2. 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.]

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
DNA is very long.
The basic shape is like a twisted ladder or zipper.
This is called a double helix.
{Show students a model of the double helix. Explain what a spiral is and a helix is.}

5. 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.

6. Nucleotide (One section of DNA)
D = deoxyribo N = Nucleic A = Acid
One deoxyribose (sugar) together with its phosphate (acid) and base make a nucleotide.
O -P O O 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
Nitrogen
base C C
Deoxyribose O

7. One Strand of DNA One strand of DNA is a chain 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. 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.

9. Base Pairs

10. DNA is coiled VERY tightly to fit inside of a cell
-nucleus of human cell contains more than
3 feet of DNA
Chromosome
Supercoils
Coils
DNA
double
helix

11. Copying the Code Occurs in the nucleus before protein synthesis.
1. DNA is unzipped by enzymes.
2. mRNA copy of the DNA is made by bases pairing.
3. After completion, mRNA breaks off—DNA strands
rejoin after a copy is made.
4. mRNA leaves nucleus and travels to ribosome
for protein synthesis.
RNA
DNA

12. Two Stranded DNA
Remember, DNA has two strands that fit together something like a zipper.
The rungs are the nitrogenous bases
{Point to the 3-D model to show the parts as you discuss them.}

13. Cytosine and Guanine always join together
Important:
Adenine and Thymine always join together
A T
Cytosine and Guanine always join together
C G

14. DNA by the numbers Each cell has about 2 m 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!

15. Copying the Genetic Code -
Protein Synthesis

16. It was made in the nucleus by transcription from a DNA molecule.
This is a molecule of messenger RNA.
It was made in the nucleus by transcription from a DNA molecule.
codon
A U G G G C U U A A A G C A G U G C A C G U U
mRNA molecule

17. A ribosome on the rough endoplasmic reticulum attaches to the mRNA molecule.
A U G G G C U U A A A G C A G U G C A C G U U

18. Amino acid
U A C
tRNA molecule
A transfer RNA molecule arrives.
It brings an amino acid to the first three bases (codon) on the mRNA.
codon
The three unpaired bases (codon) on the tRNA link up with the codon.
A U G G G C U U A A A G C A G U G C A C G U U

19. U A C
C C G
Another tRNA molecule comes into place, bringing a second amino acid.
Its codon links up with the second codon on the mRNA.
A U G G G C U U A A A G C A G U G C A C G U U

20. U A C
C C G
chemical bond
A chemical bond forms between the two amino acids.
A U G G G C U U A A A G C A G U G C A C G U U

21. U A C
C C G
The first tRNA molecule releases its amino acid and moves off into the cytoplasm.
A U G G G C U U A A A G C A G U G C A C G U U

22. C C G
The ribosome moves along the mRNA to the next codon.
A U G G G C U U A A A G C A G U G C A C G U U

23. C C G
A A U
Another tRNA molecule brings the next amino acid into place.
A U G G G C U U A A A G C A G U G C A C G U U

24. C C G
C C G
A chemical bond joins the second and third amino acids to form a chain of amino acids.
A U G G G C U U A A A G C A G U G C A C G U U

25. A C G
G U C
The process continues.
The amino acid chain gets longer.
This continues until a termination (stop) codon is reached.
The protein is then complete.
A U G G G C U U A A A G C A G U G C A C G U U

26. Each tRNA codon must pair with the proper
mRNA codon for protein synthesis to occur.
Messenger RNA
Messenger RNA is formed in the nucleus.
mRNA
Start codon
Ribosome
Methionine
Phenylalanine
tRNA
Lysine
Nucleus

27. Once pairing occurs, amino acids are attached by
chemical bonds to form a amino acid chain (protein)
that is ready to be used by the cell
mRNA
Ribosome
direction
Lysine
tRNA
Growing amino acid chain