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Ch 9 DNA Structure and Function
ENDURING
UNDERSTANDING
Living systems store, retrieve, transmit, &
respond to information essential to life’s
processes.
ESSENTIAL QUESTIONS
•
Is DNA the “essence of life”?
•
How and why does DNA replicate?
•
How is DNA a genetic code?
DNA - the book of YOU (TED
ed - 5 min video)
9.1 Identifying the Genetic Material

DNA stands for
deoxyribonucleic acid.
– DNA is pretty unusual in that it is about
the only common molecule capable of
directing its own synthesis.
Identifying the genetic material

In 1928, Frederick Griffith - pneumonia bacteria and
mice.
– This was one of the first experiments that hinted that DNA was
the genetic code material.

In 1944, Oswald Avery and his co-workers
demonstrated that DNA is the material responsible for
transformation.

In 1952, Alfred Hershey and Martha Chase
– “Hershey-Chase Experiment”.
– Showed that viral genes are made of DNA, not protein.
A=T and

C=G
In 1949, Erwin Chargaff discovered that the
same amount of Adenine & Thymine and
Guanine & Cytosine were present in organisms
– Lead to complimentary base pair knowledge
– The amount of AT, CG combos varied
between different organisms

1950s, Franklin & Wilkens, did
research bouncing x-rays off
crystals of various substances
( x-ray crystallography),
including DNA, then exposing
photographic film to the x-rays.

Studied the scatter patterns
made by the x-rays bouncing off
the crystals of various
substances

Unfortunately, Franklin died of
cancer soon afterwards, or she
might have been more famous.
Rosalind Franklin
Maurice Wilkens
X-ray crystallography

James Watson & Francis Crick saw Franklin’s
photographs of DNA x-ray crystallography, & from her
pictures, able to determine double spiral or double helix.

Based on Chargaff’s, and Franklin’s & Wilson’s data, and
their knowledge of chemical bonding in 1953, Watson
and Crick published a paper in which they proposed and
described a hypothetical structure for DNA.
Double helix – two
strands twisted around
each other like a
winding staircase.
The Nobel Prize in Physiology or Medicine 1962
"for their discoveries concerning the molecular structure of nucleic
acids and its significance for information transfer in living material"
Francis
Harry
Compton
Crick
James
Dewey
Watson
United Kingdom
b. 1916
d. 2004
USA
b. 1928
Maurice
Hugh
Frederick
Wilkins
UK and New Zealand
b. 1916
d. 2004
* Each shared 1/3 of the Nobel Prize
Mini quiz
In 1928, the experiments of Griffith demonstrated
transformation of
a. R bacteria into S bacteria.
b. S bacteria into R bacteria.
c. heat-killed S bacteria into R bacteria.
d. S bacteria into heat-killed R bacteria.
______ 2. In 1952, Hershey and Chase used the
bacteriophage T2 to determine that genetic
material is made of which of the following?
a. protein
c. DNA
b. RNA
d. 35S
______ 3. A microorganism that is virulent is
A. able to cause disease.
B. bacteriophage.
C. transformed.
D. harmless.
______ 4. Avery’s experiments showed that
a. DNA is responsible for transformation.
b. proteins are responsible for transformation.
c. bacteriophages are responsible for transformation.
d. virulent bacteria are responsible for transformation.
9.2 The DNA Molecule
DNA = Deoxyribonucleic acid
“Deoxyribo” = refers to deoxyribose sugar
“Nucleic acid” = refers to nucleotides
bonded
together
Remember nucleotides?

Nucleotides are the subunits that make up DNA.

Each nucleotide is made up of 3 parts:



phosphate group
five-carbon sugar molecule (deoxyribose
sugar)
nitrogen-containing base (nitrogenous base)

Sugar & phosphate group are the same for each
nucleotide in a molecule of DNA.
Nitrogenous base may be any one of 4 different kinds:
1. Adenine
2. Guanine
A
G
Purines- 2 rings of C and N atoms
3. Thymine
4. Cytosine
T
C
Pyrimidines- 1 ring of C and N atoms
Purines
Pyrimidines
A pairs with T
C pairs with G
Nitrogenous bases


Sugar phosphate backbone = sides of a ladder.
Paired nitrogen bases = rungs of a ladder.

Nitrogen bases face each other.

Double helix is held together by weak hydrogen bonds
between the pairs of bases.
Pairing Between Bases

Watson & Crick
determined that
purines & pyrimidines
on opposite strand
always pair
A with T
 C with G

– Pairs this way b/c of
structure and size
– Are base-pairing rules
Complimentary Base Pairs:
the sequence of bases on one strand determines
the sequence of bases on the other strand.

A 2 H -bonds w/ T

C 3 H- bonds w/ G

Hydrogen bonds
between the base
pairs keep the two
strands of DNA
together.
Mini quiz
Each nucleotide in a DNA molecule consists of a
A. sulfur group, a five-carbon sugar molecule, and a
nitrogen base.
B. phosphate group, a six-carbon sugar molecule, and a
nitrogen base.
C. phosphate group, a five-carbon sugar molecule, and an
oxygen base.
D. phosphate group, a five-carbon sugar molecule, and a
nitrogen base.
In 1953, Watson and Crick built a model of DNA with the
configuration of a
A. single helix.
B. double helix.
C. triple helix.
D. circle.
Which of the following describes the base-pairing
rules in DNA?
A. Purines pair only with purines.
B. Pyrimidines pair only with pyrimidines.
C. Adenine pairs with guanine, and thymine pairs
with cytosine.
D. Adenine pairs with thymine, and cytosines pairs
with guanine.
Which of the following researchers took key
photographs of DNA?
A. Watson
B. Crick
C. Franklin and Wilkins
D. Chargaff
Hot Dog DNA – Science Friday – 4min
9.3 DNA Replication

When the double helix structure was first
discovered, scientists were very excited about
the complimentary relationship between the
sequence of nucleotides.

Within 5 years of the discovery of DNA
structure, scientists had firm evidence that
complimentary strands of the double helix do
serve as templates for building new DNA.
p. 198 Figure 9
Step 1: Before DNA replication can begin,
must unwind double helix structure
-
-
DNA helicases – are enzymes that open
double helix by breaking the hydrogen
bonds
Proteins then hold them apart
Areas where the double helix separates is
called the replication fork
Step 1
p. 198 Figure 9
Step 2: DNA polymerase add complimentary
nucleotides to each strand
- DNA polymerase is an enzyme
- as the DNA polymerase moves along,
two new double helixes are formed
Step 2
p. 198 Figure 9
Step 3: Two DNA molecules form that are identical
to the original DNA molecule

The process of DNA polymerase making complimentary
base pairs will continue until all DNA has been copied
and the DNA polymerase receives a signal to stop.

The new DNA has a new and an original strand.

The sequences in both of these DNA molecules are
identical to each other and to the original DNA
molecule.
Step 3
DNA replication - video – 2.5min
Checking for Errors

In the course of DNA replication, errors
sometimes occur and the wrong nucleotide is
added to the new strand.

DNA polymerase has a “proofreading” role.
– Can add nucleotides to a growing strand only if
the previous nucleotide is correctly paired to its
complementary base.
– Can backtrack, remove wrong base, and add
correct base.
– Reduces errors in DNA replication to about 1 error
per 1 billion nucleotides!
The Rate of Replication
Figure 10 p. 200

Replication does not begin at one end of the DNA
molecule and end at the other.

Prokaryotes – circular DNA – has two replication forks
that begin at a single point

Eukaryotes –each chromosome contains a single long
strand of DNA
– Length = challenge!
– Each human chromosome is replicated in about 100 sections
that are 100,000 nucleotides long, each section with its own
starting point.
– An entire human chromosome can be replicated in about 8
hours.
Mini Quiz
B
A
C
Quiz
C
D
A
Review Key Concepts
p. 201
p. 203 Standardized test prep example
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videos
