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. 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?

3. The Shape of the Molecule
The basic shape is like a twisted ladder.
This is called a double helix.
It has two strands twisted together.
{Show students a model of the double helix. Explain what a spiral is and a helix is.}

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

5. NUCLEOTIDES
One deoxyribose together with its phosphate and base make 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
Deoxy
ribose
Nitrogenous
base C C
Deoxyribose O

6. ONE STRAND OF DNA One 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.}

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

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

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

10. Base Pairing Adenine and Thymine always join together A T
Cytosine and Guanine always join together
C G

11. 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 = 19.5%
C = 19.9%
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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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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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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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20. Steps of DNA replication