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8.2 DNA Structure
8.3 DNA Replication
8.2 DNA Structure
KEY CONCEPT
DNA carries the genetic information in an organism.
DNA structure is the same in all organisms.
+Watson and Crick determined the threedimensional structure of DNA by building
models.
 Overall
Shape:
They realized that DNA
is a double helix that is
made up of a sugarphosphate backbone on
the outside with bases
on the inside.
DNA is composed of four types of nucleotides.
DNA is made up of a long chain of nucleotides.
Each nucleotide has three parts:
 a phosphate group
phosphate group
 a deoxyribose sugar
 a nitrogen-containing base
deoxyribose (sugar)
nitrogen-containing
base
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DNA is held together by two types of bonds.
1. The backbone is connected by covalent bonds.
2. The bases are connected by hydrogen bonds.
hydrogen bond
covalent bond
• There are 4 types of nucleotides: thymine, adenine,
cytosine, and guanine
• The nitrogen containing bases are the only
difference in the four nucleotides.
+
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
• Because a pyrimidine
(single ring) pairs with a
purine (double ring), the
helix has a uniform
width.
G
A
C
T
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• Watson and Crick’s discovery built on the
work of Rosalind Franklin and Erwin Chargaff.
– Franklin’s x-ray images suggested that DNA
was a double helix of even width.
– Chargaff’s rules stated that A:T and C:G
ALWAYS
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Write the pairing strand:
ATC GTC ACG TCA GTA
TAG CAG TGC AGT CAT
+
8.3 DNA Replication
KEY CONCEPT
General Description: DNA replication copies
the genetic information of a cell.
+
Replication copies the genetic information.
 A single
strand of DNA serves as a template
for a new strand.
 The
rules of base pairing direct
replication.
 DNA is
replicated during the
S (synthesis) stage of the
cell cycle.
 Each
body cell gets a
complete set of
identical DNA.
+
Proteins carry out the process of replication.
 DNA serves
only as a template.
 Enzymes
and other proteins do the actual work
of replication.
nucleotide
The DNA molecule
unzips in both directions.
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Process: How does DNA replication
happen?
1. Enzymes unzip the double helix.
2. Free-floating nucleotides form hydrogen
bonds with the template strand.
nucleotide
The DNA molecule
unzips in both directions.
+
3. DNA polymerase enzymes bond the
nucleotides together to form the double
helix.
1. Sugar
Phosphate
Backbone
4. new strand
2. Nitrogen bases
3. DNA polymerase
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4. Two new molecules of DNA are formed, each with an original strand and a newly
formed strand.
•
DNA replication is semi-conservative, meaning one original strand and
one new strand.
1. original strand
3. Two molecules of DNA
2. new strand
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DNA Replication Animations

https://www.youtube.com/watch?v=dKubyIRiN84

http://highered.mheducation.com/olcweb/cgi/pluginpop.cg
i?it=swf::535::535::/sites/dl/free/0072437316/120076/bio23.s
wf::How%20Nucleotides%20are%20Added%20in%20DNA%
20Replication
+
Replication is fast and accurate.
• DNA replication starts at many points in
eukaryotic chromosomes.
There are many origins of replication in eukaryotic chromosomes.
• DNA polymerases can find and correct errors.