706_S2006_PS4key - 7.06 Problem Set Four, 2006 1. Explain...

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7.06 Problem Set Four, 2006 1. Explain the molecular mechanism behind each of the following events that occur during the cell cycle, making sure to discuss specific proteins that are involved in making each event happen. (a) During G1 phase, cells pass through the Restriction Point if growth factors are present in the cell medium. Growth factors are sensed by RTKs, which then become activated and thereby activate, in linear order, GRB2, Sos, Ras, Raf, MEK, and finally MAP kinase. MAP kinase therefore dimerizes and enters the nucleus, where it regulates transcription factors that control genes that are necessary for the progression of cells through the Restriction Point (i.e. genes necessary to promote the occurrence of events in the cell cycle). (b) During S phase, origins of replication fire. When we say that origins “fire,” what we mean is the DNA replication initiates from that origin. This occurs because the origin replication complex ORC recognizes and binds to origins of replication. ORC acts as a landing pad for Cdc6 and Cdt1. These two proteins lead to the recruitment of MCM proteins, which allow for the melting of the DNA at origins. Once the DNA is melted into single-stranded DNA, a replication bubble is formed, to which DNA polymerase is recruited. (c) During S phase, each origin of replication never fires more than once. The “firing” of origins that is described above triggers disassembly of the pre-replicative complex (the proteins bound at the origin named above). A functional pre-RC cannot be formed at the origin again until cells have progressed through the cell cycle to the next G1 phase. This is because Cdt1, an essential part of the pre-RC, is degraded after origins fire, and is not resynthesized until cells have proceeded through M phase. (d) During prophase of mitosis, chromosomes become condensed. Once cells enter S phase, the cells start to accumulate mitotic cyclins (B-type cyclins). These cyclins associate with CDKs, and upon activation of the mitotic cyclin/CDK compexes at the G2/M boundary, these complexes phosphorylate condensin complexes. Condensins are proteins that organize and compact chromatin into tightly wound structures. Phosphorylated condensins associate with chromosomes and condense chromosomes. (e) During prophase of mitosis (in many cell types, but not in yeast), the nuclear membrane breaks down.
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Once cells enter S phase, the cells start to accumulate mitotic cyclins (B-type cyclins). These cyclins associate with CDKs, and upon activation of the mitotic cyclin/CDK compexes at the G2/M boundary, these complexes phosphorylate nuclear membrane proteins, including lamins and some nuclear integral membrane proteins. Once phosphorylated, the nuclear lamin tetramer disassembles. Phosphorylation of some of the nuclear integral membrane proteins reduces their affinity for chromatin. Together, these two factors contribute to nuclear membrane breakdown.
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706_S2006_PS4key - 7.06 Problem Set Four, 2006 1. Explain...

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