WHAT DOES DNA LOOK LIKE?
Unit 5: Seventh Grade
FOR MANY YEARS, THE STRUCTURE OF A DNA
MOLECULE WAS A PUZZLE TO SCIENTISTS
In the 1950’s, two
scientists deduced the
structure while
experimenting with
chemical models.
 They later won the
Nobel Prize for
helping to solve this
puzzle.
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WHAT IS DNA? BREAKING IT DOWN
As you know, inherited characteristics are
determined by genes.
 Genes are passed from one generation to the
next.
 Genes are parts of chromosomes.
 Chromosomes are structures in the nucleus of
most cells.
 Chromosomes are made of protein and DNA
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So, DNA is the material that makes up your
chromosomes that determine your inherited
characteristics
WHAT DOES DNA STAND FOR?
DNA stands for:
 Dexoyribo
 Nucleic
 Acid
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But is written out as deoxyribonucleic acid
THE PIECES OF THE PUZZLE
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Scientists knew that the
material that makes up
our genes must do two
things:
Give instructions for
building and maintaining
cells
Must be able to be copied
each time a cell divides so
that each cell could contain
identical genes
Scientists eventually
discovered that DNA is
made of subunits called
nucleotides
NUCLEOTIDES: SUBUNITS OF DNA
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DNA is made of subunits
called nucleotides.
A “subunit” is a
subdivision of a larger
unit.
For example: a subdivision
you live in is a smaller
unit of a city or town.
It takes many subunits
working together to make
up a whole, functioning
unit.
Nucleotides are the
subunits that make up
DNA
NUCLEOTIDES: THE SUBUNITS OF DNA
Although it may look
complicated, the DNA
in a cell is really just a
pattern made up of
different parts called
nucleotides.
 Nucleotides are
molecules that, when
joined together, make
up DNA
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NUCLEOTIDES: THE SUBUNITS OF DNA
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Nucleotides are made
up of a sugar, a
phosphate, and a base.
All nucleotides are the
same except for the
base.
There are four different
kinds of bases:
Adenine
 Thymine
 Guanine
 Cytosine
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THE FOUR NUCLEOTIDES
Each of the four bases
has a different shape.
 Scientists often refer
to a nucleotide’s base
by the first letter of
it’s base
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A = Adenine
T = Thymine
G= Guanine
C = Cytosine
Annie Takes Good
Care
NUCLEOTIDES AND DNA
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Imagine a set of
blocks that has only
four shapes, or an
alphabet that has only
four letters.
DNA is a long string
of these blocks or
letters.
CHARGAFF’S RULES
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Edwin Chargaff, a
biochemist, discovered
that the amount of
adenine (A) always
equaled the amount of
thymine (T).
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A=T
He also found out that
the amount of guanine
(G) equals the amount
of cytosine (C)
G= C
 Annie Takes Good Care
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CHARGAFF’S RULES
These findings are
known as Chargaff’s
Rules
 At the time, these
findings were thought
to be relatively
unimportant, but
later on, Chargaff’s
rules helped scientists
understand the
structure of DNA
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WATSON AND CRICK
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After looking at x-ray
images of DNA showing
that DNA has a spiral
shape, scientists James
Watson and Francis
Crick concluded that
DNA must look like a
long, twisted ladder.
This discovery
eventually helped
explain how DNA is
copied and how it
functions in a cell.
DNA’S DOUBLE STRUCTURE
DNA looks like a
twisted ladder.
 This shape is known
as a double helix.
 The sides of the ladder
are made of
alternating sugar and
phosphate parts.
 The rungs are made of
a pair of bases.
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RUNGS OF THE LADDER
The rungs of the
ladder are made of the
bases adenine,
thymine, guanine, and
cytosine.
 Adenine on one side of
the rung always pairs
with thymine (A, T)
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Annie Takes
Guanine always pairs
with cytosine on a
rung (G,C)
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Good Care
MAKING COPIES OF DNA
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These pairings of the
bases allows the cell to
replicate, or make copies
of, DNA.
Each base only bonds
with only one other
base.
Furthermore, pairs of
bases are
complementary to each
other.
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For example GCAC will
bond with CGTG
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Ex: TGCA will bond
with:
HOW COPIES ARE MADE
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During replication, a
DNA molecule is split
down the middle, where
the bases meet.
As the bases are
exposed, complementary
nucleotides are added to
each side of the split
ladder.
Two new DNA
molecules are formed.
Half of each molecule is
the old DNA
 Half of each molecule is
the new DNA
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WHEN COPIES ARE MADE
DNA is copied every
time the cell divides
 Each new cell gets a
complete copy of all
the DNA.
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