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12.2 Structure of DNA
WARM UP
• Which scientists performed experiments with viruses?
– What type of viruses did these scientists use?
• Name the scientist that experimented with two different
strains of bacteria and mice?
• What final conclusion did the scientists come to after their
experiments.
12.2 Structure of DNA
KEY CONCEPT
DNA structure is the same in all organisms.
12.2
Structure
of DNA
DNA
& RNA
• Nucleic Acids
• Ex. DNA and RNA
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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12.2
Structure
of DNA
DNA
Location
and
Function
 Location - in nucleus
 Function - blueprint for life = determine genes
(heredity), make proteins, cell division
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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12.2
Structure
DNA
DNA
basicofstructure
• Discovered structure of DNA = double helix
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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2.35
12.2 Structure of DNA
DNA is composed of building blocks called nucleotides.
• DNA is made up of two long chains of nucleotides.
• Each nucleotide has three parts.
– a phosphate group
– a deoxyribose sugar
– a nitrogen-containing base (A,T,C,G)
phosphate group
deoxyribose (sugar)
nitrogen-containing
base
12.2 Structure of DNA
• The nitrogen containing bases are the only difference in
the four nucleotides.
12.2 Structure of DNA
Watson and Crick determined the three-dimensional
structure of DNA by building models.
• They realized that DNA is
a double helix that is
made up of a sugarphosphate backbone on
the outside with bases on
the inside.
12.2 Structure of DNA
• 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.
– Discovered even nucleotide ratios
– Adenine – Thymine
– Cytosine – Guanine
12.2 Structure of DNA
Nucleotides always pair in the same way
(Complementary Base Pairing)
• 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
C
A T
12.2 Structure of DNA
• The backbone is connected by covalent bonds.
• The bases are connected by hydrogen bonds.
hydrogen bond
covalent bond
12.2
of DNA
DNAStructure
Replication
Why do cells need to make a copy of their DNA?
• The process of copying the DNA molecule
• Occurs in the nucleus during interphase (S phase)
– Is a “semi-conservative” process
– DNA is used as a “Template” to make new DNA
– Results in 2 strands of DNA that are half old and half
new (2 semi conservative DNA molecules)
12.2
Structure
of DNA
DNA
Replication:
semi conservative
1. DNA helicase
uncoils and unzips
DNA
2. New (daughter)
nucleotides line up
next to both old
(parent) chains of
the DNA
Figure 3.13
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12.2 Structure of DNA
DNA Replication
3. DNA polymerase
binds the new
nucleotides together
and “proofreads” the
new strands for
errors.
4. Results in 2 semiconservative strands
of DNA
Figure 3.13
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3.32
12.2 Structure of DNA
DNA Replication
• Final Result – 2 Semi-Conservative Strands of DNA
12.2
Structure
of DNA
DNA
triplets
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12.2
Structure
of DNA
DNA:
the Genetic
Code is carried in
Triplets
 Genetic code= order of nitrogenous bases
in triplets of DNA
1 DNA triplet codes for 1 amino
acid in a protein
Gene: is a DNA segment that
carries the information for building
proteins (or polypeptide chain)
Process Discussed further in
Protein synthesis
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