Focus Questions DNA Replication 1 Why and when during the cell cycle does DNA replication occur It occurs during the S phase so that when mitosis meiosis occurs it can split into multiple cells with a sufficient amount of genetic material 2 What are the components of DNA Where do covalent and hydrogen bonds occur The three components of DNA are a 5 carbon sugar a phosphate group and a nitrogenous nitrogen containing base Covalent bonds hold those components together but hydrogen bonds connect the different types of bases C and G have three hydrogen bonds A and T have two hydrogen bonds 3 What does it mean for DNA to be replicated semi conservatively The original double helix comes 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 The enzyme 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 that one end goes from 3 5 and the other goes from 5 3 Nucleotides are added to the 3 end of the original to the 5 end meaning that the 5 end of the copy is actually being created first When DNA unwinds during semiconservative replication one leading strand with the 5 end is continuously replicated while the lagging strand with the 3 end is discontinuously replicated in short bursts 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 6 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 of which is called the replication fork The replisome attaches to the origin of replication The enzyme primase makes a primer of RNA which attaches to the template strand Proteins also prevent bases of the templates from rejoining During elongation the primer attracts DNA polymerase which brings nucleotides over from the 3 end to the 5 end of each template strand Sliding clamps help stabilize these new bonds One primer is needed for the leading strand but each okazaki fragment of the lagging strand requires its 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 a 3 end The 5 carbon sugar I mentioned above has a 3 end which normally connects to hydroxyl and another DNA and a 5 end which connects to the phosphate group The 5 end sticks out on one 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 it has U to replace T 8 8 How do errors in DNA replication get corrected DNA polymerase has a proofreading function to 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 Meiosis 7 Are any steps in the cell cycle irreversible What are the implication of this for the control of the cell cycle and the mechanics of the cell cycle There are 5 main phases to the cell cycle G1 S G2 Mitosis Meiosis and Cytokinesis with Mitosis Meiosis being further divided into Prophase Prometaphase Metaphase Anaphase and Telophase There are three irreversible points called checkpoints The first is at the end of G1 and right before S the second is at the end of G2 and right before M and the third is during Anaphase when chromosomes homologous pairs detach Therefore we have mechanics in place to make sure that everything is in order before then MPF is that mechanic during the second checkpoint and the Anaphase Promoting Complex receives signals during the third checkpoint 8 What are the phases of mitosis and what is happening during each phase Why is anaphase so important What would happen if cytokinesis was blocked During prophase the chromosomes condense and become visible The nuclear envelope breaks down and the spindle fibers are created During prometaphase the spindles attach to the chromosomes and during metaphase they align at the metaphase plate During anaphase the cohesins are removed and the chromatids come apart It is so important because this way two nucleus worth of materials can be evenly distributed across the cell During telophase the nucleus dissipates the spindles dissipate and two new nuclear envelopes form containing both sets of genetic material During cytokinesis the cell itself splits If this was blocked it would lead to huge elongated cells 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 chromosomes have sister chromatids A homolog is a chromosome specifically one half of a homologous pair which is when we have two chromosomes of the same type one from each parent A sister chromatid is a strand of genetic material that makes up a homolog Chromosomes can be 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 does meiosis II differ from mitosis The significance is that there are no homologous pairs at the end of Meiosis I It is a bunch of two chromatid haploids instead of one chromatid diploids like we started with Granted that means there s still an equivalent amount of chromatids but there 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 haploids instead of 2 one chromatid diploids like in Mitosis