5' end
of the next sugar.
•  Not only are the DNA base pairs connected via hydrogen bonding, but the
outer edges of the nitrogen-containing bases are exposed and available
for potential hydrogen bonding as well. These hydrogen bonds provide
easy access to the DNA for other molecules, including the proteins that
play vital roles in the replication and expression of DNA
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DNA and the “Dogma”
DNA has four important functions—double-helical
structure is essential:
• Genetic material stores genetic information—
millions of nucleotides; base sequence encodes
huge amounts of information.
• Genetic material is susceptible to mutation—a
change in information— possibly a simple
alteration to a sequence
• Genetic material is precisely replicated in cell
division—by complementary base pairing.
• Genetic material is expressed as the phenotype
—nucleotide sequence determines sequence of
amino acids in proteins.
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DNA and the “Dogma”
Before getting into DNA replication, it is important
to state the role of DNA in life and, to be more
precise, to state what is meant with “The Dogma
of molecular biology”:
- DNA can replicate itself exactly (Replication)
- DNA can copy its information into RNA
(Transcription)
- RNA sequence is translated into protein
(Translation)
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DNA and the “Dogma”
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