Key Concepts Genes are made of DNA When DNA is copied each strand of a DNA double helix serves as the template for the synthesis of a complementary strand DNA synthesis occurs in the 5 3 direction only and requires a large suite of specialized enzymes The leading strand is synthesized continuously but the lagging strand is synthesized as a series of fragments that are then linked together 2011 Pearson Education Inc Key Concepts Specialized enzymes repair damages to DNA and fix mistakes in DNA synthesis If these repair enzymes are defective the mutation rate increases 2011 Pearson Education Inc What Are Genes Made Of While it was known and understood that chromosomes were comprised of DNA and protein early biologists did not know whether genes were comprised of DNA or protein The general consensus at the time supported the hypothesis that genes were comprised of proteins This was because of the relative complexity and variability of proteins compared to DNA which is comprised of only four different nucleotides 2011 Pearson Education Inc The Hershey Chase Experiment To study whether genes were made of protein or DNA Hershey and Chase studied how a virus called T2 infects the bacterium Escherichia coli T2 infection of E coli begins when the virus attaches to the cell and injects its genes into the cell These genes then direct production of new virus particles During infection the protein coat or capsid of the original parent virus is left behind as a ghost attached to the exterior of the cell 2011 Pearson Education Inc 2011 Pearson Education Inc The Hershey Chase Experiment Hershey and Chase grew the virus in the presence of one of two radioactive isotopes 32P or 35S The labeled viruses were used to infect E coli cells They hypothesized If genes consist of DNA then radioactive DNA would be found inside the cells while radioactive proteins would be found only in the ghosts outside the cells If genes consist of proteins then only radioactive protein and no radioactive DNA would be found inside the cells 2011 Pearson Education Inc The Hershey Chase Experiment The radioactive protein was found in the ghosts and the radioactive DNA was found in the cells The researchers concluded that this result supports that DNA not protein is the genetic material After these results were published proponents of the protein hypothesis accepted that DNA not protein must be the hereditary material An astonishing claim that DNA contained all the information for life s complexity was correct 2011 Pearson Education Inc DNA s Primary Structure The primary structure of DNA has two major components 1 A backbone made up of the sugar and phosphate groups of deoxyribonucleotides 2 A series of nitrogen containing bases that project from the backbone DNA has directionality one end has an exposed hydroxyl group on the 3 carbon of deoxyribose and the other end has an exposed phosphate group on a 5 carbon The molecule thus has a 3 end and a 5 end 2011 Pearson Education Inc 2011 Pearson Education Inc DNA s Secondary Structure Watson and Crick proposed that two DNA strands line up in the opposite direction to each other in what is called antiparallel fashion The antiparallel strands twist to form a double helix The secondary structure is stabilized by complementary base pairing Adenine A hydrogen bonds with thymine T Guanine G hydrogen bonds with cytosine C 2011 Pearson Education Inc 2011 Pearson Education Inc DNA Strands Are Templates for DNA Synthesis Watson and Crick suggested that the existing strands of DNA served as a template pattern for the production of new strands with bases being added to the new strands according to complementary base pairing Biologists then proposed three alternative hypotheses for how the old and new DNA strands interacted during replication Semiconservative replication Conservative replication Dispersive replication 2011 Pearson Education Inc How Do the Old and New DNA Strands Interact In semiconservative replication the parental DNA strands separate and each is used as a template for the synthesis of a new strand Daughter molecules each consist of one old and one new strand In conservative replication the parental molecule serves as a template for the synthesis of an entirely new molecule In dispersive replication the parent molecule is cut into sections such that the daughter molecules contain old DNA interspersed with newly synthesized DNA 2011 Pearson Education Inc The Meselson Stahl Experiment Meselson and Stahl designed an experiment to provide more information about whether one of these hypotheses was correct They grew E coli in the presence of heavy nitrogen 15N to label the bacteria s DNA After many generations they moved the bacteria to a normal 14N containing medium and separated the DNA by density The densities of the resulting DNA samples supported semiconservative DNA replication as the mechanism by which the hereditary material is duplicated 2011 Pearson Education Inc A Comprehensive Model for DNA Synthesis Meselson and Stahl showed that each parental DNA strand is copied in its entirety but did not illustrate a mechanism for this process The discovery of DNA polymerase the enzyme that catalyzes DNA synthesis cleared the way for understanding DNA replication reactions 2011 Pearson Education Inc Characteristics of DNA Polymerases A critical characteristic of DNA polymerases is that they can only work in one direction DNA polymerases can add deoxyribonucleotides to only the 3 end of a growing DNA chain As a result DNA synthesis always proceeds in the 5 3 direction DNA polymerization is exergonic because the monomers that act as substrates in the reaction are deoxyribonucleoside triphosphates dNTPs which have high potential energy because of their three closely packed phosphate groups 2011 Pearson Education Inc 2011 Pearson Education Inc How Does Replication Get Started A replication bubble forms in a chromosome that is actively being replicated Replication bubbles grow as DNA replication proceeds because synthesis is bidirectional In bacterial chromosomes the replication process begins at a single location the origin of replication Eukaryotes also have bidirectional replication but they have multiple origins of replication and thus have multiple replication bubbles A replication fork is the Y shaped region where the DNA is split into two separate strands for copying 2011 Pearson Education Inc 2011 Pearson Education Inc How Is the Helix Opened and Stabilized Several proteins