BIOL 151 Final: Chapter 2 Notes for final biology exam

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University of North Dakota
BIOL 151
Chris Felege

DNAand the Gene: Synthesis and Repair What are genes made of? • Researchers knew and understood that chromosomes were comprised of DNAand protein but did not know whether the genes were comprised of DNAor protein • The general consensus at the time supported the hypothesis that genes were comprised of proteins ◦ There are 20 amino acids that make up proteins, and only 4 boxes that make up DNA ◦ As a result, proteins are more complex, and thus, scientists thought they were most likely the hereditary material 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 attaches to the exterior of the wall • Hershey and Chase grew the virus in the presence of one or two radioactive isotopes 32 35 ◦ P or S ◦ Labelled viruses were used to infect E. coli cells • They hypothesized that if genes consist of DNA: ◦ Then radioactive DNA( P) would be found inside the cells ◦ While radioactive proteins ( S) would be found outside the cells • If genes consists of proteins: ◦ Then only radioactive proteins ( S) found inside the cells ◦ No radioactive DNA( P) found inside the cells • The radioactive protein was found in ghosts • The radioactive DNAwas found in the cells • This result disproves that proteins are the hereditary material, and supports the proposal that DNA, not protein, is the genetic material ◦ DNAcontained all the information for life's complexity DNA's Primary Structure • The primary structure of DNAhas two major components: 1. Abackbone made up of the sugar and phosphate groups of deoxyribonucleotides 2. Aseries of nitrogen containing bases that project from the backbone • DNAhas directionality ◦ One end has an exposed hydroxyl group on the 3' carbon of deoxyribose ◦ The other end has an exposed phosphate group on a 5' carbon ◦ The molecule thus has a 3' end and a 5' end • Watson and Crick proposed that two DNAstrands line up in the opposite direction to each other. This 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) • Watson and Crick suggested that the existing strands of DNAserved as a template (pattern) for the production of the new strands ◦ Bases were added to the new strands ◦ According to complementary base pairing • Three alternative hypotheses for how the old and new DNAstrands interacted during replication: 1. Semiconservative replication 2. Conservative replication 3. Dispersive replication • In semiconservative replication, the parental DNAstrands 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 parental molecule is cut into section and the daughter molecules contain old DNAinterspersed with newly synthesized DNA • Meselson and Stahl designed and experiment that was designed to provide more information about whether one of these hypotheses was correct ◦ Grew E. coli in the presence of "heavy" nitrogen ( N) to label the bacteria's DNA ◦ After many generations they moved the bacteria to a normal N-containing medium which then separated the DNAby density ◦ The densities of the resulting DNAsamples supported semiconservative DNA replication as the mechanism and explained how the hereditary material is duplicated • Meselson and Stahl showed that each parental DNAstrand is copied in entirety and did not illustrate a mechanism for this process • DNApolymerase is the enzyme that catalyzes DNAsynthesis • The discovery of DNApolymerase cleared the way for understanding DNAreplication reactions • "ase" means enzyme • Proteins that "do a job" • Acritical characteristic of DNApolymerases ◦ They can work in only ONE direction • DNApolymerases can add deoxyribonucleotides to only the 3' end of a growing DNA chain • DNAsynthesis always proceeds in the 5' to 3' direction • DNApolymerization is exergonic because the monomers that act as substrates in the reaction are deoxyribonucleoside triphosphates (dNTPs) and have high potential energy because of their three closely packed phosphate groups • A replication bubble forms in a chromosome that is actively being replicated and grows as the DNAreplication proceeds because synthesis is bidirectional • In bacterial chromosomes, the replication process begins at a single location. This is the origin of replication • Eukaryotes also have bidirectional replication but they have multiple origins of replication and they have multiple replication bubbles • Areplication fork is the Y-shaped region where the DNAis split into two separate strands for copying • Several proteins are responsible for opening and stabilizing the double helix: ◦ Enzyme helicase catalyzes the breaking of hydrogen bonds between the two DNA strands to separate them ◦ Single stranded DNA-binding proteins (SSBPs) attach to the separated strands to prevent them from closing • Unwinding the DNAhelix creates tension farther down the helix • Enzyme topoisomerase cuts and rejoins the DNAdownstr
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