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22 Cards in this Set
- Front
- Back
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What is the reason for the complementarity occurring between the nitrogenous bases of DNA?
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The complementarity occurring between the N bases of DNA is due to the potential formation of hydrogen bonds between them.
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Explain the three proposed theoretical mode of DNA replication.
Semi conservative Conservative Dispersive |
The semi conservative model of DNA replication claims that after DNA replication, the two double-stranded DNA will consist of 1 parental and 1 daughter strand. When the parental DNA unwinds, the two single strands become templates for replication. The nucleotides of each strand attracts their corresponding complementary nucleotides.The nucleotides that attach to the template strand covalently link with each other, and thus, creating a new strand of DNA.
The conservative model of DNA replication proposed that once replication is finished, the two newly created strands come together to form a new DNA double helix while the parental strands re-associate. In dispersive replication, a cleavage occurs in the parental strands during replication. The parental strands are dispersed intro two new double helices following replication. Dispersive replication would produce two DNA molecules with sections of both old and new DNA interspersed along each strand. |
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How did the Messelson and Stahl experiment prove that the most accurate mode of DNA replication is the semiconservative one?
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First, they grew bacteria for many generations in a growth medium containing 15N. This is a heavy isotope of nitrogen (in contrast to the normal isotope, 14 N), which over many generations would be incorporated into all nitrogen-containing molecules of the cells, including DNA. DNA isolated from these cells could be distinguished from normal DNA because it would have a higher density.
The bacteria grown in heavy nitrogen were then transferred to growth medium containing 14 N for one round of replication. This lighter isotope would incorporate into any newly synthesized DNA. If semiconservative replication occurred, then each DNA molecule after replication would contain heavy nitrogen and light nitrogen, and would therefore have a density intermediate between the two. Conservative replication would produce one DNA molecule containing heavy nitrogen and one molecule containing light nitrogen, so there would be two different densities. Dispersive replication would produce a single intermediate density, just like semiconservative. The observed density of the DNA after one round of replication was intermediate. Replication was therefore either semiconservative or dispersive. These possibilities could be distinguished after a second round of replication. After two rounds, semiconservative replication would produce two DNA molecules containing only light nitrogen, and two DNA molecules containing one light strand and one heavy strand. Therefore there would be two different densities: light and intermediate. Two rounds of dispersive replication would produce four DNA molecules, each of which would contain mostly light nitrogen and some heavy nitrogen. There would be a single density (we'll call it 'slightly heavy'). When density of the DNA was measured after two rounds, two densities were observed: light and intermediate, indicating that DNA replication is semiconservative, and not dispersive or conservative. |
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Define: Origin of replication
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The origin of replication; an area along the chromosome where DNA replication initiate.
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Define: Replication Fork
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An area where DNA replication is occurring.
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Define: Replicon
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A length of DNA that possesses a replication origin and is therefore potentially capable of being replicated in a suitable cell. The length of DNA that is replicated following one initiation event at a single origin.
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John Cairns proposed that in bacteria, there is only one single region where replication is initiated. What did he call this region?
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OriC
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Arthur Kornberg and his colleagues discovered an enzyme from E. coli, called DNA polymerase I, that was able to direct DNA synthesis in an in vitro system. What are the two major requirements of in vitro DNA synthesis under the direction of this enzyme?
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1. All deoxyribonucleoside triphosphates (dNTPs) must be present.
2. DNA template strand must be present. |
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Explain the way in which each complimentary nucleotide is added to the growing chain during DNA replication.
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Each nucleotide is added to the growing chain in 5'-3' direction. The three phosphates of the nucleotide (dNTPs) are attached to the 5' carbon of the pentose sugar. The two phosphates cleave during synthesis while the remaining phosphate attached to the 5' Carbon covalently links to the 3' OH end of the deoxyribose to which it is added. The addition of one nucleotide to the growing 3' end provides newly exposed 3' OH end that can participate in the next addition of a nucleotide as DNA synthesis proceeds.
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DNA polymerase I, II, III all have the ability to elongate an existing DNA strand. However, only one of these three has the ability to initiate a synthesis. Which one is it?
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DNA poly I
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Define: primer
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an existing DNA strand capable of being replicated
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All three DNA polymerase enzymes have 3'-5' exonuclease activity. What is 3'-5' exonuclease activity?
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It means that they have the ability to polymerize in one direction and then pause, reverse their direction, and excise nucleotides just added.
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What is the importance of 3'-5' exonuclease activity?
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To proof-read newly synthesized DNA and to remove and replace mismatched nucleotides.
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Besides 3'-5' exonuclease activity, DNA poly I can also induce 5'-3' exonuclease activity. What does 5'-3' EA allow?
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It allows the enzyme to excise nucleotides, starting at the end at which replication begins and proceeding in the same direction as the synthesis.
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Which poly enzyme is mainly responsible for the 5'-3' polymerization and3'-5' exonuclease activity?
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DNA Poly III
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Which polymerase enzyme is responsible for the removal of RNA primers?
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DNA Poly I
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Define: Helicases
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The proteins responsible for the unwinding of DNA
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What are Single-Stranded Binding Proteins? (SSBPs)
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They are proteins that are responsible for stabilizing unwound DNA.
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What is the function of DNA gyrase?
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To relax supercoiling of the DNA. The gyrase makes either single- or double-stranded "cuts" and also catalyzes localized movements that have the effect of "undoing" the twists and knots created during supercoiling.
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What is the function of primase?
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directs the synthesis of RNA primer needed by DNA poly II to initiate replication
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Define: okazaki fragments
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Fragments of replicated DNA + primer containing about 1000-2000 nucleotides
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What is the function of DNA ligase?
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Catalyzes the formation of the phosphodiester bond that seals the nick between discontinuously synthesized strands.
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