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How Do We Know DNA is the
Genetic Material?
 Proteins were thought to carry genetic
information
 Fred Griffith 1928
 Experimented with pneumonia on mice
 Virulent (causes sickness)
 Non-Virulent
(does not cause sickness)
Griffith Experiment
 Proves the genetic material is not Protein
 Protein Denatures (breaks down) in heat
Hershey and Chase Experiment
EXPERIMENT
Radioactive
Phage
protein
Empty
protein
shell
Radioactivity
(phage protein)
in liquid
Bacterial cell
Batch 1:
Radioactive
sulfur
(35S)
DNA
Phage
DNA
Centrifuge
Pellet (bacterial
Radioactive
cells and contents)
DNA
Batch 2:
Radioactive
phosphorus
(32P)
Centrifuge
Pellet
Radioactivity
(phage DNA)
in pellet
Figure 16.7
C
C
5 end
G
Hydrogen bond
G
C
G
3.4 nm
A
G
C
C
1 nm
T
T
C
C
A
G
T
3 end
T
A
T
G
C
A
G
G
A
C
G
G
A
C
A
T
T
3 end
C
G
T
A
(a)Key features of
DNA structure
0.34 nm
5 end
(b) Partial chemical structure
Space-filling
(c)
model
STRUCTURE OF DNA
Single units, monomers, called
NUCLEOTIDES.
- Nucleotide consists of three parts:
 five carbon sugar - deoxyribose
 phosphate group
 nitrogen base - one of four
* Sugar & phosphate stay the same
but the nitrogen bases can
change.
-
4 NITROGEN BASES:
 Adenine
 Thymine
 Guanine
 Cytosine
 A & G are double ring (PURINES)
 T & C are single ring (PYRIMIDINES)
 They all pair specifically A-T, C-G
 Combined by a Hydrogen bond.
Base Pairing in the DNA
Molecule
 Sugar and
phosphate make up
the sides
 Nitrogen bases
make up the middle
 Two strands, that
twist around each
other, are used to
make a DNA
molecule….
Discovering DNA’s Structure
Wilkins & Franklin:
Used x-rays to
photograph DNA
Chargaff:
Discovered that A=T and G=C
Watson & Crick:
First model of DNA
 Proposed the model in
1953
 Earned the 1962 Nobel
Prize for Physiology
Medicine
 Franklin was also
named to the Nobel
Prize but had died so
could not be recognized
The Secret of Life PBS
60 Minutes: Craig Venter
Designing Life
DNA , CHROMOSOMES AND GENES
-DNA is found in the nucleus
of all cells
-Found as strands, known as
chromatin, in resting cells
-Before cell division,
chromatin forms
chromosomes.
-GENES are short segments of
DNA that contain specific
information for traits
FUNCTION OF DNA:
1. Control of cells
activities
2. Chooses the type of
proteins to be made
1. DNA REPLICATION
Process of making new copies of DNA
1. DNA untwists
2. An enzyme, DNA Helicase, “unzips” the
DNA at the nitrogen bases - breaking their
hydrogen bonds.
3. Free nitrogen bases from the
nucleoplasm attach to the newly
exposed sections of DNA by means of
DNA Polymerase.
ANTI-PARALLEL DNA
Leading and Lagging Strands
DNA Polymerase
only works in
5’3’direction
Polymerase moves
along the leading
strand
Fragments are
added in segments
of the lagging strand
Replication Animation
Okazaki Fragments
Leading and Lagging DNA
Strands and Okazaki Fragments
4. The bases are now reattached
The result is two DNA strands. 1/2 of the
old DNA and 1/2 of the new
DNA helicase
DNA polymerase
The Speed of Replication
 Replication occurs rapidly because
DNA is unzipped at a number of places
at a time
 Replication occurs
rapidly because DNA
is unzipped at a
number of places at a
time
 46 Chromosomes/cell
 Approximately 3
billion base pairs/cell
 Code would fill 1000
biology books/cell
 Takes a few hours
Proof Reading
 Average of one error per billion
nucleotides
 Mistakes in replication are check
and corrected by specialized
“proof-readers”
 Consists of 20 or so amino acids
 WHAT TYPES OF THINGS COULD
CAUSE CHANGES IN OUR CELLS
AFTER REPLICATION?
RNA
STRUCTURE/REPLICATION
*DNA has the “blueprints” for
traits
*Too big to pass through the
nuclear membrane
-Use RNA - ribonucleic acid
-sugar
-phosphate
-nitrogen bases (4)
Adenine
Uracil
Cytosine
Guanine
-RNA is a single stranded
3 Types of RNA
1. Messenger RNA:
carries code from DNA
2. Transfer RNA:
single folded strand of RNA
that carries the amino acids
3. Ribosomal RNA:
major component of
ribosome’s.
2. TRANSCRIPTION:
 Process of
transcribing or
copying the
specific sequence
of bases or code
from DNA to RNA
 DNA is considered
to be a
TEMPLATE
mRNA Synthesis
1. An enzyme, RNA
polymerase, unzips DNA
 2. Free floating RNA
nucleotides line up with the
appropriate DNA base
3.Continues until the RNA
comes to a “stop” sequence
and releases the DNA
4. RNA moves out of cell
nucleus
Protein Synthesis:
Protein Synthesis
Defined as the formation of
proteins using information
coded on the DNA and
carried out by RNA
-Proteins may consist of
hundred or thousands of
amino acids
-They are connected by
peptide bonds
The Amino
Acid Code
-20 amino
acids are
found within
our bodies
-Proteins are
read as
CODONS
Amino Acid Code
TRANSLATION:
Process of making
proteins from the mRNA
 Also referred to as
PROTEIN SYNTHESIS
 Occurs at the
Ribosomes
1. mRNA passes out of
the nucleus through
the nuclear membrane
and moves to a
ribosome

2. The mRNA attaches to the
ribosome at the start codon, (at
the P site)
3. The start codon is always
“AUG”
4. The mRNA is read
in a series of three
(3) nitrogen bases at
a time - referred to as
CODONS.
5. Ribosome signals
the tRNA with the
matching
ANTICODON to bring
in the appropriate
Amino Acids
6. The tRNA at the “P-site” is
released, but the one at the “A-site”
stays
7. The ribosome moves to open up the
A site
8. The Amino Acids bond by
PEPTIDE bonds to form
PROTEINS
Important things to remember…
-tRNA
carry specific Amino
Acids
-tRNA are recycled
-The
arrangement of the Amino
Acids determines the type of
protein that is made