Question

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1. Read page 196-197 and figure 6.20. regarding Meselson and Stahl’s experiment regarding DNA replication. And Answer the following question
2. 1. If you are using this radioactive technique in mouse cells, what would happen in each phase of G1, S, G2, mitosis and meiosis assuming that you are grown cells in 15N medium for many generations and cells in G1are then switched to 14N medium?
      1. G1
      2. S
      3. G2
      4. Prophase mitosis
      5. Metaphase mitosis
      6. Anaphase mitosis
      7. Telophase mitosis
      8. Prophase I of meiosis
      9. Metaphase I of meiosis
      10. Anaphase I of meiosis

First Figure 6.19 Three possible models of DNA replication: test tube, they dissolved the DNA in a solution of the d DNA from the original double helix is blue newly made DNA I salt cesium chloride (CCI) and spun these solutions magento (a) Semiconservative replication (the Watson-Crick moden very high speed (about 50,000 revolutions per minutes (b) Conservative replication: The parental double helix remains Intact: both strands of one daughter double hells are newly an ultracentrifuge. Over a period of two to three days. th synthesized. (c) Dispersive replicationBoth strands of both centrifugal force (roughly 250,000 times the force of daughter double helbres contain both original and newly ity) causes the formation of a stable gradient of CSC.com Synthesized material centrations, with the highest concentration, and thus the (a) Semiconservative (b) Conservative (c) Dispersive highest CsCl density, at the bottom of the tube. The DNA in the tube forms a sharply delineated band at the positio where its own density equals that of the CSCI. Becs DNA containing N is denser than DNA containing In Parent DNA pure "SN DNA will form a band lower, that is, closer to the bottom of the tube, than pure "N DNA (Fig. 6.20). As Fig 6.20 shows, when cells with pure 'N DNA transferred into N medium and allowed to divide on DNA from the resultant first-generation cells formed a band at a density intermediate between that of pure 'N DNA and that of pure "N DNA. A logical inference is that the DNA these cells contains equal amounts of the two isotopes. This generation daughter finding invalidates the conservative model, which predicts DNA the appearance of bands reflecting only pure N and pure "SN with no intermediary band. In contrast, DNA extracted from second-generation cells that had undergone a second round of division in the 'N medium produced two observa ble bands, one at the density corresponding to equal amounts of ''N and ''N, the other at the density of pure *N. This re- sult invalidates the dispersive model, which predicts a single Second- band between the two bands of the original generation. generation daughter Meselson and Stahl's observations are consistent only a DNA with semiconservative replication. In the first generation after transfer from the N to the 'N medium, one of the two strands in every daughter DNA molecule carries the heavy isotope label; the other, newly synthesized strand of DNA replication (Fig. 6.20). The experiment depended carries the lighter 'N isotope. The band at a density inter on being able to distinguish preexisting parental DNA from mediate between that of 'N DNA and ''N DNA represents newly synthesized DNA. To accomplish this, Meselson and this isotonic hybrid. In the second generation after transfer, Stahl controlled the isotopic composition of the nucleo - half the DNA molecules have one "N strand and one 'N tides incorporated in the newly forming strands by taking St a ining half carry two 'N strands. The advantage of the fact that the purine and pyrimidine bases and one at the hybrid position, the of DNA contain nitrogen atoms. They grew E. coli bacterias 'N position-reflect this mix. By con- for many generations on media in which all the nitrogen m o tions of semiconservative replication, the was the normal isotope "N; these cultures served as a cor Mescla Stahl experiment disproved the conservative and trol. They grew other cultures of E. coll for many genera- dispersive alternatives. We now know that the semicon tions on media in which the only source of nitrogen was the servative replication of DNA is nearly universal heavy isotope N. After several generations of growth on Let's consider precisely how semiconservative replica heavy-isotope medium, essentially all the nitrogen atoms in tion relates to the structure of chromosomes in eukaryotic the DNA of these bacterial cells were labeled with (that is, cells during the mitotic cell cycle (review Fig. 4.9). Early contained) "N. The cells in some of these cultures were interphase, each eukaryotic chromosome contains a single then transferred to new medium in which all the nitrogen continuous linear double helix of DNA. Later, during the was "N. Any DNA synthesized after the transfer would S-phase portion of interphase, the cell replicates the double contain the lighter isotope. helix semiconservatively, after this semiconservative repe Meselson and Stahl isolated DNA from cells grown in cation, each chromosome is composed of two sister chro the different nitrogen-isotope cultures and then subjected matids joined at their centromeres. Each sister chromatid these DNA samples to equilibrium density gradient centrif- a double helix of DNA, with one strand of parental DNA neation, an analytic technique they had just developed. In a one strand of newly synthesized DNA. At the conclusion WW

Answer 1

> On G, Phase all DNA strand containing wit i On s phase, due to oDNA replication through semiconservative model, one strand of DNA containing wis and other strand containing Ni4 > In G2 Phase, one DNA strand containing wis and I other strand costring containing wil o ivyovy gn prophase and metaphase mitosis the chromosome contain on a which are hybrid DNA, because the & Strand contain N15 and other strand contain Ni4. riy seeing anaphase, two chromatics are separated and which are a contain hybrid DNA.. two viit on telophase, two nucleous are form, so, nencleus contain hybrid DNA. erili, ix, x> on prophase II, metaphase I, anaphase-I of meosis all DNA are my brid Dra.