Cell Biology:
DNA
Lesson 1 – DNA Replication (Inquiry into Life pg. 489-493)
Today’s Objectives
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Describe DNA Replication, including:
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Describe the three steps in the semi-conservative
replication of DNA:
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“unzipping” (DNA helicase)
Complementary base pairing (DNA polymerase)
Joining of adjacent nucleotides (DNA polymerase)
Describe the purpose of DNA replication
Identify the site of DNA replication within the cell
Background information
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It is important to recall the information we
learned about DNA from the last 2 units:
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DNA stands for deoxyribonucleic acid
DNA is the molecule that makes up
chromosomes and serves as hereditary
information
Nucleic Acids
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There are two types of nucleic acids, DNA and RNA
Both DNA and RNA are polymers of nucleotides
(chains of joined nucleotides)
They form genetic material and are involved in the
functioning of chromosomes and protein synthesis
DNA Structure:
The shape of DNA is a double helix made up of
repeating nucleotide units
Nucleotide Structure
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Nucleotides are composed of 3
parts:
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1) phosphate molecule
2) 5 carbon sugar
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Deoxyribose in DNA
Ribose in RNA
3) Nitrogen containing base
Nitrogen containing bases
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Two types:
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Purines – adenine (A) and guanine (G)
Pyrmidines – thymine (T), uracil (U) and cytosine (C)
DNA has A,G,T,C, RNA has A,G,U,C
Purines are larger than pyrimidines and have a double
ring structure
Nitrogen containing bases
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Pyrimidines are smaller than purines and have a single
ring structure
When the bases bond
together to form the “rungs
of the DNA ladder” they do
so in a set pattern.
The alternating sugar and
phosphates make up the
rails (backbone).
The bases make up the
rungs.
Complementary Base Pairing
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Adenine always bonds to thymine
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Guanine always bonds to cytosine
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Two hydrogen bonds
Three hydrogen bonds
This bonding of bases is called complementary base
pairing
They cannot bond any other way because 2 purines
would overlap and 2 pyrimidines would be too short to
form the rungs of the ladder
The double strand is
held in place by
hydrogen bonds
between the bases.
It is the number and
order as well as the
type of the bases that
determine what kind of
organism will develop.
Order of Bases
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Example:
ATCCGATT means something entirely different than
ACCGTTAT, just as the words hate and heat mean
different things even though they contain the same letters
As a DNA strand lengthens, it twists into a double spiral
called a double helix
Candy DNA Model
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Next class you will build a DNA molecule using CANDY!
(you can eat it after you are finished)
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DNA model…
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….
Functions of DNA
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1) Replicates (duplicates) itself so each new cell has a
complete, identical copy
2) Controls the activities of a cell by producing
proteins
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The combination of proteins determines the characteristics of
each living organism
3) Undergoes occasional mutations (mistakes in
replication) which accounts for the variety of living
things on earth
DNA Replication
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Cool website demonstrating the steps in DNA replication
http://www.pbs.org/wgbh/aso/tryit/dna/shockwave.html
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Steps in DNA Replication
1. The DNA molecule
becomes untwisted by the
enzyme DNA helicase
breaking the H-bonds. The
2 strands that make up
DNA become unzipped.
Each side acts as a
template. (ie. The weak
hydrogen bonds between
the paired bases are
broken by an enzyme)
2. New complementary
nucleotides, always
present in the nucleus,
move into place and pair
with complementary
bases on the exposed
strands.
T joins to A
C joins to G
3. The adjacent
nucleotides, through
their sugar-phosphate
components become
joined together along
the newly forming
chain. The enzyme
DNA polymerase helps
this. (Adds nucleotides
to template).
4.
When the process
is finished, 2 complete
DNA molecules are
present, identical to
each other and to the
original molecule.
5. Both DNA will now wind back up into their helical shape.
 DNA replication is called semi-conservative because
each new double helix is composed of an old (parental)
strand and a new (daughter) strand.
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Each strand conserves a length of the old strand
Enzymes assist the unwinding process, join together the
nucleotides, and assist the rewinding process and many
others.
When errors are made in replication. A mutation can
arise.
Meet the enzymes involved in DNA replication
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DNA presentation