1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 16: The Molecular Basis of Inheritance

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.1 Watson and Crick with their DNA model

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.2 Can the genetic trait of pathogenicity be transferred between bacteria? Bacteria of the “S” (smooth) strain of Streptococcus pneumoniae are pathogenic because they have a capsule that protects them from an animal’s defense system. Bacteria of the “R” (rough) strain lack a capsule and are nonpathogenic. Frederick Griffith injected mice with the two strains as shown below: Griffith concluded that the living R bacteria had been transformed into pathogenic S bacteria by an unknown, heritable substance from the dead S cells. EXPERIMENT RESULTS CONCLUSION Living S (control) cells Living R (control) cells Heat-killed (control) S cells Mixture of heat-killed S cells and living R cells Mouse diesMouse healthy Mouse dies Living S cells are found in blood sample.

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.3 Viruses infecting a bacterial cell Phage head Tail Tail fiber DNA Bacterial cell 100 nm

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.4 Is DNA or protein the genetic material of phage T2? In their famous 1952 experiment, Alfred Hershey and Martha Chase used radioactive sulfur and phosphorus to trace the fates of the protein and DNA, respectively, of T2 phages that infected bacterial cells. EXPERIMENT Radioactivity (phage protein) in liquid Phage Bacterial cell Radioactive protein Empty protein shell Phage DNA Centrifuge Pellet (bacterial cells and contents) Radioactive DNA Centrifuge Pellet Radioactivity (phage DNA) in pellet Batch 1: Phages were grown with radioactive sulfur ( 35 S), which was incorporated into phage protein (pink). Batch 2: Phages were grown with radioactive phosphorus ( 32 P), which was incorporated into phage DNA (blue). 1 2 3 4 Agitated in a blender to separate phages outside the bacteria from the bacterial cells. Mixed radioactively labeled phages with bacteria. The phages infected the bacterial cells. Centrifuged the mixture so that bacteria formed a pellet at the bottom of the test tube. Measured the radioactivity in the pellet and the liquid

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phage proteins remained outside the bacterial cells during infection, while phage DNA entered the cells. When cultured, bacterial cells with radioactive phage DNA released new phages with some radioactive phosphorus. Hershey and Chase concluded that DNA, not protein, functions as the T2 phage’s genetic material. RESULTS CONCLUSION

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.5 The structure of a DNA strand

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.6 Rosalind Franklin and her X-ray diffraction photo of DNA (a) Rosalind Franklin Franklin’s X-ray diffraction Photograph of DNA (b)

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.7 The double helix

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Unnumbered Figure p. 298 Purine + Purine: too wide Pyrimidine + pyrimidine: too narrow Purine + pyrimidine: width Consistent with X-ray data

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.8 Base pairing in DNA H N H O CH 3 N N O N N N NH Sugar Adenine (A) Thymine (T) N N N N Sugar O H N H N H N O H H N Guanine (G) Cytosine (C)

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.9 A model for DNA replication: the basic concept (layer 1) (a) The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C. A C T A G T G A T C

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.9 A model for DNA replication: the basic concept (layer 2) (a) The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C. (b) The first step in replication is separation of the two DNA strands. A C T A G A C T A G T G A T C T G A T C

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.9 A model for DNA replication: the basic concept (layer 3) (a) The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C. (b) The first step in replication is separation of the two DNA strands. (c) Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand. A C T A G A C T A G A C T A G T G A T C T G A T C A C T A G T G A T C T G A T C

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.9 A model for DNA replication: the basic concept (layer 4) (a) The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C. (b) The first step in replication is separation of the two DNA strands. (c) Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand. (d) The nucleotides are connected to form the sugar-phosphate backbones of the new strands. Each “daughter” DNA molecule consists of one parental strand and one new strand. A C T A G A C T A G A C T A G A C T A G T G A T C T G A T C A C T A G A C T A G T G A T C T G A T C T G A T C T G A T C

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.10 Three alternative models of DNA replication Conservative model. The two parental strands reassociate after acting as templates for new strands, thus restoring the parental double helix. (a) Semiconserva- tive model. The two strands of the parental molecule separate, and each functions as a template for synthesis of a new, comple- mentary strand. (b) Dispersive model. Each strand of both daughter mol- ecules contains a mixture of old and newly synthesized DNA. (c) Parent cell First replication Second replication

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.11 Does DNA replication follow the conservative, semiconservative, or dispersive model? Matthew Meselson and Franklin Stahl cultured E. coli bacteria for several generations on a medium containing nucleotide precursors labeled with a heavy isotope of nitrogen, 15 N. The bacteria incorporated the heavy nitrogen into their DNA. The scientists then transferred the bacteria to a medium with only 14 N, the lighter, more common isotope of nitrogen. Any new DNA that the bacteria synthesized would be lighter than the parental DNA made in the 15 N medium. Meselson and Stahl could distinguish DNA of different densities by centrifuging DNA extracted from the bacteria. EXPERIMENT The bands in these two centrifuge tubes represent the results of centrifuging two DNA samples from the flask in step 2, one sample taken after 20 minutes and one after 40 minutes. RESULTS Bacteria cultured in medium containing 15 N Bacteria transferred to medium containing 14 N 2 1 DNA sample centrifuged after 20 min (after first replication) 3 DNA sample centrifuged after 40 min (after second replication) 4 Less dense More dense

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CONCLUSION Meselson and Stahl concluded that DNA replication follows the semiconservative model by comparing their result to the results predicted by each of the three models in Figure 16.10. The first replication in the 14 N medium produced a band of hybrid ( 15 N– 14 N) DNA. This result eliminated the conservative model. A second replication produced both light and hybrid DNA, a result that eliminated the dispersive model and supported the semiconservative model. First replicationSecond replication Conservative model Semiconservative model Dispersive model

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.12 Origins of replication in eukaryotes Replication begins at specific sites where the two parental strands separate and form replication bubbles. The bubbles expand laterally, as DNA replication proceeds in both directions. Eventually, the replication bubbles fuse, and synthesis of the daughter strands is complete. 1 2 3 Origin of replication Bubble Parental (template) strand Daughter (new) strand Replication fork Two daughter DNA molecules In eukaryotes, DNA replication begins at many sites along the giant DNA molecule of each chromosome. In this micrograph, three replication bubbles are visible along the DNA of a cultured Chinese hamster cell (TEM). (b) (a) 0.25 µm

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Parental DNA DNA pol Ill elongates DNA strands only in the 5 3 direction. 3 5 5 3 3 5 2 1 Okazaki fragments DNA pol III Template strand Leading strand Lagging strand 3 2 1 Template strand DNA ligase Overall direction of replication One new strand, the leading strand, can elongate continuously 5 3 as the replication fork progresses. The other new strand, the lagging strand must grow in an overall 3 5 direction by addition of short segments, Okazaki fragments, that grow 5 3 (numbered here in the order they were made). DNA ligase joins Okazaki fragments by forming a bond between their free ends. This results in a continuous strand. 2 3 1 4 Figure 16.14 Synthesis of leading and lagging strands during DNA replication

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.15 Synthesis of the lagging strand Overall direction of replication 3 3 3 3 5 3 5 3 5 3 5 3 5 3 5 3 5 3 5 5 1 1 2 1 1 2 5 5 1 2 3 5 DNA ligase forms a bond between the newest DNA and the adjacent DNA of fragment 1. 6 The lagging strand in this region is now complete. 7 DNA pol 1 replaces the RNA with DNA, adding to the 3 end of fragment 2. 5 After the second fragment is primed. DNA pol III adds DNA nucleotides until it reaches the first primer and falls off. 4 After reaching the next RNA primer (not shown), DNA pol III falls off. 3 DNA pol III adds DNA nucleotides to the primer, forming an Okazaki fragment. 2 Primase joins RNA nucleotides into a primer. 1 Template strand RNA primer Okazaki fragment

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 16.17 Nucleotide excision repair of DNA damage Nuclease DNA polymerase DNA ligase A thymine dimer distorts the DNA molecule. 1 Repair synthesis by a DNA polymerase fills in the missing nucleotides. 3 DNA ligase seals the Free end of the new DNA To the old DNA, making the strand complete. 4 A nuclease enzyme cuts the damaged DNA strand at two points and the damaged section is removed. 2