DNA contains the genetic code for building and maintaining organisms. It is made up of nucleotides that pair together in a double helix structure, with adenine pairing with thymine and guanine pairing with cytosine. Erwin Chargaff discovered these base pairing rules, and Rosalind Franklin's X-ray images of DNA suggested its spiral structure. Using this evidence, Watson and Crick constructed a double helix model of DNA that showed its ladder-like shape formed from paired nucleotides. DNA replicates by splitting and using each strand as a template to form two new DNA molecules.

1. What Does DNA Look Like?

2. The Pieces of the Puzzle

3. DNA is sometimes
called "the blueprint of
life" because it contains
the code, or instructions
for building an organism
and ensuring that
organism functions
correctly.
Just like a builder uses
a blueprint to build a
house, DNA is used as
the blueprint, or plans,
for the entire organism.

4. Scientists knew that the material that
makes up genes must be able to do 2
things:
1. Give instructions for building and
maintaining cells
2. Be able to be copied each time a cell
divides and contain identical genes

5. It is the chemical
component of
chromosomes, which are
located in the nucleus of
every cell.
Stretches of DNA (or
stretches of
chromosomes) code for
genes.
Gene - a segment of DNA
that codes for a protein,
which in turn codes for a
trait (skin tone, eye color,
etc), a gene is a stretch of
DNA.

6. DNA is made up of subunits called
nucleotides
Nucleotide = consists of a sugar, a
phosphate, and a base
Four bases adenine, thymine, guanine,
and cytosine (A, T, G, C)
Each base has a different shape

8. Chargaff’s Rule
In 1950’s,
biochemist named
Erwin Chargaff
found that the
amount of adenine
in DNA always
equals amount of
thymine

9. Also found that
amount of guanine
always equals
amount of cytosine
Chargaff’s Rule
helped scientists
understand the
structure of DNA

10. Franklin’s Discovery
Chemist Rosalind
Franklin was able
to make images
of DNA
molecules

11. X ray diffraction =
X rays are aimed
at DNA molecules
and the ray
bounces off
The pattern made
by the bouncing
rays is captured on
film, suggesting
that DNA has a
spiral shape

12. Watson and Crick’s Model
James Watson
and Francis Crick
used Franklin’s X
ray images to
conclude that
DNA must look
like a long,
twisted ladder

13. They built a
model of DNA in
their laboratory

14. DNA’s Double Helix
Double helix =
shaped like a twisted
ladder
Two sides of the
ladder are made up of
alternating sugar
parts and phosphate
parts

15. Each rung of the
ladder is made of a
pair of bases
The rungs of the
ladder can occur in
any order (as long as
the base-pair rule is
followed)—A always
with T, G always with
C

16. For instance, a stretch of DNA could be
AATGACCAT (which would code for a
different gene than a stretch that read:
GGGCCATAG).
All in all, there are billions of bases in
cells, which code for all the things an
organism needs to function.

17. Making Copies of DNA
The pairing of bases allows the cell to
“replicate”, or make copies of, DNA
Pairs of bases are complementary—
meaning they only pair with each
other, and both sides of the DNA
ladder pair together

18. During replication
(copying process),
DNA molecule
splits down the
middle, where the
bases meet
The bases on each
side of the molecule
are used as a
pattern for a new
strand
How Copies are
Made

19. As the bases are
exposed,
complementary
nucleotides are
added to each
side of the ladder
Two DNA
molecules are
formed half of
each DNA is old
and half is new

20. When Copies are Made
DNA is copied
every time a cell
divides
The job of
unwinding,
copying, and rewinding the DNA is
done by proteins
within the cell

21. DNA is usually
found with several
kinds of proteins
Other proteins help
with the process of
carrying out the
instructions written
in the code of DNA

22. ScientiStS’ contributionS
Erwin Chargaff: discovered base-pair
rule
Rosalind Franklin: used X-rays to find
DNA’s spiral shape
Watson and Crick: built 3-D model of
DNA