Lesson 1: Discovery of DNA Structure Flashcards
Phoebus Levene
(1910)
An American biochemist, isolated two different types of nucleic acids that could be distinguished by the sugars involved in their composition.
One contained the sugar ribose which he termed “ribonucleic acid”
The other contained an unknown sugar that was similar to ribose but was missing an oxygen atom, therefore he called it deoxyribose and termed the nucleic acid “deoxyribonucleic acid”
Showed that DNA and RNA were composed of long chains of nucleotides Levene also discovered the structure of
the nucleotide – a phosphate group, a sugar and one of five nitrogenous bases.
Adenine, Thymine, Guanine and
Cytosine are the nitrogenous bases in DNA.
In RNA, Thymine is replaced by Uracil.
However, Levine concluded equal
amounts of each base must be present in a chain of nucleic acid and accounted for this by proposing the following
structure:
DNA Structure
DNA is made up of nucleotides.
Each Nucleotide is made up of 3
parts;
Phosphate group
Nitrogenous base
Pentose sugar
The Sugar
In DNA the sugar is called
deoxyribose.
Deoxyribose is a pentose sugar
because it has 5 carbon atoms in
its structure.
The Phosphate Group
The acidic part of the nucleotide
Connects the 5’ carbon of one
nucleotide to the hydroxide
(-OH) group on the 3’ carbon of
the neighbouring nucleotide.
The Nitrogenous Bases
These are bases that contain nitrogen
The nucleotide is named for the nitrogenous base attached
There are 4 nitrogenous bases found in DNA
These fall into two categories:
Purines – Adenine and Guanine
Pyrimidines – Thymine and Cytosine
Purines
The nitrogenous bases that
have a double-ringed
structure.
Pyrimidines
The nitrogenous bases that
have a single-ringed
structure.
Fred Griffith (1928), Oswald Avery (1944)
Conducted experiments that showed that
DNA is probably the “genetic” material
Griffith’s experiment used
transforming factors in
pneumococcus bacteria in mice
Griffith was killed in World War II before
he could confirm what the transforming
factors were and publish his results
Avery completed Griffith’s work and
proposed that the transforming factors
were DNA.
Alfred Hershey and Martha Chase (1953)
Clearly showed that DNA was the genetic
material of the cell (not protein)
Conducted the “Blender Experiment”
Tagged bacteriophages with radioactive
isotopes
Placed the bacteriophages in a blender
with non radioactive E. Coli bacteria
Discovered that only the radioactively
tagged viral DNA was transferred to the E.
Coli.
Results showed that DNA was the
transforming factor
Erwin Chargaff
(1952)
Believed that DNA was more than the
repetitive tetranucleotide blocks proposed
by Levine.
Noticed that the number of adenine bases
always equaled the number of thymine
bases and that the number of cytosines
always equaled the number of guanines.
BUT the number of adenines and thymines
DID NOT always equal the number of
cytosines and guanines
This became known as Chargaff’s Ratios
Rosalind Franklin and Maurice Wilkins
(1952 – 1953)
Franklin was the expert on X-ray
crystallography
Her techniques produced two sets of
X-ray images of DNA
Used X – ray crystallography to
suggest that DNA was a helical
structure BUT could not figure out
how the nucleotides fit the structure
in a stable pattern
Wilkins gave one set of X – ray images
to another group of scientists
Franklin died of cancer in 1953
James Watson and Francis Crick
(1953)
Used Franklin and Wilkins’ X – ray
images and Pauling’s technique to
determine the structure of DNA
Proposed the DNA structure was a
double helix
Found a base pairing method that used
hydrogen bonds and covalent bonds
that agreed with Chargaff’s ratios
Found that the two strands of DNA in
the double helix were “anti – parallel”
(ran in opposite directions)
DNA
DNA is a thread – like molecule made
up of 2 long strands of nucleotides
bound together in the shape of a
double helix
If the helix was unwound it would look
like a “ladder”
The Sugar – Phosphate Backbone
would be the “rails” and are held
together by covalent bonds
The Nitrogenous Base Pairs would
be the “rungs” and are held
together by weaker hydrogen
bonds
Dimensions
The distance between nucleotides is constant (0.34 nm or 3.4 angstroms)
The diameter of the molecule is 2nm or 20 angstroms
Each turn contains 10 nucleotides
Complementary Base Pairing
A Purine is ALWAYS bonded to a Pyrimidine
In order to fit the double helix structure:
A can only form a stable bond with T
C can only form a stable bond with G
Nucleotide strands are anti – parallel
The 2 nucleotide strands that make up the DNA molecule run in OPPOSITE directions
At the end of each DNA molecule will be the 5’ end of one strand and the 3’ end of the other
Naming
By rule, we read DNA from the 5’ end
to the 3’ end.
We name the strand of DNA by listing
the bases in series from the 5’ end to
the 3’ end
The name is called a DNA Sequence
Terminology
Genes - The functional subunits of DNA
Specific sequences of DNA that have the potential to be “expressed” in order to guide an organism’ development
Often called the “Code” because it provides instructions for the assembly of proteins, enzymes and complex
polypeptides
Genes have 2 regions:
Exons
These are coding regions
These regions of the gene are active
Introns
Unused portions of DNA
May have had a purpose in an earlier form of the organism but are no longer required
Genome - The sum of all DNA carried in an organism’s cells
The human genome contains about 6 billion base pairs
If we could arrange DNA end to end, the length would be 2 x 1010 km
(about 100x the distance between the Earth and the Sun)
