DNA has three main functions: storing genetic information, copying that information during cell division, and transmitting the information to offspring. DNA is made up of nucleotides containing a sugar, phosphate, and one of four nitrogenous bases. Rosalind Franklin discovered that DNA has a double helix structure with the bases on each strand bonding with their complement on the other strand in a regular pattern. Watson and Crick were the first to model DNA as a double helix with antiparallel strands held together by hydrogen bonding between complementary nucleotide base pairs. DNA replication copies the DNA before cell division, unwinding the double helix and using DNA polymerase to add complementary bases to each strand.

1. Chapter 12: DNA

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.