DNA Structure &
Replication
DNA Structure
The discovery of the structure
Francis Crick and James Watson
• The deoxyribonucleic acid (DNA) molecule is the
genetic blueprint for each cell and ultimately the
blueprint that determines every characteristic of a
living organism. The DNA molecule was discovered
in 1951 by Francis Crick, James Watson, and
Maurice Wilkins using X-ray diffraction.
Photo
Researchers,
Inc.
• "Francis Crick
and James
Watson,"
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Encarta® 98
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In 1953 Crick, left, and Watson,
right, described the structure of
the DNA molecule as a double
helix, somewhat like a spiral
staircase with many individual
steps. In 1962 Crick, Watson, and
Wilkins received the Nobel Prize
for their pioneering work on the
structure of the DNA molecule.
A little extra info
• Although Maurice Wilkins
from Cambridge is credited
as a key player in the
discovery, it was actually
Rosalind Franklin, who was
working in his lab, that
used a technique called Xray diffraction to her
determine the structure of
DNA – she got no credit for
the discovery!
Watson/Crick Model of DNA
• 1. 2 chains of nucleotides coiled around
each other to form a double helix.
• 2. The nitrogen bases of the 2 chains
are joined together by weak hydrogen
bonds. (easily broken)
Watson/Crick Model of DNA
• 3. A specific purine base is paired
with a specific pyrimidine base.
Adenine with Thymine (A-T)
Guanine with Cytosine (G-C)
• 4. The sequence of base pairs along
the DNA molecule determine the
genetic code.
Structure
Structure
• Looks like a twisted ladder
• The “handrails” or backbone is made of the phosphate and 5-C
(pentose) sugar called deoxyribose
• The “rungs” are made of the joined nitrogen bases
• The nucleotides are joined together by covalent bonds into a
single strand
Nitrogen Bases
There are 4 different nitrogen bases
• Adinine,Thymine,Guanine,Cytosine
• As Chargaff’s rule indicates:
• Adinine bonds with Thymine
• Guanine bonds with Cytosine
• Therefore there will always be equal
amounts of A and T , G&C.
Purines
• Nitrogen bases with a double ring structure
• Adenine and Guanine
Pyrimidines
• Nitrogen bases that have a single ring structure
• Thymine and Cytosine (and Uracil of RNA)
Complementary Base Pairs
• A two ring base will bind with a one
ring base so that there are always
three rings that separate the backbone
• Three hydrogen bonds attach cytosine
to guanine
• Two hydrogen bonds attach thymine
to adenine.
Antiparallel
• The two strands run opposite to each other
Hydrogen bonds
One end of the chain is 3’ (sugar end) the other end
is 5’ (phosphate end)
Draw and label a simple diagram of the molecular
structure of DNA
Un-seeable Biology
What is a “genome”?
• the genome is the entirety
of an organism's hereditary
information
• The genome includes both
the genes and the noncoding sequences of the
DNA/RNA.
RNA
• Not the same as DNA because:
• The sugar component of RNA is ribose
rather than deoxyribose
• Uracil instead of Thymine
• Remains single stranded, though it can
fold back on itself to produce regions of
complementary base pairs
THE CENTRAL DOGMA
Table 12.01
12.08
DNA Replication
Interphase of Mitosis/Meiosis
• Semi conservative
• Meaning one old strand combines with a new strand to produce two new
double strands of DNA
Parent
Molecule
Separation of
strands
“Daughter” DNA molecules each
consisting of one parent strand
and one new strand
Animation
• http://www.mcgrawhill.ca/school/applets/abbio/ch18/dna_replicatio
n.swf
DNA Replication
• semi-conservative replication-new DNA molecule
made of one parent and one newly replicated strand.
• In general a DNA molecule ‘unzips’ down the middle
of the paired bases, 2 individual strands are made
that will become the ‘templates’ for new complete
DNA stands
The Steps for DNA Replication (during S-phase of
Interphase):
1. Initiation starts at a specific nucleotide
sequence, a group of enzymes called DNA
helicases breaks hydrogen bonds between
bases to unzip the double helix
2. Proteins bind to keep strands apart
3) RNA primers attach to a spot on the original DNA
stand
4) DNA polymerase III – starts at where the primer
attached to the DNA and makes new strand in 5’ to
3’ direction (always)
5) DNA polymerase 1 – removes primers and replaces
with nucleotide
6) DNA ligase – joins DNA fragments
• DNA Replication
Remember—Replication 3’-5’
Build a DNA
DNA Replication
DNA makes DNA
Un-seeable Biology 2:52