Download 11.2 What Is the Structure of DNA?

Chapter 11 At a Glance
 11.2 What Is the Structure of DNA?
 11.3 How Does DNA Encode Genetic Information?
 11.4 How Does DNA Replication Ensure Genetic
Constancy During Cell Division?
 11.5 What Are Mutations, and How Do They
Occur?
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DNA: The Molecule of Heredity
Ch. 11
11.2 What Is the Structure of DNA?
 Knowing that DNA is made up of genes does not
provide an answer to the critical questions about
inheritance
 The secrets of DNA function and, therefore, of
heredity itself are found in the three-dimensional
structure of the DNA molecule
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11.2 What Is the Structure of DNA?
 DNA is composed of four nucleotides
– DNA is made of chains of small subunits called
nucleotides
– Each nucleotide has three components
1. A phosphate group
2. A deoxyribose sugar
3. One of four nitrogen-containing bases
1. Thymine (T)
2. Cytosine (C)
3. Adenine (A)
4. Guanine (G)
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Figure 11-3 DNA nucleotides
phosphate
phosphate
base  thymine
sugar
base  adenine
phosphate
sugar
phosphate
base  cytosine
sugar
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base  guanine
sugar
11.2 What Is the Structure of DNA?
 DNA is composed of four nucleotides (continued)
– In the 1940s Erwin Chargaff, a biochemist at
Columbia University, analyzed the amounts of the
four bases in DNA from diverse organisms
– He discovered a consistency in the equal amounts of
adenine and thymine, and equal amounts of guanine
and cytosine for a given species, although there was a
difference in proportion of the bases
– This finding was called “Chargaff’s rule”
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11.2 What Is the Structure of DNA?
 DNA is a double helix of two nucleotide strands
– In the 1940s, several other scientists investigated
the structure of DNA
– Rosalind Franklin and Maurice Wilkins studied the
structure of DNA crystals using X-ray diffraction
– They bombarded crystals of purified DNA with X-rays
and recorded how the X-rays bounced off the DNA
molecules
– The resulting pattern does not provide a direct picture
of the DNA structure, but the researchers were able to
extract specific information
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11.2 What Is the Structure of DNA?
 DNA is a double helix of two nucleotide strands
Wilkins and Franklin deduced the following
information about DNA from the patterns they found:
1. A molecule of DNA is long and thin, with a uniform
diameter of 2 nanometers
2. DNA is a helical, twisted like a
Corkscrew or a spiral staircase
3. DNA is a double helix
4.DNA has repeating subunits
5. Phosphates are probably on the outside of the helix
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11.2 What Is the Structure of DNA?
 DNA is a double helix of two nucleotide strands
(continued)
– James Watson and Francis Crick combined the
X-ray data with bonding theory to deduce the
structure of DNA
– They proposed that a single strand of DNA is a
polymer consisting of many nucleotide subunits
– Within each DNA strand, the phosphate group of one
nucleotide bonds to the sugar of the next nucleotide in
the same strand
– The deoxyribose and phosphate portions make up the
sugar-phosphate backbone
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11.2 What Is the Structure of DNA?
 DNA is a double helix of two nucleotide strands
(continued)
– The nucleotide bases protrude from the sugarphosphate backbone
– All the nucleotides within a single DNA strand are
oriented in the same direction, and thus have an
unbonded sugar at one end and an unbonded
phosphate at the other end
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11.2 What Is the Structure of DNA?
 Hydrogen bonds between complementary bases
hold two DNA strands together in a double helix
– Watson and Crick’s findings provided the following
insight about the DNA model:
– The DNA model consists of two DNA strands,
assembled like a twisted ladder
– The bases protrude inward toward each other from
the sugar-phosphate backbone like rungs on a ladder
– Hydrogen bonds hold the base pairs together,
composing the rung
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11.2 What Is the Structure of DNA?
 Hydrogen bonds between complementary bases
hold two DNA strands together in a double helix
(continued)
– The two strands in a DNA double helix are said to be
antiparallel; that is, they are oriented in opposite
directions
– From one end of the DNA molecule, if one strand
starts with the free sugar and ends with the free
phosphate, the other strand starts with the free
phosphate and ends with the free sugar
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11.2 What Is the Structure of DNA?
 Hydrogen bonds between complementary bases
hold two DNA strands together in a double helix
(continued)
– Because of their structures and the way they face
each other, adenine (A) bonds only with thymine (T)
and guanine (G) bonds only with cytosine (C)
– Bases that bond with each other are called
complementary base pairs
– Thus, if one strand has the base sequence
CGTTTAGCCC, the other strand must have the
sequence GCAAATCGGG
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11.2 What Is the Structure of DNA?
 Hydrogen bonds between complementary bases
hold two DNA strands together in a double helix
(continued)
– Complementary base pairing explains Chargaff’s rule
that for a given molecule of DNA, adenine equals
thymine and guanine equals cytosine
– Since every adenine, for example, is paired with a
thymine, no matter how many adenines are in the DNA
molecule, there will be an equal number of thymines
© 2014 Pearson Education, Inc.
11.2 What Is the Structure of DNA?
 Hydrogen bonds between complementary bases
hold two DNA strands together in a double helix
(continued)
– Adenine and guanine are large molecules; thymine
and cytosine are relatively smaller
– Because base pairing always places a large
molecule with a small one, the diameter of the double
helix remains constant
– In 1953, James Watson and Francis Crick
consolidated all the historical data about DNA into an
accurate model of its structure
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Figure 11-5 The Watson-Crick model of DNA structure
nucleotide
free
phosphate
nucleotide
free
sugar
phosphate
base
(cytosine)
sugar
hydrogen
bonds
free sugar
free
phosphate
Hydrogen bonds hold complementary
base pairs together in DNA
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Two DNA strands
form a double helix
Four turns of
a DNA double
helix
BUILD DNA
http://learn.genetics.utah.edu/content/molecules/builddna/
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