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 or Protein?
Walter Sutton discovered chromosomes were made of DNA and Protein
However, scientists were NOT sure which one (protein or DNA) was the actual genetic material of the cell

3. DNA!
Frederick Griffith in 1928 showed the DNA was the cell’s genetic material

4. 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?

5. Why do we study DNA? We study DNA for many reasons:
Important to all life on Earth
Medical benefits such as cures for diseases
Better food crops
About better food crops, this area is controversial. There is a Dr. Charles Arntzen who is working on bioengineering foods with vaccines in them. People in poor countries could be immunized against diseases just by eating a banana, for instance.

6. What is a Gene?
Gene – segment of DNA that is located in a chromosome and that codes for a specific hereditary trait.
Basic unit of heredity.
Each cell in the human body contains about 25,000 to 35,000 genes, which carry information that determines your traits.
Our genes are on our chromosomes.
{Ask students where the chromosomes are in this picture. Or ask them where the DNA is. Remind them that the mitochondria also have DNA.}

7. Chromosomes and DNA
Chromosomes are made up of a chemical called DNA and proteins.
Chromosomes come in pairs, and there are hundreds, sometimes thousands, of genes in one chromosome.
There are 46 chromosomes in each human cell.
Draw and explain Figure 2 Chromosome structure on p.119 on the left side of your notebook.

9. DNA Deoxyribonucleic acid Stores hereditary information
Found in all cells
Organic Comound made up of C, H, O, N and P

10. The Shape of DNA DNA is a very long polymer.
The basic shape is like two strands twisted around each other, like a winding staircase.
This is called a double helix.
Read the first paragraph on p. 194; then summarize what you read in your notes.
{Show students a model of the double helix. Explain what a spiral is and a helix is.}

11. Watson & Crick in the 1950’s built the 1st model of DNA

12. The Double Helix Molecule
The DNA double helix has two strands twisted together.
We will take apart the DNA molecule to see how it is put together.
First, we will look at one strand.

13. What are Nucleotides? Each DNA strand is made of linked nucleotides.
Nucleotides are the subunits of DNA
Each nucleotide is made up of 3 parts:
1. phosphate group
2. 5 carbon sugar (deoxyribose)
3. nitrogen base

14. One Strand of DNA
The backbone of the molecule is alternating phosphates and deoxyribose sugar
The teeth are nitrogenous bases.
phosphate
deoxyribose
{Point to the 3-D mode, if you have one, to show the parts as you discuss them.}
bases

15. Nucleotides
O -P O O
ATP
One deoxyribose together with its phosphate and base make a nucleotide.
O -P O O
Nitrogenous
base C O
Phosphate
{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.} C C C
Deoxyribose
ribose O

16. 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.}

17. 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.

18. Two Kinds of N Bases in DNA
Pyrimidines are single ring bases.
Purines are double ring bases. N C O C C N C C N

19. Thymine and Cytosine are pyrimidines
Thymine and cytosine each have one ring of carbon and nitrogen atoms. C N O
cytosine C N O
thymine

20. Adenine and Guanine are purines
Adenine and guanine each have two rings of carbon and nitrogen atoms. C N O
Guanine C N
Adenine

21. Two Stranded DNA
Remember, DNA has two strands that fit together something like a zipper.
The teeth are the nitrogenous bases but why do they stick together?
{Point to the 3-D model to show the parts as you discuss them.}

22. Hydrogen Bonds The bases attract each other because of hydrogen bonds.
Hydrogen bonds are weak but there are millions and millions of them in a single molecule of DNA.
The bonds between cytosine and guanine are shown here with dotted lines

23. Hydrogen Bonds, cont.
When making hydrogen bonds, cytosine always pairs up with guanine
Adenine always pairs up with thymine
Adenine is bonded to thymine here C N O

24. Chargraff’s Rule: Adenine and Thymine always join together A T
Cytosine and Guanine always join together
C G

25. DNA by the Numbers Each cell has about 2m of DNA.
The average human has 75 trillion cells.
The average human has enough DNA to go from the earth to the sun more than 400 times.
DNA has a diameter of only m.
The earth is 150 billion m
or 93 million miles from
the sun.
If you unravel all the DNA in the chromosomes of one of your cells, it would stretch out 2 meters. If you did this to the DNA in all your cells, it would stretch from here to sun more than 400 hundred times!

26. DNA Replication What: The process of making a copy of DNA. When:
DNA replication occurs during the synthesis (S) phase of the cell cycle, before a cell divides.

27. Step 1 : DNA replication Double helix unwinds
DNA helicases open the double helix by breaking the hydrogen bonds that link the complementary nitrogen bases.

28. Replication Fork
The areas where the double helix separates are called replication forks because of their Y shape.

29. Step 2: DNA replication
At the replication fork, enzymes known as DNA polymerases add nucleotides to the exposed nitrogen bases, according to the base-pairing rules.
As it moves along, two new double helixes are formed.

30. Step 3: DNA replication
Process continues until all of the DNA has been copied and the polymerases are signaled to detach.
Results in two identical DNA strands.

31. Checking for Errors
Errors sometimes occur and the wrong nucleotide is added to the new strand.
An important feature of DNA replication is that DNA polymerases have a “proofreading” role.
It can backtrack to remove the incorrect nucleotide.
Reduces errors in DNA replication to about one error per 1 billion nucleotides.