1. DNA
Chapter 16

2. Griffith
Streptococcus pneumoniae - bacteria that causes pneumonia in mammals
R strain – harmless
S strain – pathogenic
mixed heat-killed S strain with live R strain bacteria and injected this into a mouse
the mice died

3. Transformation
change in genotype and phenotype due to the assimilation of a foreign substance (now known to be DNA) by a cell.

4. Avery and other scientists discovered that DNA is the nucleic acid that stores and transmits the genetic information from one generation of an organism to the next.
Oswald Avery 1944
World knows a molecule carries the genetic information.
Doesn’t know if the molecule is a: protein, lipid, carbohydrate, RNA, or DNA
Avery performs Griffith’s experiment again with a twist.

5. What is the transforming substance?
Track the infection of bacteria by viruses
Viruses consist of a DNA enclosed by a protective coat of protein
To replicate, a virus infects a host cell and takes over the cell’s metabolic machinery

6. Bacteriophages (phage)
Virus that specifically attacks bacteria

7. Hershey and Chase
T2 phage, consisting almost entirely of DNA and protein, attacks Escherichia coli
label protein and DNA and then track which entered the E. coli cell during infection

8. How? Grew one batch of T2 phage in the presence of radioactive sulfur
marking the proteins but not DNA
Grew another batch in the presence of radioactive phosphorus
marking the DNA but not proteins
Allowed each batch to infect separate E. coli cultures

10. Chargaff
Developed a series of rules based on a survey of DNA composition in organisms
DNA is a polymer of nucleotides
consists of a nitrogenous base, deoxyribose, and a phosphate group
The bases = adenine (A), thymine (T), guanine (G), or cytosine (C).
The four bases are found in ratios

11. Wilkins and Franklin
X-rays are diffracted as they passed through aligned fibers of purified DNA
Used to deduce the three-dimensional shape of molecules

12. Watson and Crick Double Helix

13. Purine and Pyrimidine
nitrogenous bases are paired in specific combinations: adenine with thymine and guanine with cytosine

14. DNA Replication
Because each strand is complementary to each other, each can form a template when separated

15. DNA Replication

16. semiconservative replication
each of the daughter molecules will have one old strand and one newly made strand

17. DNA replication

18. Human Cells
Copy its billion base pairs and divide into daughter cells in a few hours
One error per billion nucleotides

19. Origin of Replication
Single specific sequence of nucleotides that is recognized by replication enzymes
enzymes separate the strands, forming a replication “bubble”
Replication proceeds in both directions until the entire molecule is copied

20. Anti-Parallel Each DNA strand has a
3’ end with a free hydroxyl group attached to deoxyribose and a
5’ end with a free phosphate group attached to deoxyribose.

21. Bubbles and Forks

22. DNA Polymerase

23. DNA polymerases can only add nucleotides to the free 3’ end of a growing DNA strand
leading strand - used by polymerases as a template for a continuous complimentary strand
lagging strand - copied away from the fork in short segments (Okazaki fragments)

24. DNA polymerase and Primers
Polymerase cannot initiate synthesis of a polynucleotide
can only add nucleotides to the end of an existing chain
To start a new chain requires a primer
a short segment of RNA

25. Animation: Lagging Strand Animation: Leading Strand
Replication Fork
Topoisomerases
Helicases
Single-strand binding proteins
Primases (RNA primers)
DNA Polymerases
Ligases

26. Animation: DNA Replication Review
DNA polymerase later replaces the primer with deoxyribonucleotides complimentary to the template

28. Mismatched nucleotides
Reactive chemicals, radioactive emissions, X-rays, and ultraviolet light can change nucleotides in ways that can affect encoded genetic information
Each cell continually monitors and repairs its genetic material
over 130 repair enzymes identified in humans

29. In mismatch repair, special enzymes fix incorrectly paired nucleotides
In nucleotide excision repair, a nuclease cuts out a segment of a damaged strand

30. telomeres = The ends of eukaryotic chromosomal DNA molecules – long repetitive sequences (no genes)

31. Telomerase
uses a short molecule of RNA as a template to extend the 3’ end of the telomere

32. Difference between bacterial chromosomes and eukaryotic chromososes

33. Animation: DNA Packing
Chromatin is a complex of DNA and protein, and is found in the nucleus of eukaryotic cells
Histones are proteins that are responsible for the first level of DNA packing in chromatin
For the Cell Biology Video Cartoon and Stick Model of a Nucleosomal Particle, go to Animation and Video Files.
Animation: DNA Packing
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

34. Nucleosomes, or “beads on a string” (10-nm fiber)
Fig a
Nucleosome
(10 nm in diameter)
DNA double helix (2 nm in diameter) H1
Histone tail
Histones
Figure 16.21a Chromatin packing in a eukaryotic chromosome
DNA, the double helix
Histones
Nucleosomes, or “beads on a string” (10-nm fiber)

35. Looped domains (300-nm fiber) Metaphase chromosome
Fig b
Chromatid
(700 nm)
30-nm fiber
Loops
Scaffold
300-nm fiber
Figure 16.21b Chromatin packing in a eukaryotic chromosome
Replicated chromosome (1,400 nm)
30-nm fiber
Looped domains (300-nm fiber)
Metaphase chromosome

36. Chromatin is organized into fibers 10-nm fiber
DNA winds around histones to form nucleosome “beads”
Nucleosomes are strung together like beads on a string by linker DNA
30-nm fiber
Interactions between nucleosomes cause the thin fiber to coil or fold into this thicker fiber
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

37. 300-nm fiber Metaphase chromosome
The 30-nm fiber forms looped domains that attach to proteins
Metaphase chromosome
The looped domains coil further
The width of a chromatid is 700 nm
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

38. Loosely packed chromatin is called euchromatin
Most chromatin is loosely packed in the nucleus during interphase and condenses prior to mitosis
Loosely packed chromatin is called euchromatin
During interphase a few regions of chromatin (centromeres and telomeres) are highly condensed into heterochromatin
Dense packing of the heterochromatin makes it difficult for the cell to express genetic information coded in these regions
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

39. Histones can undergo chemical modifications that result in changes in chromatin organization
For example, phosphorylation of a specific amino acid on a histone tail affects chromosomal behavior during meiosis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings