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Purdue BIOL 10200 - Bio Focus Questions #1

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Focus Questions: DNA Replication1. Why and when during the cell cycle does DNA replication occur? It occurs during the S phase sothat when mitosis/meiosis occurs, it can split into multiple cells with a sufficient amount ofgenetic material.2. What are the components of DNA? Where do covalent and hydrogen bonds occur? The threecomponents of DNA are: a 5-carbon sugar, a phosphate group and a nitrogenous(nitrogen-containing) base. Covalent bonds hold those components together, but hydrogen bondsconnect the different types of bases. C and G have three hydrogen bonds, A and T have twohydrogen bonds.3. What does it mean for DNA to be replicated semi-conservatively? The original double helixcomes apart into two separate strands, then each of those strands are paired with a copy. It’s“semi-conservative” because copies and the original coexist.4. What are primers, how are they produced, and why are they necessary for DNA replication? Theenzyme primase creates primers, and the primer is necessary because it attracts DNA polymerase,an enzyme that accelerates the replication of DNA.5. Why are there leading and lagging strands in DNA replication? DNA is antiparallel, meaning thatone end goes from 3-5 and the other goes from 5-3. Nucleotides are added to the 3’ end of theoriginal to the 5’ end (meaning that the 5’ end of the copy is actually being created first). WhenDNA unwinds during semiconservative replication, one “leading” strand (with the 5’ end) iscontinuously replicated, while the “lagging” strand (with the 3’ end) is discontinuously replicatedin short bursts.6. Identify the steps by which DNA gets replicated. Fully explain what happens during initiation,elongation, and termination. Include the role of all necessary enzymes and other proteins.The main enzymes/proteins are: primase, helicase, ligase and DNA polymerase.During initiation, the DNA unwinds into two template strands because of helicase, the site ofwhich is called the replication fork. The replisome attaches to the origin of replication. Theenzyme primase makes a primer of RNA, which attaches to the template strand. Proteins alsoprevent bases of the templates from rejoining.During elongation, the primer attracts DNA polymerase, which brings nucleotides over from the3’ end to the 5’ end of each template strand. Sliding clamps help stabilize these new bonds. Oneprimer is needed for the leading strand, but each okazaki fragment of the lagging strand requiresits own primer.Finally, during termination, DNA ligase connects adjacent strands of DNA and the process ends.7. Compare and contrast the structures of DNA and RNA. What does it mean to have a 5’ end and a3’ end? The 5-carbon sugar I mentioned above has a 3’ end (which normally connects to hydroxyland another DNA) and a 5’ end (which connects to the phosphate group). The 5’ end sticks out onone end, and the 3’ sticks out on the other.RNA can be double-stranded like DNA but it can also be single-stranded. If double-stranded, ithas U to replace T.8. 8. How do errors in DNA replication get corrected? DNA polymerase has a proofreading functionto find any errors. The polymerase usually removes errors, but if not, the excision repair does it;either way, polymerase fills in the resulting gap.Focus Questions: Mitosis/Meiosis7. Are any steps in the cell cycle irreversible? What are the implication of this for the controlof the cell cycle and the mechanics of the cell cycle? There are 5 main phases to the cellcycle (G1, S, G2, Mitosis/Meiosis and Cytokinesis) with Mitosis/Meiosis being further dividedinto Prophase, Prometaphase, Metaphase, Anaphase and Telophase.There are three irreversible points, called “checkpoints.” The first is at the end of G1 and rightbefore S; the second is at the end of G2 and right before M; and the third is during Anaphasewhen chromosomes/homologous pairs detach. Therefore, we have mechanics in place tomake sure that everything is in order before then. MPF is that mechanic during the secondcheckpoint, and the Anaphase-Promoting Complex receives signals during the thirdcheckpoint.8. What are the phases of mitosis and what is happening during each phase? Why isanaphase so important? What would happen if cytokinesis was blocked?During prophase, the chromosomes condense and become visible. The nuclear envelopebreaks down and the spindle fibers are created.During prometaphase, the spindles attach to the chromosomes; and during metaphase, theyalign at the metaphase plate.During anaphase, the cohesins are removed and the chromatids come apart. It is soimportant because this way two nucleus’ worth of materials can be evenly distributed acrossthe cell.During telophase, the nucleus dissipates, the spindles dissipate, and two new nuclearenvelopes form containing both sets of genetic material.During cytokinesis, the cell itself splits. If this was blocked, it would lead to huge, elongatedcells with twice as much material but no use for all the extra chromosomes.9. What is the difference between a homolog and a sister chromatid? Do all chromosomeshave sister chromatids? A homolog is a chromosome, specifically one half of a homologouspair, which is when we have two chromosomes of the same type (one from each parent). Asister chromatid is a strand of genetic material that makes up a homolog. Chromosomes canbe made up of 1 or 2 chromatids, but all chromosomes have them.10. What is the significance of meiosis I and how does it differ from mitosis? How doesmeiosis II differ from mitosis? The significance is that there are no homologous pairs at theend of Meiosis I. It is a bunch of two-chromatid haploids instead of one-chromatid diploidslike we started with. Granted, that means there’s still an equivalent amount of chromatids, butthere aren’t any actual pairs.Meiosis II differs from mitosis, in turn, because there is no replication. Instead, the two cells’chromatids start splitting without synthesis, so we end up with 4 one-chromatid haploidsinstead of 2 one-chromatid diploids like in


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