Transcript DNA Structure PowerPoint

The Structure of DNA
(Chapter 13.1)
DNA: The Genetic Material
 Genes
are made up of small
segments of deoxyribonucleic acid
or “DNA”
 DNA is the primary material that
causes recognizable, inheritable
characteristics in related groups of
organisms.
Searching for the Genetic
Material
3 major experiments lead to the
conclusion that DNA is the genetic
material in cells.
1. Frederick Griffith - 1928
 Worked
with 2 types of bacteria in
mice (S bacteria & R bacteria)
 Discovered “transformation” – the
transfer of genetic material in the
form of DNA fragments from one
cell to another.
Conclusion of Griffith’s
Experiment:
Something is present in the Smooth
bacteria that is capable of transforming
the Rough into Smooth
 This “mystery substance” is capable of
changing the genetics of the organism.

Hmmm…what could
this mystery
substance be?
2. Avery, McCarty & MacLeod

1944—Wanted to determine whether
the transforming agent in Griffith’s
experiments was a carbohydrate, lipid,
protein, or DNA.

Used enzymes to destroy each of these
molecules in the Smooth (diseasecausing) bacteria.
Avery, McCarty & MacLeod’s
Experiment, Step by Step:
Step 1: Using enzymes, they took the
heat-killed smooth bacteria and
removed the carbohydrates. After
adding this to the Live Rough bacteria,
they injected mice.
 The mice died of pneumonia. This
means the Live Rough still turned into
Smooth.

Avery, McCarty & MacLeod’s
Experiment, Step by Step:
Step 2: This time, they used enzymes to
destroy the lipids in the heat-killed
smooth. After adding it to the Live
Rough, they injected mice.
 The mice died of pneumonia. This
means the Live Rough still turned into
Smooth, even without the lipids.

Avery, McCarty & MacLeod’s
Experiment, Step by Step:
Step 3: This time, they used enzymes to
destroy the proteins in the heat-killed
smooth. After adding it to the Live
Rough, they injected mice.
 The mice died of pneumonia. This
means the Live Rough still turned into
Smooth, even without the proteins.

Avery, McCarty & MacLeod’s
Experiment, Step by Step:
Step 4: This time, they used enzymes to
destroy the DNA in the heat-killed
smooth. After adding it to the Live
Rough, they injected mice.
 The mice lived! This means the Live
Rough stayed Rough! Without the DNA
from the smooth, the Rough did not turn
into smooth.

Conclusion of Avery’s
Experiment:

Concluded that DNA is responsible for
transformation in bacteria
Were their findings
accepted by the
scientific community?
3. Alfred Hershey & Martha
Chase - 1952

Worked with bacteriophages (infectious
viruses) made up of proteins and DNA.

Found that DNA is the hereditary
material in viruses.
Life Cycle of a Virus
Thanks to
the HersheyChase
Experiment,
we now
know what
part of the
virus is
injected into
the cell.
Hershey-Chase
Experiment:
Conclusion of the Hershey/Chase
Experiment:

DNA, (and only DNA) is the genetic
material of the cell.
The Race is On……
 In
the 1950s, we knew genes
were made of DNA but knew
nothing about its structure.
 Scientists around the world
were actively searching for
more information about DNA
James Watson & Francis Crick
Received his doctoral degree at age 22
in Zoology, with an emphasis on
genetics. He also did post-doctoral
research in biochemistry.
 Francis Crick, 36, a physicist working at
Cambridge University in England

James
Watson
Francis
Crick
James Watson & Francis Crick

Created the “double helix” model that is
composed of 2 nucleotide chains.

The chains are twisted around a
central axis and are held together
by weak hydrogen bonds
How did Watson & Crick
determine this structure?
(3 contributions)
1. In 1949, Erwin Chargaff found that
there was always the same amount of
adenine as thymine and the same
amount of cytosine as guanine.
2. In 1952, Rosalind Franklin & Maurice
Wilkins developed high quality x-ray
images of strands of DNA.
Images resembled a tightly coiled helix
of 2 chains.
3. In 1953, Watson & Crick used the
findings of Chargaff & Franklin to create
a 3-D model of DNA.
Nucleotide Subunits
(Review – 3 parts)
The 4 Nitrogen Bases:
Nitrogenous Bases

Purines – Have 2 rings of
carbon & nitrogen atoms per
base.
A – adenine
G – guanine

Pyrimidines – Have 1 ring of
C & N atom per base
T – thymine
C – cytosine
Base- Pairing Rules
A purine on one strand of DNA is
always paired with a pyrimidine on the
other
 A always with T
 G always with C

DNA Structure