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?
***************************************************************
[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 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!

3. DNA – deoxyribonucleic acid is
the nucleic acid that stores and transmits genetic info. from one generation to the next.
present in all organisms, but different (unique) in each individual, except for
identical twins.

5. 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?

6. History of DNA

7. History of DNA
Early scientists thought protein was the cell’s hereditary material because it was more complex than DNA
Proteins were composed of 20 different amino acids in long polypeptide chains

8. Transformation
Griffith 1928-worked with virulent S and nonvirulent R strain Pneumoccocus bacteria
found that R strain could become virulent (transform) when it took in DNA from heat-killed S strain
Avery suggested that DNA was probably the genetic material that was “transformed”

9. Griffith Experiment

10. History of DNA Chromosomes are made of both DNA and protein
Hershey & Chase experiments on bacteriophage viruses proved that DNA was the cell’s genetic material
Radioactive 32P was injected into bacteria!

11. Chargaff’s Rule Adenine pairs with Thymine Guanine pairs with Cytosine
The bases form weak hydrogen bonds T A G C

12. Discovery of DNA Structure
Erwin Chargaff 1950 showed the amounts of the four bases on DNA
(A,T,C,G)
In a body or somatic cell: If…
A = 15% then…
T = __% so…
G = 35%
C = __%

13. DNA Structure
Rosalind Franklin & Maurice Wilkins took diffraction x-ray photographs of DNA crystals (2 sides, twisted)
Watson & Crick built the first model of DNA using Franklin’s x-rays
(double helix)

14. Rosalind Franklin

15. Watson and Crick

16. Helix
Most DNA has a right-hand twist with 10 base pairs in a complete turn
Left twisted DNA is called Z-DNA or southpaw DNA
Hot spots occur where right and left twisted DNA meet producing mutations

17. Antiparallel Strands One strand of DNA goes from 5’ to 3’ (sugars)
The other strand is opposite in direction going 3’ to 5’ (sugars)

18. DNA Structure

19. The Shape of the Molecule
DNA is a very long
double stranded
polymer of
nucleotides.
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.}

20. One Strand of DNA Each nucleotide monomer contains 1 phosphate group
1 sugar (deoxyribose)
*1 nitrogenous base
One strand of DNA has many millions of nucleotides.
nucleotide S B P S B P S B
{Point to the 3-D mode, if you have one, to show the parts as you discuss them.} P S B P S B

21. DNA Nucleotide O=P-O O N CH2 O C1 C4 C3 C2 Phosphate Group O
Nitrogenous base
(A, G, C, or T) O
CH2 O C1 C4 C3 C2 5
Sugar
(deoxyribose)

22. DNA P O 1 2 3 4 5 P O 1 2 3 4 5 G C T A

23. One Strand of DNA The backbone of the molecule is alternating
phosphates &
deoxyribose sugar
(phosphodiester bond)
The “rungs/teeth” are nitrogenous
bases (4 possible)
C, T, A, or G
phosphate
deoxyribose
{Point to the 3-D mode, if you have one, to show the parts as you discuss them.}
bases

24. 4 possible Nitrogenous Bases
Double ring PURINES
Adenine (A)
Guanine (G)
Single ring PYRIMIDINES
Thymine (T)
Cytosine (C)
A or G
T or C

25. Base-Pairings Purines only pair with Pyrimidines
Hydrogen bonds required to bond
Guanine & Cytosine C G
3 H-bonds

26. Hydrogen bonds are required to bond Adenine & Thymine
2 H - bonds

27. DNA Double Helix “Rungs of ladder” Nitrogenous Base (A,T,G or C)
“Legs of ladder”
Phosphate &
Sugar Backbone

28. Two Stranded DNA
DNA has two strands that fit together something like a ladder or zipper.
The rungs or 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.}

29. DNA Two strands coiled called a double helix
Sides made of a pentose sugar Deoxyribose bonded to phosphate (PO4) groups
Middle made of nitrogen bases bonded together by weak hydrogen bonds

30. Hydrogen Bonds The bases attract each other because of hydrogen bonds.
Hydrogen bonds are
weak but there are
millions of them in a
single molecule of
DNA. Cytosine always
pairs with Guanine C N H G

31. Hydrogen Bonds, cont. When making hydrogen bonds, Cytosine
always pairs
with Guanine
Adenine
with Thymine A C N T H O

32. Remember the Strands are AntiparallelO 1 2 3 4 5 P O 1 2 3 4 5 G C T A

33. The nitrogen bases of each strand pair with the bases on the
complementary
strand
The order of the bases
makes up the genetic
code. A T C G

34. Question: DNA –C G T A T G-
What would be the complementary DNA strand for the following DNA sequence?
DNA –C G T A T G-

35. Answer:
DNA –C G T A T G-
comp DNA –G C A T A C-

36. Question:
If there is 30% Adenine, how much Cytosine is present?

37. Answer: If 30% Adenine then 30% Thymine If 60% A-T; then 40% C-G
Therefore,40% C-G would be
20% Guanine = __% Cytosine

38. DNA Replication

39. DNA REPLICATION

40. DNA Replication When a cell divides, DNA preserves individuality
by passing exact copies of
itself to the new cell

41. 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

42. Synthesis Phase (S phase)
S phase during interphase of the cell cycle
Nucleus of eukaryotes
Mitosis
-prophase
-metaphase
-anaphase
-telophase G1 G2 S
phase
interphase
DNA replication takes
place in the S phase.

43. 5’ to 3’ Sugars .
When the DNA double helix unwinds, it resembles a ladder
The sides of the ladder are the sugar-phosphate backbones
The rungs of the ladder are the complementary paired bases
The two DNA strands are anti-parallel (they run in opposite directions)

44. Steps in DNA Replication
Occurs when chromosomes duplicate (make copies)
Enzyme DNA Helicase unwinds & separates the 2 DNA strands by breaking the weak
hydrogen bonds as enzymes “unzip” the molecule
Each old strand of nucleotides serves as a template for each new strand
New nucleotides move into complementary positions are joined by DNA polymerase
DNA polymerase is an enzyme.

45. Anti-Parallel Strands of DNA
On the left is the DNA double helix. When the helix is unwound, a ladder configuration shows that the uprights are composed of sugar and phosphate molecules and the rungs are complementary bases. Notice that the bases in DNA pair in such a way that the phosphate-sugar groups are oriented in different directions. This means that the strands of DNA end up running antiparallel to one another, with the 3’ end of one strand opposite the 5’ end of the other strand.

46. DNA Replication Begins at Origins of Replication
One strand serves as a mold for another strand to be copied
Two strands open forming Replication Forks (Y-shaped region)
New strands grow at the forks
Replication
Fork
Parental DNA Molecule 3’ 5’

47. Two New, Identical DNA Strands Result from Replication
Replication is called semiconservative because each new double helix is composed of an old (parental) strand and a new (daughter) strand.

48. Another View of Replication
Use of the ladder configuration better illustrates how complementary nucleotides available in the cell pair with those of each old strand before they are joined together to form a daughter strand.

49. DNA Replication Begins at Origins of Replication
One strand serves as a mold for another strand to be copied
Two strands open forming Replication Forks (Y-shaped region)
New strands grow at the forks
Replication
Fork
Parental DNA Molecule 3’ 5’

50. DNA Replication 1.Enzyme DNA Helicase unwinds & separates the 2 DNA
strands by breaking the weak
hydrogen bonds to unzip the chain
Single-Strand Binding Proteins attach and keep the 2 DNA strands separated and untwisted
2. Free nucleotides match up &
form H bonds to complete
complementary base strand

51. 3. Base pairs bond, DNA polymerase
links phosphate of one nucleotide to
the sugar of another.
4. Pairing continues until each
“original” DNA strand has a
complete matching strand
(result 2 identical DNA strands)
T A G C A T
A T C G T A
IF…
T A G C A T
and
A T C G T A
T A G C A T
A T C G T A
THEN…

53. Replication of Strands
Replication Fork
Point of Origin

54. Semiconservative Model of Replication
Idea presented by Watson & Crick
The two strands of the parental molecule separate, and each acts as a template for a new complementary strand
New DNA consists of 1 PARENTAL (original) and 1 NEW strand of DNA
DNA Template
New DNA
Parental DNA

55. DNA Replication DNA polymerase can then add the new nucleotides
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

56. Direction of Replication
DNA Replication
DNA polymerase can only add nucleotides to the 3’ end of the DNA
This causes the NEW strand to be built in a 5’ to 3’ direction
RNA
Primer
DNA Polymerase
Nucleotide 5’ 3’
Direction of Replication

57. Synthesis of the New DNA Strands
The Leading Strand is synthesized as a single strand from the point of origin toward the opening replication fork
RNA
Primer
DNA Polymerase
Nucleotides 3’ 5’

58. Synthesis of the New DNA Strands
The Lagging Strand is synthesized discontinuously against overall direction of replication
This strand is made in MANY short segments It is replicated from the replication fork toward the origin
RNA Primer
Leading Strand
DNA Polymerase 5’ 3’
Lagging Strand 5’ 3’

59. Remember HOW the Carbons Are Numbered!
O=P-O
Phosphate
Group N
Nitrogenous base
(A, G, C, or T)
CH2 O C1 C4 C3 C2 5
Sugar
(deoxyribose)

60. Lagging Strand Segments
Okazaki Fragments - series of short segments on the lagging strand
Must be joined together by an enzyme
Lagging Strand
RNA
Primer
DNA
Polymerase 3’ 5’
Okazaki Fragment

61. Joining of Okazaki Fragments
The enzyme Ligase joins the Okazaki fragments together to make one strand
Lagging Strand
Okazaki Fragment 2
DNA ligase
Okazaki Fragment 1 5’ 3’

62. Proofreading New DNA
DNA polymerase initially makes about 1 in 10,000 base pairing errors
Enzymes (helicase) proofread and correct these mistakes
The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors

63. DNA Damage & Repair
Chemicals & ultraviolet radiation damage the DNA in our body cells
Cells must continuously repair DAMAGED DNA
Excision repair occurs when any of over 50 repair enzymes remove damaged parts of DNA
DNA polymerase and DNA ligase replace and bond the new nucleotides together

64. DNA makes proteins that are needed
for growth, repair and all life functions
Ex:
collagen - cartilage and tendons
hemoglobin – blood carries oxygen
through the body
keratin - hair and fingernails
insulin – metabolizes blood sugars
…muscles, skin, etc…

65. DNA  RNA  Protein Prokaryotic Cell DNA mRNA Ribosome Protein
Transcription
Translation
DNA
mRNA
Ribosome
Protein
Prokaryotic Cell

67. Remember the Strands are AntiparallelO 1 2 3 4 5 P O 1 2 3 4 5 G C T A

68. DNA Stands for Deoxyribonucleic acid
Made up of subunits called nucleotides
Nucleotide made of:
1. Phosphate group
2. 5-carbon sugar
3. Nitrogenous base

69. Question: DNA 5’-CGTATG-3’
What would be the complementary DNA strand for the following DNA sequence?
DNA 5’-CGTATG-3’

70. Answer:
DNA 5’-CGTATG-3’
DNA 3’-GCATAC-5’