Chapter 2: How We Learned the Function of DNA

1. Chapter 2:How We Learned the Function of DNA

2. How does the structure of DNA describe replication? • Watson and Crick’s discovery of the structure of DNA made it apparent how DNA could hold information. • The model also explained replication through complementary DNA pairs. • DNA is made of 2 separate strands. • The strands could separate to create two complementary halves. • Each half was the template for the other half. • In 1958, Matthew Meselson and Frank Stahl tested this hypothesis. • They tried to prove that DNA replication was semiconservative.

3. Sadava, David. (2008). The Mehelson Stahl Experiment. [Diagram]. Retrieved from http://www.nature.com/scitable/content/18551/sadava_11_11_large_2.gif .

4. How is DNA copied over and over? • The study of the mechanism of DNA started in vitroin 1960, with Alex Kornberg. • He isolated DNA polymerase I (pol I) from E. coli. • He also identified the 4 things required for DNA synthesis: • DNA Polymerase • The four deoxyribonucleic triphosphates or dNTPs, structures similar to nucleotides. • A template strand of DNA. • Mg++ • He later found out that the primary DNA polymerase was DNA polymerase III (pol III).

5. DNA replication always occurs in the 5’ to 3’ direction. • 3’ to 5’ is theoretically possible, but not found in nature. • Not enough energy to continue synthesis if 3’ to 5’. • No phosphate groups to break off. • DNA replication involves two strands: the leading strand and the lagging strand. • Reiji Okazaki found that the lagging strand had many fragments of DNA. • He coined these small fragments “Okazaki fragments“. • These parts are later joined together by DNA ligase.

6. Upper Left: The basic structure of a dNTP. Bottom: How dNTPs are added during DNA synthesis. Two phosphates are removed from the DNA, and the splitting of the phosphate provides the energy to continue synthesis. This is why DNA doesn’t replicate in nature from 3’ to 5’. (2006). dNTP molecule.[Diagram]. Retrieved from http://www.ocf.berkeley.edu/~edy/genome/deoxy.jpg Toxchick. DNA Synthesis. [Diagram]. Retrieved from http://www.angelfire.com/sc3/toxchick/images/R/replicationreaction01.gif

7. DNA needs a piece of something to attach to when replicating. • A small RNA primer is created and attached to DNA. • Later removed by RNase H and replaced with DNA. • DNA helicase unwinds the DNA strand initially. • dNTPs are added by DNA polymerase. • On the lagging strand, Okazaki fragments are formed. • These fragments are later joined by DNA ligase. • Origins of replication are places where DNA starts to replicate. • In eukaryotes, a similar process occurs, except at the ends of chromosomes. • Telomerase forms DNA from an RNA template. • This is why telomeres become shorter with each replication.

8. How does information move from DNA to Proteins? • In the 1950’s, Paul Zamecnik established that protein synthesis occurs at structures made of RNA and protein, called microsomes (now known as ribosomes). • These studies would eventually lead to the one-gene one-ribosome one-protein theory, which was disproved. • Later, Benjamin Hall and Sol Spiegelman experimented and discovered that DNA and RNA were complementary. • They experimented with the weights of DNA and RNA after injecting bacteria with T2 bacteriophages.

9. Later, other scientists disproved Zamecnik’s theory. • Rather than each gene creating one ribosome, they discovered that each gene codes for a strand of RNA, which transfers information to the ribosomes. • They did this by labeling ribosomes with heavy isotopes after infecting bacteria with T4 phages, which causes the cells to produce T4 specific RNA and proteins. • They submerged the bacteria in “heavy isotopes” and let them divide. • Later, they switched to “light isotopes” for a short time. • It was found that none of the ribosomes were marked with the “light isotopes”. • However, the same experiment showed that RNA was formed with the “light isotopes”.