2. 12.1 The Role of DNA
• DNA has 3 functions:
– Storing information
– Copying information
– Transmitting information
3. 12.2 The Structure of DNA
• DNA – deoxyribonucleic acid
• Nucleotides – monomers that make
up a DNA molecule
– 3 parts:
• 5 Carbon sugar (deoxyribose)
• Phosphate group
• Nitrogenous base
– Adenine (A)
– Guanine (G)
– Cytosine (C)
– Thymine (T)
4. Solving the Structure of DNA
Edwin Chargaff (1949)
Chargaff’s Rule
[A]=[T]
[C]=[G]
Adenine Thymine Guanine Cytosine
35%
45%
5. • Rosiland Franklin used X-ray diffraction
to visualize DNA structure (1952).
6. • Watson and Crick
were the first to
model DNA as a
double helix in 1953.
7. The Double Helix Model
• All the previous discoveries show us the
structure of DNA and how DNA can
function as a carrier of genetic
information.
• Characteristics of the double helix model:
– Antiparallel strands
– Hydrogen Bonding
– Base Pairing
8. Antiparallel Strands
Run in opposite
directions
Base Pairing
Bases are held together
by hydrogen bonds.
Base Pairing
A bonds with T, and C
bonds with G
Complimentary base
pairs
hydrogen bond covalent bond
9. Practice Base Pairing
• If the sequence of bases on one strand of a
DNA molecule is TCGAACTGA, the sequence
on the other (complimentary) strand is:
10. 12.3 DNA Replication
• Why does DNA replicate?
(Hint: Think back to Mitosis)
– DNA is replicated in the S phase of the cell
cycle, before the cell divides.
11. Copying the Code
• Step 1
– The DNA double
helix unwinds
• DNA helicase
(enzyme).
– breaks the H bonds
between the bases.
• The areas where the
double helix
separates are called
replication forks.
12. • Step 2
– DNA polymerase
(enzyme) moves along
each DNA strand adding
complimentary bases
according to the base
pairing rules.
– DNA polymerase also
proofreads the DNA
molecule to reduce
errors.
13. • Step 3
– The process
continues until all of
the DNA has been
copied.
– DNA polymerase
detaches.
14. • This process produces two DNA
molecules each composed of one new
and one original strand.
• Both DNA molecules produced are
identical to each other.
• DNA replication takes place at many
points on a eukaryotic chromosome.
– Allows replication to happen more quickly.
