8.2
Structure of DNA
TEKS 3F, 6A, 6B
The student is expected to:
3F research and describe
the history of biology and
contributions of scientists;
6A identify components of DNA,
and describe how information for
specifying the traits of an organism
is carried in the DNA;
6B recognize that components that
make up the genetic code are
common to all organisms
8.2
Structure of DNA
TEKS 3F, 6A, 6B
KEY CONCEPT
DNA structure is the same in all organisms.
8.2
Structure of DNA
DNA
• DNA is one of the 4
types of
macromolecules
known as a nucleic
acid.
• DNA stands for
deoxyribonucleic
acid.
TEKS 3F, 6A, 6B
8.2
Structure of DNA
TEKS 3F, 6A, 6B
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
TEKS 3F, 6A, 6B
• The nitrogen containing bases are the only difference in
the four nucleotides.
8.2
Structure of DNA
TEKS 3F, 6A, 6B
8.2
Structure of DNA
The chain has a direction determined
by whether the deoxyribose is right
side up, or upside down.
The “top” of the molecule will have a
phosphate sticking out. It is known
as the 5’ end (5 prime).
TEKS 3F, 6A, 6B
8.2
Structure of DNA
If there is no phosphate sticking out,
it is the 3’ end (3 prime).
TEKS 3F, 6A, 6B
8.2
Structure of DNA
TEKS 3F, 6A, 6B
Constructing the DNA Polymer
• 5’
3’
• 3’
5’
8.2
Structure of DNA
TEKS 3F, 6A, 6B
It looks like a
twisted ladder. The
strands run in
opposite directions;
called antiparallel
(like opposite flows
of traffic)
8.2
Structure of DNA
TEKS 3F, 6A, 6B
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.
8.2
Structure of DNA
TEKS 3F, 6A, 6B
• 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.
8.2
Structure of DNA
TEKS 3F, 6A, 6B
Chargaff’s Rules:
•
Erwin Chargaff (1905-2002) discovered that in any
DNA sample the amount of guanine is always equal to
the amount of cytosine, the amount of adenine was
always equal to the amount of thymine.
A=T
G=C
8.2
Structure of DNA
TEKS 3F, 6A, 6B
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
C
A T
8.2 Structure
ofbase
DNApairing rules: TEKS 3F, 6A, 6B
• This eventually
led to our
1. Each base pair is made of 1 purine and 1 pyrimidine.
2. Adenine from one strand and thymine from the other always
pair together with two hydrogen bonds
8.2
Structure of DNA
3. guanine on one strand and cytosine
on the other strand always bond
together with three hydrogen bonds.
TEKS 3F, 6A, 6B
8.2
Structure of DNA
TEKS 3F, 6A, 6B
• The backbone is connected by covalent bonds.
• The bases are connected by hydrogen bonds.
hydrogen bond
covalent bond