DNA Structure
(November 2nd)
Learning Goals
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Students will learn about nucleotides, the monomer of DNA molecules, and how they are
bound to form strands
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Students will understand how complementary base pairing can lead to the formation of
two polynucleotide strands that twist to form a double helix
What is DNA?
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Deoxyribonucleic acid (DNA)
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DNA is the hereditary material – passed on from generation to generation
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Contains information that controls the production of proteins
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Has to be able to replicate itself with great accuracy
Structure of DNA
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Structure: DNA is made up of repeating units of ________________ (monomer)
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Nucleotides are composed of the following 3 components:
1. A sugar group
2. A phosphate group
3. A nitrogenous base
1. Sugar Group
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In DNA, the sugar group is a _______________ sugar
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The sugar group is a _________________ molecule
2. Phosphate Group
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The deoxyribose sugar binds with phosphate at both its 3’ and 5’ (three prime and five
prime) carbons
3. Nitrogenous Base
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A nitrogenous base attaches to the deoxyribose sugar group
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There are two different forms of nitrogenous bases:
1. _____________________
2. _________________
Thymine and Cytosine are pyrimidines
Have a ______________
Adenine and Guanine are purines
Have ____________________
Drawing of a nucleotide:
Nucleoside vs. Nucleotide
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A sugar + a nitrogenous base = nucleoside
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A sugar + a nitrogen base + phosphate = nucleotide
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Phosphorylating a nucleoside makes a nucleotides
Primary Structure
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Each nucleotide is attached to the next by a ____________________ _____________
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The phosphodiester bond is the linkage between the 3’ carbon atom of one sugar
molecule and the 5’ carbon atom of the next
Think: Phospho (phosphate) + diester (two esters)
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This bonding creates a polynucleotide (multiple nucleotides) strand
Sugar Phosphate Backbone
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Each polynucleotide strand has a backbone of alternating
phosphate groups and sugars
called the ______________ ___________________
__________________
Binding of two strands
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Complementary base pairing: nitrogenous bases on opposite strands pair
through hydrogen bond formation such that
Adenine pairs with Thymine
Cytosine pairs with Guanine
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A-T held by _____ hydrogen bonds
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C-G held by _____ hydrogen bonds
Question: Which pair do you think is stronger?
Answer: _______________________________________
Chargaff’s Rule
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The amount of adenine is __________ to the amount of thymine
AND
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The amount of cytosine is equal to the amount of guanine
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Question: are purines binding to one another and pyrimidines binding to one another?
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Answer:_________________________________________________
Antiparallel
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Antiparallel: The two strands run in _______________ directions
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Each strand will have a 5’ end and a 3’ end
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The 5’ end of one strand lies across from the 3’ end of the complementary strand
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The 5’ and 3’ come from the numbering of the carbons on the deoxyribose sugar
Antiparallel – Analogy
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This is similar to how sports teams face in a different direction when they shake hands at
the end of a game
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Let’s say the goalie is always at the front of the line (5’)
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And the coach is always at the back of the line (3’)
Drawing of two antiparallel strands:
Secondary Structure – Double Helix
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The two bound polynucleotide strands twist around each other to form a double helix
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Resources in case you’d like a visual (also on page 213 in textbook):
http://www.johnkyrk.com/
-On left hand side click “Structure” under DNA
-You can move through the pictures using the arrows in the bottom left corner
http://www.dnatube.com/video/3447/DNA-double-helix
Questions!
1.
2.
3.
4.
How many bonds between A-T? C-G?
What kinds of bonds are these?
What is a nucleotide and what is a nitrogenous base?
Are the two polynucleotide strands that twist around each other identical?