8.2 Structure of DNA
KEY CONCEPT (K.C.)
MAIN IDEA S (M.I.)
DNA structure is the
same in all organisms.
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8.2 Structure of DNA
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Covalent Bond
Chemical bond formed
when two atoms share one
or more pairs of electrons
Nucleotide
Small units, or monomers, that make up
DNA. Each has a phosphate group, a
sugar, and a nitrogen-containing base.
Double Helix
Shape that DNA creates when 2 strands
of DNA wind around each other like a
twisted ladder.
• Model that compares the structure of a
DNA molecule.
Base Pairing Rules
Rule that describes how nucleotides
form bonds in DNA; adenine (A) always
pairs with thymine (T), while guanine
(G) always pairs with cytosine (C).
Hydrogen Bond
Attraction between a
slightly positive hydrogen
atom and a slightly
negative atom.
8.2 Structure of DNA
Main Idea (M.I.)
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
– a deoxyribose sugar
– a nitrogen-containing base
phosphate group
deoxyribose (sugar)
nitrogen-containing
base
8.2 Structure of DNA
• The nitrogen containing bases are the only difference in
the four nucleotides.
8.2 Structure of DNA
Main Idea (M.I.)
Watson and Crick developed an accurate model of
DNA’s three-dimensional structure.
• Watson and Crick’s discovery built on the work of Rosalind
Franklin and Erwin Chargaff.
• Franklin studied DNA using x-rays; the x-ray images
suggested that DNA was a double helix of even width.
• For a long time scientists hypothesized that DNA in all
organisms was exactly the same.
• Chargaff found that the same 4 bases are found in the DNA
of all organisms.
• It is the proportion of the 4 bases that differs from
organism to organism.
• Chargaff’s rules stated that A=T and C=G.
8.2 Structure of DNA
• Working with these
findings of other
scientists, Watson and
Crick realized that 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
Main Idea (M.I.)
Nucleotides always pair in the same way.
• DNA nucleotides of a single strand are
joined together by covalent bonds.
– These connect the sugar of one
nucleotide to the phosphate of the
next nucleotide.
– The sugars and phosphates
alternate to form the sides of the
double helix.
– The double helix is held strongly
together by the many hydrogen
bonds between the bases in the
middle.
– This is how the DNA structure is
maintained.
G
C
A T
8.2 Structure of DNA
• The base-pairing rules show how
nucleotides always pair up in DNA.
• A pairs with T : Apples grown on Trees
• C pairs with G: Cars need Gas
• Because a pyrimidine
(single ring) pairs with a
purine (double ring), the
helix has a uniform width.
G
C
A T
8.2 Structure of DNA
• The backbone is connected by covalent bonds.
• The bases are connected by hydrogen bonds.
• A and T form 2 hydrogen bonds.
• C and G form 3 hydrogen bonds.
hydrogen bond
covalent bond