Download DNA

Watson & Crick & Franklin
 In 1953, Watson and Crick deduced the
structure of DNA by using x-ray diffraction of
DNA crystals
 Watson & Crick only figured out the
structure, but relied upon the work of
Rosalind Franklin who produce the
photographs and crystals
 Watson & Crick received Nobel prizes, but
Franklin didn’t since she had already passed
away
Watson
& Crick
Franklin
X-Ray
Crystallography
of DNA
DNA Structure
 DNA consists of two molecules that are arranged
into a ladder-like structure called a Double
Helix.
 A molecule of DNA is made up of millions of
tiny subunits called Nucleotides.
 Each nucleotide consists of:
1. Phosphate group
2. Pentose sugar – in DNA this is Deoxyribose
3. Nitrogenous base
DNA Subunits
Nucleotides
Phosphate
Nitrogenous
Base
O
-O
O
P
-O
O
NH
N
O
H
H
O
H
HO
H
H
Pentose
Sugar
Nucleotides
 The phosphate and sugar form the backbone
of the DNA molecule, whereas the bases form
the “rungs”.
 There are four types of nitrogenous bases.
Nitrogenous Bases
Nucleotides
 In 1949, Erwin Chargaff determined that each
base will only bond with one other specific
base.
Base-Pairing Rules
 Adenine (A)
Straight letters go
 Thymine (T)
together!
 Cytosine (C)
 Guanine (G)
Curvy letters go
together!
DNA Structure
 Because of this complementary base pairing,
the order of the bases in one strand
determines the order of the bases in the other
strand.
 The bases on one side of the helix form
hydrogen bonds with the complementary
base on the other side of the strand
DNA Structure
 The order of nitrogenous bases on one chain
for the DNA molecules is complementary to
the order of bases on the opposite chain
 The order of nitrogenous bases of DNA is
called its base sequence
AAGCT
TTCGA
A
T
C
G
T
A
C
G
A
T
G
C
T
A
Why is complementary base
pairing important?
 The hydrogen bonds between the base pairs
help hold the two strands of a DNA molecule
together
 The complementary nature of DNA helps
explain how DNA replicates before a cell
divides. One strand of DNA can serve as a
template for making a new complementary
strand
DNA stands for deoxyribonucleic acid
Purines are nitrogenous bases made of two rings
of carbon and nitrogen atoms. Pyrimidines are
nitrogenous bases made of a single ring of carbon
and nitrogen atoms.
Their photographs suggested that the DNA
molecule resembled a tightly coiled helix and was
composed of two or three chains of nucleotides.