1. The Structure of DNA
All life on earth uses a chemical called DNA to carry its genetic code or blueprint. In this lesson we be examining the structure of this unique molecule.
{Point out the alligator’s eyes in the first picture.} By the way, can you make out what this is?
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[The goal of this presentation is to introduce high school biology students to the chemical structure of DNA. It is meant to be presented in the classroom while accompanying the teacher’s lecture, under the control of the teacher.]

2. Learning Objective
We will identify the components of DNA and Describe how information for specifying the traits of an organism is carried in the DNA.
I will use notes and class discussions with an interactive coloring lab to identify the components of DNA.
Why is DNA called the blueprint of life?

3. DNA DNA is often called the blueprint of life.
In simple terms, DNA contains the instructions for making proteins within the cell.
Why is DNA called the blueprint of life?

4. The Shape of the Molecule
The basic shape is like a twisted ladder.
Two strands twist around each other to form a double helix.
{Show students a model of the double helix. Explain what a spiral is and a helix is.}

5. Hydrogen Bonds C N O
The two strands of DNA stick to each other because of hydrogen bonds.
Hydrogen bond

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DNA STRUCTURE
Rosalind Franklin took diffraction x-ray photographs of DNA crystals
In the 1950’s, Watson & Crick built the first model of DNA using Franklin’s x-rays
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7. ONE STRAND OF DNA Each strand of DNA is a polymer of nucleotides.
One strand of DNA has many millions of nucleotides.
{Point to the 3-D mode, if you have one, to show the parts as you discuss them.}

8. NUCLEOTIDES
One deoxyribose (5-Carbon Sugar) together with its phosphate group and nitrogenous base make up a nucleotide.
O -P O O C O
{Ask students where they have seen a similar molecule before in this class.
Answer: ATP Emphasize that nucleotides are the basic building blocks or units of a DNA molecule and that a single molecule has many millions of nucleotides.}
Phosphate C
Deoxyribose
Nitrogenous
base C C
Deoxyribose O

9. Four nitrogenous bases
DNA has four different bases:
Adenine A
Thymine T
Cytosine C
Guanine G
These four bases are abbreviated by using their respective first letters.

10. Two Kinds of Bases in DNA
AnGels have wings.
AnGels are pure – a way to remember which bases are purines!
Purines are double ring bases.
Adenine
Guanine
Pyrimidines are single ring bases.
Thymine
Cytosine C N
Adenine C N O
Guanine C N O
cytosine C N O
thymine

11. ONE STRAND OF DNA
The backbone of the molecule is alternating phosphate and deoxyribose (5-Carbon sugar) parts.
The “rungs” or “steps” are nitrogenous bases.
phosphate
deoxyribose
{Point to the 3-D mode, if you have one, to show the parts as you discuss them.}
bases

12. Chargaff’s rule of Base Pairing
Adenine and Thymine always join together
A T
Cytosine and Guanine always join together
C G

13. DISCOVERY OF DNA STRUCTURE
Erwin Chargaff showed the amounts of the four bases on DNA ( A,T,C,G)
In a body or somatic cell:
A = 30.3%
T = 30.3%
G = 19.7%
C = 19.7%
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CHARGAFF’S RULE
Adenine must pair with Thymine
Guanine must pair with Cytosine
The bases are held together by weak hydrogen bonds T A G C
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ROSALIND FRANKLIN
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REPLICATION FACTS
DNA has to be copied before a cell divides
DNA is copied during the S or synthesis phase of interphase
New cells will need identical DNA strands
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DNA REPLICATION
Begins at Origins of Replication
Two strands open forming Replication Forks (Y-shaped region)
New strands grow at the forks
Replication
Fork
Parental DNA Molecule 3’ 5’
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DNA REPLICATION
As the 2 DNA strands open at the origin, Replication Bubbles form
Prokaryotes (bacteria) have a single bubble
Eukaryotic chromosomes have MANY bubbles
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DNA REPLICATION
Enzyme Helicase unwinds and separates the 2 DNA strands by breaking the weak hydrogen bonds
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DNA REPLICATION
Before new DNA strands can form, there must be RNA primers present to start the addition of new nucleotides
Primase is the enzyme that synthesizes the RNA Primer
DNA polymerase can then add the new nucleotides
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21. Steps of DNA replication