8.2 Structure of DNA
KEY CONCEPT
DNA structure is the same in all organisms.
8.2 Structure of DNA
DNA is the molecule of Life
• Deoxyribonucleic Acid
• Contains genetic information for cell
functions, growth, and division
• Shape – Double Helix
• Watson & Crick –
Discovered the shape of
DNA
8.2 Structure of DNA
• DNA is made up of a
phosphate group
long chain of
nucleotides.
• Each nucleotide has
three parts.
– a phosphate group
– a deoxyribose sugar
deoxyribose (sugar)
– a nitrogen-containing
base
nitrogen-containing
base
8.2 Structure of DNA
DNA is composed of four types of nucleotides.
-Adenine
-Cytosine
-Thymine
-Guanine
8.2 Structure of DNA
Watson and Crick determined the threedimensional structure of DNA by building models.
• DNA is a double
helix that is made
up of a sugarphosphate
backbone on the
outside with bases
on the inside.
8.2 Structure of DNA
Nucleotides always
pair in the same way.
• The base-pairing rules
show how nucleotides
always pair up in DNA.
– A pairs with T
– C pairs with G
G
A
C
T
8.2 Structure of DNA
• The backbone is connected by covalent bonds.
• The bases are connected by hydrogen bonds.
hydrogen bond
covalent bond
8.2 Structure of DNA
History of Replication
• Watson & Crick – realized that
complementary base pairing
provided a way for DNA to copy itself
- base pairing could allow a new
strand to be built on an old strand
8.2 Structure of DNA
DNA Replication –
• Replication = DNA making copies of itself
– DNA must be copied before a cell can divide
– Each new cell will have
a complete set of DNA
8.2 Structure of DNA
DNA Replication: Process
• Replication begins when the enzyme DNA helicase
opens the DNA forming replication bubbles
8.2 Structure of DNA
DNA Replication- The Big Picture
Each Bubble has 2 Forks – each fork has a leading and
lagging strand
8.2 Structure of DNA
DNA Replication – DNA Polymerase
• The enzyme DNA polymerase
brings new nucleotides to the
replication fork
- it pairs them according to base pairing
rules A pairs with T
C pairs with G
8.2 Structure of DNA
DNA Replication
• The process continues until 2 complete copies of the
DNA are produced
• Each copy of the DNA contains one strand of DNA from
the original DNA molecule and one new strand that was
produced by replication
• Known as semi-conservative replication
8.2 Structure of DNA
DNA Replication