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. What is DNA?
DNA (or Deoxyribonucleic Acid) is often called “The blueprint of life”.
DNA, like Proteins, Carbohydrates, and Fats, are biochemical polymers. The monomer is a “nucleotide”
DNA contains all of the instructions to make each organism the DNA is contained in.
Why is DNA called the blueprint of life?

3. The Discovery of the structure of DNA
The structure of DNA was discovered in 1952 by Rosalind Franklin (who showed DNA was a helix using X-rays) and Jim Watson and Francis Crick who built models.
They realized that the structure told us much about how DNA functioned as the genetic material.

4. Why do we study DNA?
DNA is in ALL living cells, it is centrally important to ALL life on Earth.
It works the same way in all of these organisms. If we understand the principles of function in one organism we can often extrapolate those functions to other organisms.
Understanding how DNA works has, and will continue to have, profound medical implications and tell us much about the history of life on this planet.

5. DNA, Genes, & Chromosomes
Chromosomes are long strands of DNA (each of ours are on average 200 million nucleotides long).
Individual instructions on the DNA are called genes.
Each gene has instructions to build 1 protein.
Our genes are contained on our 46 chromosomes (E. coli has 2000 genes and only 1 chromosome) and are passed on when organisms reproduce.
{Ask students where the chromosomes are in this picture. Or ask them where the DNA is. Remind them that the mitochondria also have DNA.}

6. The Shape of the Molecule
DNA is a very long polymer often consisting of millions of nucleotides.
Its basic shape is that of a twisted ladder or a twisted zipper.
The poles of the ladder are the sugar-phosphate backbones and the rungs are the bases
Each pole of the ladder is arranged in the shape of a helix and because the two poles are intertwined with each other it is called a double helix.
{Show students a model of the double helix. Explain what a spiral is and a helix is.}

7. One Strand of DNA
The backbone of the molecule is made of alternating molecules of phosphate and deoxyribose sugar.
The “rungs of the ladder” are called nitrogenous bases.
phosphate
deoxyribose
{Point to the 3-D mode, if you have one, to show the parts as you discuss them.}
bases

8. Nucleotides
One deoxyribose, together with its phosphate and base, make a nucleotide.
Nucleotides are the monomer of DNA
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 O
Deoxyribose

9. One Strand of DNA One strand of DNA is a polymer of nucleotides.
One strand of DNA can have many millions of nucleotides within that polymer.
{Point to the 3-D mode, if you have one, to show the parts as you discuss them.}
A nucleotide monomer.

10. 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.

11. Two Kinds of Bases in DNA
Pyrimidines are single ring (Hexagon) bases.
Purines are double (Hexagon and pentagon) ring bases. N N C O C C N C C N

12. 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

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

14. Nucleotide sequence
The order (or sequence) of the nucleotides on the DNA is important.
The sequence of bases is a code for the instructions to make an organism.
Each organisms DNA has a unique sequence.
{Point to the 3-D model to show the parts as you discuss them.}

15. Base Pairing and bonding
The bases attract each other because they have shapes that fit with each other.
Hydrogen bonds link the bases together (like a magnet).
H bonds are a special type of weak ionic bond.
They are formed when hydrogen is “sandwiched” between two electronegative atoms (e.g. N or O)

16. Hydrogen Bonds.
When making hydrogen bonds, cytosine always pairs up with guanine
Adenine always pairs up with thymine
These are known as Chargraff’s Rule… After the man who discovered them.

17. DNA’s incredible Numbers
Each human cell has about 6 feet of DNA (enough for 3 billion bases).
The average human has about 100 trillion cells.
The average human has enough DNA to go from the earth to the sun and back about 500 times.
DNA has a diameter of only meters.
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!