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. Watson & Crick’s Model

4. Why do we study DNA? We study DNA for many reasons, e.g.,
its central importance 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.

5. Chromosomes and DNA Our genes are on our chromosomes.
Chromosomes are made up of a chemical called DNA.
{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.
The basic shape is like a twisted ladder.
This is called a double helix.
{Show students a model of the double helix. Explain what a spiral is and a helix is.}

7. 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.

8. One Strand of DNA
The backbone (sides) of the DNA molecule are alternating phosphates and deoxyribose sugar
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

9. 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

10. 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.}

11. 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.

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

13. Thymine and Cytosine are pyrimidines
Thymine and cytosine each have one ring of carbon and nitrogen atoms.
They are the smaller. C N O
cytosine C N O
thymine

14. Adenine and Guanine are purines
Adenine and guanine each have two rings of carbon and nitrogen atoms.
They are the larger bases C N O
Guanine C N
Adenine

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

16. Hydrogen Bonds
The bases attract to 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 C N O

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

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

19. DNA Double Helix P O 1 2 3 4 5 P O 1 2 3 4 5 G C T A

20. BASE-PAIRINGS C G
H-bonds T A

21. The Code of Life… A T C G T A T G C G G…
The “code” of the chromosome is the SPECIFIC ORDER that bases occur.
A T C G T A T G C G G…

22. 1. Why is replication necessary? 2. When does replication occur?
Replication Quiz
1. Why is replication necessary?
2. When does replication occur?
3. Describe how replication works.
Use the complementary rule to
create the complementary strand:
A---?
G---?
C---?
T---?

23. Replication Quiz 1. Why is replication necessary?
So both new cells will have the correct DNA
2. When does replication occur?
During interphase (S phase).
3. Describe how replication works.
Enzymes unzip DNA and complementary nucleotides join each original strand.
4. Use the complementary rule to
create the complementary strand:
A---T
G---C
C---G
T---A

24. DNA Trivia 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.
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. ✔ Check for Understanding Figure out the complementary DNA strand for the following:
A A T T G G C G A T C C G G C G T G G C C

27. SUPER JOB! A A T T G G C G A T C C G G C G T G G C C
T T A A C C G C T A G G C C G C A C C G G
SUPER JOB!

28. Before Replication can occur:
STEP-1
Double helix must be unwound.
This occurs by the enzymes known as helicases.
Helicases break the hydrogen bonds that link the complementary bases together.

29. Step-2
Next DNA polymerases are enzymes that move along the exposed DNA strands adding nucleotides according to the base paring rules.
A=T
C=G
As the DNA polymerases move along the strand 2 NEW double helixes are formed.
The Enzymes will remain attached until a stop signal is reached.

30. DNA polymerase also acts as a proof reader
DNA polymerase also acts as a proof reader. This enzyme can backtrack and remove an incorrect nucleotide and replace it with the correct one.

31. ✔ Check for Understanding
What happens when the DNA polymerase doesn’t catch a mistake?

32. A mutation occurs!

33. ✔ Check for Understanding What are some mutations we have already discussed?

34. Inversion
Deletion
Duplication Translocation

35. DNA is a Prisoner:
If DNA can’t get out of the nucleus some other molecule has to be able to do so.
What do you think it is?

36. What do you notice about RNA?

37. What is RNA? RNA DNA Ribose sugar Deoxyribose sugar A,U,C,G A,T,C,G
RNA is chemically similar to DNA but there are a few differences.
RNA
DNA
Ribose sugar
(extra oxygen atom)
Deoxyribose sugar
A,U,C,G
A,T,C,G
Single strand
Double strand
✔ Check for Understanding
Why do you think that RNA being only a single strand is so important?

38. Fantastic! So it can carry the genetic information out of the nucleus.
RNA takes copies of genes out of the nucleus, into the cytoplasm so proteins can be produced.

39. RNA is present in the cell in 3 different forms:
1. Messenger RNA (mRNA) comes from the nucleus
2. ribosomal RNA (rRNA) makes up ribosomes
3. transfer RNA (tRNA) is found in the cytoplasm of the cell

40. mRNA
Transcription is the process in which the hereditary information of the DNA is copied into the mRNA (enzymes are needed)
messenger RNA passes from the nucleus, where it is synthesized, into the cytoplasm and becomes attached to the ribosomes
messenger RNA is a short-lived intermediate in protein production
messenger mRNA can be produced very quickly and broken down just as quickly

41. ✔ Check for Understanding
Practice Problem:
DNA: A A C C G C G A T T T A G G C C A T A T
DNA
Comp:
mRNA:

42. Ribosomal and transfer RNA
Ribosomes are made up of rRNA and some proteins
Transfer RNA is found in the cell’s cytoplasm, tRNA picks up amino acids and carries them to the ribosomes

43. ✔ Check for Understanding
DNA: A A T G C G T C A T
DNA:
Comp
mRNA:

44. Gene Expression works through two stages:
The Genetic Code and Protein Synthesis DNA (gene) transcription RNA (mRNA) translation Protein (peptide sequence)
Gene Expression: Is the use of genetic information to produce proteins.
Gene Expression works through two stages:
Transcription
Translation

45. How RNA Makes Proteins Translation Takes genetic information and
gets it deciphered, changes the language
During translation
mRNA + rRNA + tRNA work together to build proteins by joining amino acids

46. Translation occurs at the ribosomes (rRNA)

47. Translation occurs at the ribosomes
Ribosomes are made up of 2 sections or subunits
mRNA attaches to the smaller subunit and then the larger section or subunit connects forming a complete ribosome
2 pieces + mRNA= ribosome

48. Every 3 nucleotides in mRNA codes for an amino acid.
The Genetic Code
Every 3 nucleotides in mRNA codes for an amino acid.
Each triplet of mRNA is called a codon.

49. The Genetic Code Continued
There are 64 possible codon combinations but only 20 amino acids. MORE than ONE codon can specify a specific amino acids.
Stop codon specify the end of the amino acid sequence.

50. TRANSFER RNA (tRNA) attaches to the ribosome at a special location
attaches to the ribosome at a special location
tRNA carries the amino acid to the ribosome
one end of the tRNA has the amino acid and the other end is a 3 nucleotide sequence called an anticodon.

51. Anticodon is complementary to the mRNA
As the mRNA passes through the ribosomes one tRNA after another is matched to the mRNA codons
Amino acids are added to the protein chain until a stop message is reached

53. Using the amino acid chart:
1. A U G Amino Acid=
2. C A A Amino Acid=
3. G U U Amino Acid=

54. ✔ Check for Understanding
mRNA CCG
Amino Acid
mRNA TAT
mRNA UGC