1. Biology, 9th ed,Sylvia Mader
Ch. 12 -
DNA - Structure
& Function
Biology, 9th ed,Sylvia Mader
Chapter 13
Chapter 13
DNA Structure & Function
DNA Structure & Function

2. DNA as Genetic Material
Biology, 9th ed,Sylvia Mader
DNA as Genetic Material
Chapter 13
DNA Structure & Function
Johann Miescher (1869)
Removed nuclei from pus cells
Found they contained a chemical he called nuclein
This was rich in phosphorus and had no sulfur; thus it could not be a protein
Later scientists realized there were two types of nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid)

3. Biology, 9th ed,Sylvia Mader
Frederick Griffith (1931)
Chapter 13
DNA Structure & Function
Investigated virulence of Streptococcus pneumoniae in mice in following manner:
1. S strain bacteria have a smooth capsule & are capable of killing mice
2. R strain have no capsule & don’t kill mice
3. Injected heat-killed S strain bacteria into mice; they did not die
4. Injected mice with mixture of heat-killed S strain & live R strain. These mice had living S strain bacteria & died
Concluded that virulence passed from the dead strain to the living strain; transformation had occurred

4. Griffith’s Transformation Experiment
Biology, 9th ed,Sylvia Mader
Chapter 13
DNA Structure & Function
Griffith’s Transformation Experiment

5. Avery, MacLeod & McCarty (1944)
Biology, 9th ed,Sylvia Mader
Avery, MacLeod & McCarty (1944)
Chapter 13
DNA Structure & Function
(Refer to transparency here first)
Discovered that DNA is the transforming substance.
1. Took DNA only from the S bacteria and mixed it with R bacteria.
2. S strain DNA was then incorporated into genome of living R strain bacteria and they were then transformed into S strain bacteria.
3. Enzymes that degrade proteins or RNA did not prevent transformation while those that digest DNA did.

6. Reproduction of Viruses
Biology, 9th ed,Sylvia Mader
Reproduction of Viruses
Chapter 13
DNA Structure & Function
Viruses consist of a protein coat (capsid) surrounding a nucleic acid core
Bacteriophages are viruses that infect bacteria

7. Biology, 9th ed,Sylvia Mader
Hershey and Chase (1952)
Chapter 13
DNA Structure & Function
Did an experiment to determine whether the bacteriophages inject the protein or DNA into the bacteria.
Radioactively labeled the DNA core and protein capsid of a bacteriophage
1. Radioactive P (found in DNA & not in protein) was found inside cells
2. Radioactive S (found in protein & not in DNA) was found mainly outside of cells
Results indicated that DNA, not the protein, enters the host
The DNA of the phage contains genetic information for producing new phages

8. Hershey and Chase Experiments
Biology, 9th ed,Sylvia Mader
Hershey and Chase Experiments
Chapter 13
DNA Structure & Function

9. Biology, 9th ed,Sylvia Mader
Structure of DNA
Chapter 13
DNA Structure & Function
DNA contains:
Two nucleotides with purine bases. These are double ring nitrogenous bases.
Adenine (A)
Guanine (G)
Two nucleotides with pyrimidine bases. These are single ring nitrogenous bases.
Thymine (T)
Cytosine (C)

10. Nucleotide Composition of DNA
Biology, 9th ed,Sylvia Mader
Nucleotide Composition of DNA
Chapter 13
DNA Structure & Function

11. Biology, 9th ed,Sylvia Mader
Chargaff’s Rules
Chapter 13
DNA Structure & Function
The amounts of A, T, G, and C in DNA:
Identical in identical twins
Varies between individuals of a species
Varies more from species to species
In each species, there are equal amounts of:
A & T
G & C
All this suggests DNA uses complementary base pairing to store genetic information.
Human chromosome estimated to contain, on average, 140 million base pairs.
Number of possible nucleotide sequences 4^140,000,000.

12. Biology, 9th ed,Sylvia Mader
Diffraction Data
Chapter 13
DNA Structure & Function
Rosalind Franklin:
Studied structure of DNA using X-rays.
Found that if a concentrated solution of DNA is made it forms into a crystal like structure.
When X-rayed, an X-ray diffraction pattern results.
The pattern of DNA shows that it is a helix.

13. X-Ray Diffraction of DNA
Biology, 9th ed,Sylvia Mader
X-Ray Diffraction of DNA
Chapter 13
DNA Structure & Function

14. Watson and Crick Model (1953)
Biology, 9th ed,Sylvia Mader
Watson and Crick Model (1953)
Chapter 13
DNA Structure & Function
Using data provided by Franklin’s X-ray diffraction and other knowledge about DNA, they eventually determined that DNA is a double-helix
Sugar-phosphate backbones make up the sides
Hydrogen-bonded bases make up the rungs. Complementary bases (A-T; C-G) pair up.
Model matched data of both Franklin & Chargaff
Received a Nobel Prize in 1962

15. Watson/Crick Model of DNA
Biology, 9th ed,Sylvia Mader
Watson/Crick Model of DNA
Chapter 13
DNA Structure & Function

16. Biology, 9th ed,Sylvia Mader
DNA Replication:
Chapter 13
DNA Structure & Function
Replication = process of copying a DNA molecule
1. During DNA replication, each old DNA strand of the parental molecule (original double helix) serves as a template for a new strand in a daughter molecule.
2. DNA replication is termed semiconservative replication because one of the old strands is conserved, or present, in each daughter DNA molecule.

17. Biology, 9th ed,Sylvia Mader
Steps of Replication
Chapter 13
DNA Structure & Function
1. Unwinding
DNA replication begins at numerous points along linear chromosome called replication forks.
DNA unwinds and unzips into two strands. Weak hydrogen bonds between paired bases are broken.
A special enzyme, DNA helicase, unwinds the DNA.

18. Biology, 9th ed,Sylvia Mader
Replication (cont’d)
Chapter 13
DNA Structure & Function
2. Complementary base pairing
Each old strand of DNA serves as a template for a new strand
New complementary nucleotides are positioned by process of complementary base pairing
A special enzyme, called DNA polymerase, helps to position the complementary base pairs

19. Biology, 9th ed,Sylvia Mader
Replication (cont’d)
Chapter 13
DNA Structure & Function
3. Joining
The complementary nucleotides join to form new strands.
This is also helped by DNA polymerase

20. Semiconservative Replication of DNA
Biology, 9th ed,Sylvia Mader
Semiconservative Replication of DNA
Chapter 13
DNA Structure & Function

21. Meselson & Stahl’s experiment (1958)
Biology, 9th ed,Sylvia Mader
Meselson & Stahl’s experiment (1958)
Chapter 13
DNA Structure & Function
Confirmed semiconservative replication theory
They grew bacteria in a medium containing heavy N-15 so only heavy DNAs were found.
Switched bacteria to N-14 medium.
After 1 division, only hybrid DNA was found
After 2 divisions, half the DNA is light & half is hybrid
These are the results expected if DNA replication is semiconservative.

22. Meselson and Stahl’s DNA replication experiment
Biology, 9th ed,Sylvia Mader
Meselson and Stahl’s DNA replication experiment
Chapter 13
DNA Structure & Function

23. DNA Replication Animation
DNA Replication Video
DNA Replication Animation

24. Details of DNA Replication
Biology, 9th ed,Sylvia Mader
Details of DNA Replication
Chapter 13
DNA Structure & Function
Carbon atoms are numbered in the deoxyribose molecule.
DNA strands are antiparallel. One of the strands runs from 3’ to 5’ in one direction, and the other strand runs from 3’ to 5’ in the opposite direction.
During replication, DNA polymerase has to synthesize the daughter strand in the 5’ to 3’ direction.
Why? DNA polymerase can only join a nucleotide to a free 3’ end of a previous nucleotide.

25. Biology, 9th ed,Sylvia Mader
Chapter 13
DNA Structure & Function

26. Details of DNA Replication (cont’d)
Biology, 9th ed,Sylvia Mader
Details of DNA Replication (cont’d)
Chapter 13
DNA Structure & Function
This also means that DNA polymerase cannot start the synthesis of a DNA chain.
An RNA polymerase lays down a short amount of RNA, called an RNA primer, that is complementary to DNA.
Then DNA polymerase can join DNA nucleotides to the 3’ end of the growing daughter strand.

27. Details of DNA Replication (cont’d)
Biology, 9th ed,Sylvia Mader
Details of DNA Replication (cont’d)
Chapter 13
DNA Structure & Function
As helicase unwinds DNA, one parental strand runs in the 3’ to 5’ direction toward the fork. Thus, the new complementary daughter strand will be synthesized from the 5’ to 3’ direction. This strand is called the leading strand.
The other parental strand, however, is running in the opposite direction (3’ to 5’ AWAY from the fork). The daughter strand must begin at the fork and run in the opposite direction to the leading strand. This is called the lagging strand.

28. Antiparallel Replication of DNA
Biology, 9th ed,Sylvia Mader
Chapter 13
Antiparallel Replication of DNA
DNA Structure & Function

29. Details of DNA Replication (cont’d)
Biology, 9th ed,Sylvia Mader
Details of DNA Replication (cont’d)
Chapter 13
DNA Structure & Function
Replication of the lagging strand is discontinuous.
It results in segments called Okazaki fragments.
While proofreading, DNA polymerase will remove the RNA primers and replace them with complementary DNA nucleotides.
DNA ligase will then join the fragments together.

30. Antiparallel Replication of DNA
Biology, 9th ed,Sylvia Mader
Chapter 13
Antiparallel Replication of DNA
DNA Structure & Function

31. DNA Replication: Prokaryotic
Biology, 9th ed,Sylvia Mader
DNA Replication: Prokaryotic
Chapter 13
DNA Structure & Function
Prokaryotic Replication
Bacteria have a single circular loop
Replication moves around the circular DNA molecule in both directions. Takes about 40 minutes.
Produces two identical circles
Cell divides between circles, as fast as every 20 minutes

32. Replication: Prokaryotic vs. Eukaryotic
Biology, 9th ed,Sylvia Mader
Replication: Prokaryotic vs. Eukaryotic
Chapter 13
DNA Structure & Function

33. Biology, 9th ed,Sylvia Mader
Replication Errors
Chapter 13
DNA Structure & Function
Genetic variations are the raw material for evolutionary change
Mutation:
A permanent (but unplanned) change in base-pair sequence
Some due to errors in DNA replication. Proofreading occurs which eliminates most errors. Mistake rate is only 1 per 1 billion base pairs.
Others are due to DNA damage like UV radiation
DNA repair enzymes are usually available to reverse most errors

34. DNA Replication Animation II
Videos for Chapter 13 I
DNA Replication Animation II