DNA Structure,
Function and
Replication
B5 – Describe DNA replication:
• Review DNA structure and function
• Describe the purpose of DNA replication
• Identify the site of DNA replication in a cell
• Describe the three steps in DNA replication
• Explain why DNA replication in called “semiconservative”
Comic by Kate Beaton, http://harkavagrant.com/
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Double helix
Backbone is sugar and phosphate joined by
covalent bonds
Rungs are complementary base pairs joined
by hydrogen bonds (purine-pyrimidine: A-T,
G-C)
Note that the nucleotides in the two strands
are laid out in opposite directions
(antiparallel)
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DNA carries the genetic code
Replicates to pass on genetic information
Instructions for synthesis of proteins
Proteins are required to build an organism
and catalyze all of its biochemical reactions
(therefore controlling all of the functions of
the cell and organism)
Questions for discussion:
• What is meant by the term DNA replication?
• Why is DNA replication necessary?
• Where does it take place?
• When does it take place?
• What is the function of the genetic code?
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DNA must be replicated in every cell before
the cell divides
This is necessary so that each new cell has a
complete copy of the genetic code
The method of DNA replication is known as
“semi-conservative”
(what do you think this means?)
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The double-stranded
DNA molecule “unzips”
when the enzyme DNA
helicase breaks the
hydrogen bonds between
complementary bases
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Each strand acts as a
template to make new
double helix
New complementary
nucleotides attach to the
exposed bases
Catalyzed by the enzyme
DNA polymerase
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Adjacent phosphate
and sugar groups
are attached by
another DNA
polymerase
We now have two
DNA molecules,
each identical to the
original
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This is called semi-conservative replication
because half of each DNA strand is “old” and
the other half is “new”