Transcript Deoxyribonucleic acid, DNA

DNA:
The Blueprint
of Life
DNA &
Scientists
Griffith and Transformation
In 1928, British scientist Fredrick Griffith was trying to learn how
certain types of bacteria caused pneumonia.
He isolated two different strains of pneumonia bacteria from mice and
grew them in his lab.
Griffith made two observations:
(1) The disease-causing strain of bacteria grew into smooth colonies on
culture plates.
(2) The harmless strain grew into colonies with rough edges.
Griffith's Experiments
Griffith set up four individual
experiments. Griffith injected mice with
four different samples of bacteria. When
injected separately, neither heat-killed,
disease-causing bacteria nor live,
harmless bacteria killed the mice. The
two types injected together, however,
caused fatal pneumonia. From this
experiment, biologists inferred that
genetic information could be transferred
from one bacterium to another.
Experiment 1: Mice were injected with the
disease-causing strain of bacteria. The mice
developed pneumonia and died.
Experiment 2: Mice were
injected with the harmless strain
of bacteria. These mice didn’t get
sick.
Harmless bacteria
(rough colonies)
Lives
Experiment 3: Griffith
heated the disease-causing
bacteria. He then injected the
heat-killed bacteria into the
mice. The mice survived.
Heat-killed diseasecausing bacteria (smooth
colonies)
Lives
Experiment 4: Griffith
mixed his heat-killed,
disease-causing bacteria
with live, harmless
bacteria and injected the
mixture into the mice.
The mice developed
pneumonia and died.
Heat-killed diseasecausing bacteria
(smooth colonies)
Harmless bacteria
(rough colonies)
Live disease-causing
bacteria
(smooth colonies)
Dies of pneumonia
Griffith concluded that
the heat-killed bacteria
passed their diseasecausing ability to the
harmless strain.
Heat-killed diseasecausing bacteria
(smooth colonies)
Harmless bacteria
(rough colonies)
Live disease-causing
bacteria
(smooth colonies)
Dies of pneumonia
Transformation
Griffith called this process transformation because one
strain of bacteria (the harmless strain) had changed
permanently into another (the disease-causing strain).
Griffith hypothesized that a factor must contain
information that could change harmless bacteria into
disease-causing ones.
Avery and DNA
Oswald Avery repeated Griffith’s work to determine which
molecule was most important for transformation.
Avery and his colleagues made an extract from the heatkilled bacteria that they treated with enzymes.
The enzymes destroyed proteins, lipids, carbohydrates, and other
molecules, including the nucleic acid RNA. Transformation still
occurred.
Avery and other scientists repeated the experiment using
enzymes that would break down DNA.
When DNA was destroyed, transformation did not occur.
Therefore, they concluded that DNA was the transforming
factor.
Avery and other scientists discovered that the nucleic acid
DNA stores and transmits the genetic information from one
generation of an organism to the next.
Chargaff's Rules
Erwin Chargaff discovered that:
 The percentages of guanine [G] and cytosine [C] bases are
almost equal in any sample of DNA.
 The percentages of adenine [A] and thymine [T] bases are
almost equal in any sample of DNA.
X-Ray Evidence
This X-ray diffraction photograph of
DNA was taken by Rosalind Franklin in
the early 1950s. The X-shaped pattern in
the center indicates that the structure of
DNA is helical.
Rosalind Franklin used X-ray
diffraction to get information about
the structure of DNA. She aimed an
X-ray beam at concentrated DNA
samples and recorded the scattering
pattern of the X-rays on film.
The Double Helix
Using clues from Franklin’s pattern, James Watson and Francis Crick built
a model that explained how DNA carried information and could be
copied.
Watson and Crick's model of DNA was a double helix, in which two
strands were wound around each other.
What is
DNA ?
Deoxyribonucleic acid, DNA
• Double-stranded molecule that carries information that determines an
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organism’s traits.
Composed of a four-letter nucleotide/molecule alphabet referred to as A, T,
C, and G.
Order of the alphabet determines the characteristics of the living organism,
much like the order of letters in our alphabet determines the words.
Each cell in the human body contains >3 BILLION letters.
The only difference between living organisms is the amount and order of the
DNA alphabet.
DNA is like a fingerprint, in that it is unique
to each individual and can be used to identify
them, even though it is not visible to the
naked eye.
Parts of DNA
 DNA is made up of nucleotides,
building blocks of DNA.
 A nucleotide is a monomer of
nucleic acids made up of:
 Deoxyribose – 5-carbon Sugar
 Phosphate Group
 Nitrogenous Base
 DNA molecule looks like a
twisted ladder - shaped like a
double helix.
The Double Helix
 In the “ladder,” bases –
nitrogen-containing
compounds are arranged
in pairs.
 The sides of the ladder
are composed of sugar
molecule and phosphate
group.
 Each of the building
blocks of DNA is called a
nucleotide.
The Bases
There are four kinds of
bases in DNA:
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Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
These bases can combine in
billions of different ways, but
they always pair this way:
 Hydrogen bonds form and
hold these bases.
 This principle is called
base pairing.
Where do
you find
DNA?
Prokaryotic Cells
 In prokaryotic cells, DNA is located in the cytoplasm.
 Most prokaryotes have a single DNA molecule containing
nearly all of the cell’s genetic information.
Eukaryotic Chromosome Structure
Chromosome
Nucleosome
DNA
double
helix
Coils
Supercoils
Histones
Eukaryotic DNA is located in the cell nucleus inside
chromosomes.
DNA
Replication?
DNA Replication
 When a cell divides, it makes new cells with exactly the same
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genetic information as the original cell.
DNA cannot divide in two, instead the chromosomes must be
copied. The process is called replication.
Replication ensures that each new cell has the same genetic
information as the original cell.
Replication ensures that each resulting cell will have a complete set
of DNA.
Replication proceeds in both directions until each chromosome is
completely copied.
DNA Replication (Cont.)
 During replication, the DNA
molecule “unzips,” exposing
each half of the base pairs.
 Free bases in the cytoplasm pairs
up with the exposed bases on the
each half of the DNA strand.
 The result is the formation of
two strands of DNA identical to
the original.
DNA FACT
FACT:
You have about 9 million kilometers of DNA.
That's enough to reach to the moon and
back 13 times!
FACT:
Strands of DNA are so tiny, you could fit
about 5 million strands through the eye of
a needle.
FACT:
Each cell in your body contains 3 billion
letters in the DNA: they would fill a stack
of paperback books 200 ft high.
FACT:
Each cell contains 9 feet of DNA. In an
average meal, you eat approximately
55,000,000 cells or
about 93,205 miles of DNA.