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G1/S transition
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<p>The G1/S transition is a stage in the <a href="/facts/Cell_cycle/nLMT7K4r">cell cycle</a> at the boundary between the <a href="/facts/G1_phase/92lq1LdT">G1 phase</a>, in which the cell grows, and the <a href="/facts/S_phase/vplz8LTU">S phase</a>, during which <a href="/facts/DNA_replication/wrU6RuR3">DNA is replicated</a>. It is governed by <a href="/facts/Cell_cycle_checkpoint/lLda9WDW">cell cycle checkpoints</a> to ensure cell cycle integrity and the subsequent S phase can pause in response to improperly or partially replicated DNA. During this transition the cell makes decisions to become quiescent (enter <a href="/facts/G0_phase/PwxUzrWr">G0</a>), <a href="/facts/Cellular_differentiation/bCVqUG7w">differentiate</a>, make DNA repairs, or proliferate based on environmental cues and molecular signaling inputs. The G1/S transition occurs late in G1 and the absence or improper application of this highly regulated checkpoint can lead to cellular transformation and disease states such as <a href="/facts/Cancer/PVW6n1D0">cancer</a>. </p><p>During this transition, G1 <a href="/facts/Cyclin_D/XrAPyeCw">cyclin D</a>-Cdk4/6 dimer phosphorylates <a href="/facts/Retinoblastoma_protein/qc2WKTc4">retinoblastoma</a> releasing <a href="/facts/Transcription_factor/wro0DPHe">transcription factor</a> <a href="/facts/E2F/18zluKwA">E2F</a>, which then drives the transition from G1 to S phase. The G1/S transition is highly regulated by transcription factor <a href="/facts/P53/YEGULLgM">p53</a> in order to halt the cell cycle when DNA is damaged. </p><p>It is a "point of no return" beyond which the cell is committed to dividing; in yeast this is called the <a href="/facts/Start_point_(yeast)/JebUqfHR">Start point</a>, and in <a href="/facts/Multicellular/rk8QBF7o">multicellular</a> <a href="/facts/Eukaryotes/SkwyPLMA">eukaryotes</a> it is termed the <a href="/facts/Restriction_point/74Kqfghl">restriction point</a> (R-Point). If a cell passes through the G1/S transition the cell will continue through the cell cycle regardless of incoming mitogenic factors due to the positive feed-back loop of G1-S transcription. Positive feed-back loops include G1 cyclins and accumulation of E2F in multicellular eukaryotes, and the accumulation of SBF in yeast cells. </p>