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DNA STRUCTURE
What are the Parts and Shape of
DNA?
What is the Structure of DNA?
• Backbone?
• Rungs of
DNA?
• Purines?
• Pyrimidines?
BONDING
RULES??
History
• Began with F. Griffith trying to
figure out how certain types of
bacteria cause pneumonia.
Griffith and Transformation
• For grins and giggles, Griffith
mixed heat-killed, disease causing
bacteria with live, harmless ones
and injected the mice.
• What do you think happened?
• Yep, the mice died.
• Hypothesized that somehow there
was a factor passed from bad to
good.
Hershey Chase
Wanted to determine what part of a
virus caused infection in host…
Concluded the genetic material that
caused infection was the DNA not
the protein coat.
DNA Structure
• 1950’s R. Franklin used x-ray
diffraction to show there were
two twisted strands to a DNA
molecule.
Watson and Crick
• Used Franklin’s
work to
complete DNA
structure of a
DOUBLE
HELIX.
DNA Replication
DNA makes an exact copy of itself when a
CELL DIVIDES
Cell division: parent cell divides into 2 or
more daughter cells
Cell division in eukaryotic cells  MITOSIS
DNA Replication
The most important
concern with cell division is
the maintenance of the
cell’s GENETIC
information.
Before a CELL can divide,
genetic information in
chromosomes must be
replicated
(i.e. DNA replication)
How The Heck?
• Enzyme (DNA helicase)
breaks nucleotides
(strands separate)
• Another enzyme
(DNA Polymerase)
attach free floating
nucleotides to the open
parts of the DNA strand
• Base pairs bond (rungs)
• Sugars and phosphates
bond (backbone)
Definitions
• Enzyme: proteins that catalyzes
(increases RATE (or speed) of a
chemical reaction)
• DNA Helicase: an ENZYME that
unzips the parent DNA strand (breaks
H bonds between bases)
More Definitions
• Polymerase: ENZYME whose
function deals with the polymers
(large molecule of repeating
structures connected by COVALENT
(strong) bonds) of DNA and RNA)
• DNA Polymerase: adds
complementary nucleotides only to
the 3’ end of the newly-formed strand
DNA Polymerase- 2 jobs
“Glue” that joins nucleotides
together
“Proofreads” each new DNA
strand
Complementary Process
• Each new
strand has the
information to
reconstruct the
other half
through base
pairing.
Making the Complementary
Strand
Where are the
Complementary
Strands?http://www.johnkyrk.
com/DNAreplication.html
Semi-conservative Process
• Each double helix strand serves as a
TEMPLATE for a new strand.
• Sites where replication and separation
occur are called REPLICATION
FORKS.
• DNA HELICASE unzips the molecule
• DNA POLYMERASE proofreads new
copies for errors.
5’ & 3’… HUH?
Direction of DNA
• LEADING strand refers to
the strand is being copied in the
direction that the double helix unwinds
• LAGGING stand refers to the strand being
copied in the opposite direction (away
from the unwinding parts)
Okazaki Fragments
• Short DNA fragments that are created by
the LAGGING strand being copied
DNA Replication Steps
1. Begins at ORIGIN of replication
2. DNA Helicase unzips parent DNA strand
3. DNA Polymerase adds complementary
nucleotides to 3’ end of leading strand (in
the 5’  3’ direction) (continuous)
4. The opposite happens for the lagging
strand, 5’  3’ direction (discontinuous
“backstitching”)
More Terms
• Gene: sequence of DNA that codes for a
protein & determines a trait
• Chromosome: threadlike structure in
nucleus that contains genetic info
• Centromere: region on chromosome
where sister chromatids attached
• Telomere: repetitive DNA at end of
chromosome
• Chromatid: one of the 2 identical “sister”
parts of a duplicated chromosome
Chromosome & Parts