GEN 3000 Exam 2 2016 Study Guide Chapters 10 and 12 Dr Tsai Clemson University Modeled around Dr Tsai s recommended subjects to study color coded organized by chapter Exam 2 Chapters 10 and 12 Chapter 10 DNA Replication Note Unlike most everything else we study in Genetics we can use the word ALWAYS in these chapters Not everywhere but in some ideas Original Hypotheses of DNA Replication o Conservative Replication This hypothesis stated that the entire original double strand is conserved After 1 replication both strands of the original stay together and there are two strands of new together Then in the second replication have original still together and there are 3 sets of entirely new replicated strands o Dispersive Replication The idea that replication occurs in pieces creating a patchwork of original and new replicated DNA in the new product Semiconservative Replication The idea that the new product conserves a single strand Have original double strand and those two original strands split to act as templates for new strands The new product consists of one original strand and one new replicated strand Messelson and Stahl o These two scientists set up an experiment to see how DNA replicates in bacteria It was based on the fact that DNA contains nitrogenous bases so it would take up Nitrogen to build these bases Messelson and Stahl allowed DNA to take up heavy Nitrogen 15N or light Nitrogen 14N which would separate in a vial of liquid differently due to the different number of neutrons o First they put bacterial cells where the only nitrogen source was only heavy nitrogen Cells incorporated heavy nitrogen into DNA Allowed all bacteria to take up heavy Nitrogen Then when cells were moved to media containing light nitrogen the bacteria then incorporated 14N o When the DNA from cells were put into a tube of liquid they found they had DNA settle in the middle not toward the top where it would be if it was made of all light nitrogen and not at the bottom where DNA with all heavy nitrogen would be This suggested that the DNA strands were half old and half new This finding allowed them to rule out conservative method which they would have expected to show that DNA containing 15N only stayed intact after replication takes place giving some DNA at the bottom In addition there would be DNA containing only light nitrogen at the top and nothing in the middle of the tube This DID NOT HAPPEN o This did not rule out dispersive so they did another round of replication in light Nitrogen media and got a hybrid molecule as well as DNA containing only light Nitrogen As more rounds of replication were allowed to occur the small amount of hybrid DNA persisted and the DNA containing light Nitrogen built up at the top of the tube This allowed them to rule out dispersive Dispersive would have resulted in a gradual movement of DNA toward the light end but always keep some of the heavy nitrogen due to chopped up pieces of original strands They did NOT GET THIS o These findings led them to conclude that Bacterial cells replicate in a semiconservative fashion Old strand serves as template for new strand Semiconservative replication o The process of DNA replication in which a strand of original DNA is used as a template to create a strand of new DNA The resulting double helix DNA is made of a single original strand and a single new strand Messelson and Stahl showed this to be true for Bacterial cells but another experiment was required to prove it for Eukaryotic cells o Thymidine Experiment Eukaryotic cells were allowed to replicate in the presence of thymidine after a round of replication saw that both sister chromatids were labeled Then they followed this through anaphase without thymidine present and saw division with chromatid separation In the next generation without thymidine present they found that only 1 strand would be labelled and the other whole strand would be unlabeled This proved Semi conservative replication for eukaryotic cells Fun Fact Experimenters were able to see was the exchange between sister chromatids in meiotic interphase due to the labelling differences Normally we can t see this exchange due to identical genetic information Replication of DNA o All organisms use semiconservative replication but there are differences in replication types across organisms o Theta rolling circle linear Theta Prokaryotes have the singular circular chromosome use theta replication A chromosome has to have an origin of replication there is ALWAYS an origin of replication where the chromosome will begin unwinding where replication will begin If a chromosome loses this origin it cannot be replicated When the origin opens up there is a bubble formed called the replication bubble There is bidirectional expansion the replication bubble will expand out from between the two replicated strands This is what gives the circular chromosome a theta appearance There are some prokaryotes that use unidirectional replication where the bubble opens in one direction However the replication is most commonly bidirectional With bacterial replication since there s just a single origin the entire organism is called a single replicon A replicon is the newly replicated fragment from one origin and the number of bases replicated in one set For E coli have a single origin so a single replicon of 4 600 000 base pairs Rolling Circle Another mechanism that viruses and the F factor replicate by is the rolling circle replication Instead of a bubble forming at the replication origin the strand will actual cut and peel away as it is replicated The cycle may be repeated Do NOT see a replication bubble because this process is unidirectional Linear In eukaryotes there are multiple replication bubbles that open up and are all bidirectional moving away from the origin There are multiple origin points due to the extensive amount of DNA to replicate Therefore there are multiple replicons or segments of DNA replicated in one go o Origin The place at which replication begins There ALWAYS must be an origin of replication in order for genetic material to be replicated If the origin somehow malfunctions or is lost replication will not take place at all Typically an origin regardless of the type of replication taking place o Steps is a distinctive sequence of base pairs DNA is antiparallel we can only make DNA in a 5 to 3 direction Bases will be added in the opposite direction of the strand In a bidirectionally replicating DNA strand one strand will