As you can see from the
model, the structure is
basically a long, …twisty,
…uh laddery type thing.
1953: The structure of the DNA molecule is first
described.
DNA - Introduction
DNA
• How do genes work? What are they made of,
and how do they determine the characteristics
of organisms?
• In the middle 1900’s questions like these were
on the minds of biologists everywhere.
Gregor Mendel “Father of
Genetics” (1864)
Genetics – the scientific study of
heredity.
The Components and Structure of DNA
• When scientists found out that the molecule DNA
was responsible for our genetics they still were
not satisfied.
• How could DNA or any molecule for that matter
have the ability to carry, translate, copy, and be
responsible for our heredity?
DNA(Deoxyribose Nucleic Acid)
• DNA is a long molecule made up of units called
nucleotides.
• Nucleotide is made up of three basic structures:
- A 5-carbon sugar called deoxyribose
- A phosphate group
- And a combination of four nitrogenous
bases.
The Backbone of DNA
• The “backbone” of DNA
is made of up of sugar
and phosphate.
• This backbone allows
for the nitrogeneous
bases (adenine,
cytosine, guanine, and
thymine) to combine
and construct our
genetic code.
Nitrogenous Bases
• DNA’s nitrogenous bases are the letters of the
words which make up our genetic code.
• Adenine always connects with Thymine.
• Cytosine always connects with Guanine.
Together these bases make up the steps of a
ladder.
Nitrogenous Bases
- Two nitrogenous bases belong to a group called the
purines. (Adenine and Guanine) A and G
- The other two nitrogenous bases belong to a group
called the pyrimidines. (Cytosine and Thymine) C and T
- Each base pair contains one purine and one pyrimidine.
Adenine and Thymine
Straights with Straights
A-T
Guanine and Cytosine
Curves with Curves
G–C
DNA
• Each DNA molecule is made up of a series of
monomers called nucleotides. Each nucleotide has
three parts:
• A deoxyribose sugar
• A phosphate group
• A nitrogenous base.
DNA’s History
• Now that you know what DNA is and how it is
constructed, how do you think it was found?
Chargaff’s Rule
• One of the puzzling facts about DNA was a curious
relationship between its nucleotides.
• Erwin Chargaff (an American biochemist) had
discovered that the percentages of guanine and
cytosine bases are almost always equal in any
sample of DNA.
X-Ray Evidence
• In the early 1950’s a British scientist named
Rosalind Franklin began to study DNA.
• Just like anyone else that is curious Rosalind
wanted to see what she was studying, so she took
pictures of DNA with an X-ray.
The Double Helix
• At the same time that Franklin was continuing her
research, Francis Crick and James Watson were
trying to build a three-dimensional model of the
molecule.
• Watson and Crick used Rosalind’s picture to figure
out DNA’s puzzle and the solved what is known as
the double helix.
• DNA is a double helix in which two strands are
wound around each other. Each strand is made up
of a chain of nucleotides. The two strands are held
together by hydrogen bonds between adenine and
thymine and between guanine and cytosine.
100 Greatest Discoveries - DNA
Read Section 12-1 / pp. 287-294
(Relationship between genes & DNA / Structure)
Questions to turn in…
Page 294 - 2, 3 & 4
Page 315 - 12 & 13