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When a species reproduces, there is genetic
continuity that is maintained from one
generation to the next.
 WHAT is responsible for this continuity?
 HOW does this continuity happen?
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Genetic material present in ALL organisms
Usually found within the nucleus
Different for each individual within a species
DNA = DeoxyriboNucleic Acid
Polymer made of repeating NUCLEOTIDE
monomers
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A molecule made of the following 3 parts:
 1. a 5-Carbon sugar group (deoxyribose)
 2. a phosphate group
 3. a nitrogen base – 4 different possibilities!
(5 Carbon sugar)
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Adenine (A)
Guanine (G)
 Purines (2 carbon rings)
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Cytosine (C)
Thymine (T)
 Pyrimidines (1 carbon ring)
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It is the arrangement of
these 4 different
nucleotides that allows
DNA to be so unique!
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Adenine ALWAYS pairs
with Thymine
(double bond)
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Cytosine ALWAYS pairs
with Guanine
(triple bond)
To Help you Remember: AT C-G
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2 Scientists credited with
discovering the structure
of DNA in 1953:
 James Watson & Francis
Crick (Watson & Crick)
Watson & Crick Model of
structure = DOUBLE
HELIX
 They received the Nobel
Prize (Medicine) in 1962
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The key to Watson and Crick’s
discovery was an X-ray diffraction
image (known as Photo 51) of DNA
recorded by Rosalind Franklin
 Both the image and Franklin’s
interpretation of the image were
shown to Watson without her
permission
 Watson, Crick, and Wilkins (the man
who gave Watson the image) received
the Nobel Prize
▪ Franklin had died 4 years earlier
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Characteristics:
 1. DOUBLE stranded molecule
 2. phosphate & sugar portion of
nucleotides form the “backbone”
of each strand
 3. Nitrogen base of one strand
forms a HYDROGEN BOND with
the nitrogen base of a second
strand
 4. In 3-Dimensional space, it forms
a “twisted ladder” (spiral staircase)
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DNA must be able to make a copy of itself in
order for genetic material to be passed on to
the next generation and to preserve genetic
continuity.
DNA Replication
This replication process occurs in 4 basic
steps.
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Step 1 – HELICASE (an enzyme) cause the
hydrogen bonds between the nitrogen bases
to break (“Unzips the ladder”)
Step 2 – DNA POLYMERASE (another
enzyme!) is responsible for reading the DNA
template and matching up the
complimentary nucleotide from the free
floating nucleotides present in the nucleus
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Step 3 – Base pairs form hydrogen bonds and
the phosphate group of one nucleotide bonds
with the sugar of another to form the
“backbone”
Step 4 – Bonding continues along the strands
until 2 new strands are formed
RESULT = 2 new EXACT copies of the
original DNA molecule!
Pink =
parent
strand
Blue =
daughter
strands
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DNA Replication Simulation