1. Structure of DNA and Replication
Discovery of DNA

2. I. Scientist Frederick Griffith Oswald Avery Alfred Hershey
Martha Chase
Erwin Chargaff
Rosalind Franklin
Maurice Wilkins
James Watson
Francis Crick

3. I. Frederick Griffith’s Experiment 1928
S (smooth bacteria-pneumonia) bacteria
Kills Mouse

4. b. R (rough bacteria-pneumonia) bacteria DOES NOT KILL MOUSE

5. c. Heat-Killed S bacteria DOES NOT KILL MOUSE

6. d. Heat-Killed S bacteria and Live R bacteria KILL MOUSE

7. e. Results Live, disease causing bacteria (S strain) from dead mouse
WHY?
Transformation – one type of bacteria (the harmless form) has been changed permanently into another (the disease-causing form)
Thus Griffith concluded that the transforming factor had to be a gene

8. f. Vaccine
A substance that is prepared from a KILLED or WEAKENED disease-causing agents, including certain bacteria

9. II. Oswald Avery Experiment 1944
Repeated Griffith’s Experiment
i. Observing bacterial transformation, Avery and his team discovered that the nucleic acid DNA stores and transmits genetic information from one generation of bacteria to the next.

10. III. Alfred Hershey and Martha Chase’s Experiment 1952
Hershey and Chase’s experiment convinced many scientist that DNA was the genetic material found in genes
Not just in viruses and bacteria, in all living cells
Bacteriophage – is a virus that infects bacteria
i. Remember this is NOT a bacteria it is a VIRUS

11. IV. Erwin Chargaff Observations 1949
Showed that the amount of Adenine ALWAYS equals the amount of Thymine i.
b. Showed that the amount of Guanine ALWAYS equals the amount of Cytosine
A = T OR
T = A
C = G OR
G = C

12. 1. So if there is 50 micrograms of adenine in a strand of DNA, there would also be 50 micrograms of ___________________.
Thymine

13. V. Rosalind Franklin’s & Maurice Wilkins Photographs, Early 1950’s
Wilkins and Franklin developed high-quality x-ray diffraction of strands of DNA
The photograph suggested that the DNA molecule resembled a TIGHTLY COILED HELIX

14. VI. James Watson and Francis Crick Early 1950’s – published 1953
Discovered the structure of DNA
Watson and Crick built a model of DNA with the configuration of a Double Helix (Spiral Staircase)
of two strands of nucleotides twisting around a central axis

15. II. Structure of DNA DNA – Deoxyribonucleic Acid
i. Five-carbon sugar found on DNA is called deoxyribose

16. b. Nucleotides
Watson and Crick determined that a DNA molecule is made up of a chain of nitrogen bases called Nucleotides
3 parts of DNA nucleotide
1 - phosphate group
1 - 5-carbon sugar – deoxyribose
1 – nitrogen base (A, T, C, G)

17. iii. Label the illustration
Phosphate
Nitrogen base
Five-Carbon Sugar

18. The phosphate and five-carbon sugar group are always the same for each nucleotide
The nitrogenous base may be any one of the 4 bases
Adenine
Guanine
Cytosine
Thymine

19. c. Purines Adenine and Guanine are purines
Nitrogen bases are made of two rings of carbon and nitrogen

20. 2. How do we remember that?
a. Aggies - AGs are pure = purines

21. d. Pyrimidine Thymine and Cytosine are pyrimidines
Nitrogen bases are made up of a single ring of carbon and nitrogen

22. 2. How do we remember that?
a. pYrimidine = cYtosine
and thYmine
i. Hint – for the Y’s

23. e. Pairing and Structure
Watson and Crick, with the help of Chargaff’s work, realized that purines are always paired with pyrimidines
Adenine ALWAYS pairs with Thymine and Guanine ALWAYS pairs with Cytosine this is called Complementary Base Pairs
Complementary base pair – the sequence of bases on one strand determines the sequence of bases on the other strand

24. a. Adenine (purine) = Thymine (pyrimidine)
AT or TA
b. Guanine (purine) = Cytosine (pyrimidine)
GC or CG
c. Example:
T C G T A C
___ ___ ___ ___ ___ ___ A G C A T G

25. A G C A T G C A A C A T G T C G
A G T C A G T A A C C G G T A A C
G G T C

26. ii. Sides of DNA (backbone)
1. Phosphate and five-carbon sugar (deoxyribose)
iii. Middle
1.Nitrogen Bases (A, T, C, G)
2. Hydrogen bond holding the helix together

27. iv. Bonds Adenine forms two hydrogen bonds with Thymine
Guanine forms three hydrogen bonds with Cytosine
The bonds help keep the DNA strands (2) together

28. v. Label the DNA illustration
Deoxyribose
Adenine
Guanine
Hydrogen Bond
Cytosine
Thymine
Phosphate
Adenine – Blue
Phosphate – purple
Thymine – Yellow
5-Carbon Sugar – Orange
Guanine – Pink
2 Hydrogen Bond – red
Cytosine – Green
3 Hydrogen Bond - brown

29. EOC TEST PREP QUESTION:
# 1. Which pattern show how bases pair in complementary strands of DNA? a. A-C and T-G c. A-G and T-C d. A-A and C-C
b. A–T and C-G

30. # 2. If the sequence of nucleotides on one strand of DNA molecule is GCCATTG, the sequence of the complementary strand is
a. GGGTAAG
b. CCCTAAC
c. CGGTAAC
d. GCCATTC

31. III.The Replication of DNA
a. Roles of enzymes in DNA Replication
i. The complementary strands of the double helix
serve as a TEMPLATE for building new DNA.
ii. The process of making a copy of DNA
is called REPLICATION.
1. Remember that DNA replication
occurs in the SYNTHESIS (S)
phase of the cell cycle, before a
cell DIVIDES.

32. i. Step 1: b. Steps of Replication a. ASE = enzyme
1. The double helix UNWINDS because of enzymes called DNA HELICASES.
a. ASE = enzyme
2. DNA helicase breaks open the DOUBLE HELIX that link the complementary bases between 2 strands.
3. PROTEINS keep the strands separated.
4. The areas where the double helix separates are called REPLICATION FORKS because of the Y-SHAPE.

33. ii. Step 2:
At the replication fork, ENZYMES known as DNA POLYMERASES move along each of the DNA strands and add NUCLEOTIDES to the exposed nitrogen bases, according to the BASE-PAIRING rules.

34. iii. Step 3 :
1. Once all of the DNA has been copied, the POLYMERASES are signaled to DETACH.
2. This process produces TWO DNA molecules (each strand is made up of one OLD and one NEW) that are IDENTICAL to each other.

38. c. Checking for ERRORS
iv. Sometimes ERRORS occur and the wrong NUCLEOTIDE is added to the new strand.
v. An important feature of DNA replication is that DNA polymerases have a “PROOFREADING ROLE”

39. 1. DNA Polymerase can only add nucleotides to a growing
strand only if the PREVIOUS NUCLEOTIDE is
CORRECTLY PAIRED to its complementary base.
2. In the event of a mismatched
NUCLEOTIDE, the DNA polymerase can
BACKTRACK and remove the incorrect
nucleotide and replaces it with the
CORRECT one.
a. This proofreading reduces error in
DNA replication to about ONE error per ONE BILLION nucleotides.

40. d. The Rate of Replication
i. Replication DOES NOT begin at one end and end at the other.
Prokaryotic DNA (ex: Bacteria)
a. Their circular DNA has TWO replication
forks that begins at a single point.

41. 2. Eukaryotic DNA (Ex: animals and plants)
a. Their linear DNA has MANY
replication forks
b. If replication in eukaryotic DNA
started at a single point it would take 33 days for a single strand to replicate.

42. c. Each human chromosome is
replicated in about 100 SECTIONS that are 100,000 NUCLEOTIDES long, each section with its own STARTING POINT.
d. An entire human chromosome can be replicated in about 8 hours.

44. EOC PREP QUESTIONS
# 3. How many DNA molecules exist after one molecule of DNA has been replicated?
A. 1
B. 2
C. 4
D. 8

45. EOC PREP QUESTIONS # 4. Multiple replication forks along the DNA
a. correct replication errors
b. reduce DNA replication time
c. ensure that the new and old DNA strands are
complementary
d. signal DNA polymerase to stop