SAT阅读材料 gene regulatory network
<p> <span word="A">A</span> <span word="gene">gene</span> <span word="regulatory">regulatory</span> <span word="network">network</span> <span word="or">or</span> <span word="genetic">genetic</span> <span word="regulatory">regulatory</span> <span word="network">network</span> <span word="is">is</span> <span word="a">a</span> <span word="collection">collection</span> <span word="of">of</span> <span word="DNA">DNA</span> <span word="segments">segments</span> <span word="in">in</span> <span word="a">a</span> <span word="cell">cell</span> <span word="which">which</span> <span word="interact">interact</span> <span word="with">with</span> <span word="each">each</span> <span word="other">other</span> <span word="and">and</span> <span word="with">with</span> <span word="other">other</span> <span word="substances">substances</span> <span word="in">in</span> <span word="the">the</span> <span word="cell">cell</span>, <span word="thereby">thereby</span> <span word="governing">governing</span> <span word="the">the</span> <span word="rates">rates</span> <span word="at">at</span> <span word="which">which</span> <span word="genes">genes</span> <span word="in">in</span> <span word="the">the</span> <span word="network">network</span> <span word="are">are</span> <span word="transcribed">transcribed</span> <span word="into">into</span> <span word="mRNA">mRNA</span>. <span word="In">In</span> <span word="general">general</span>, <span word="each">each</span> <span word="mRNA">mRNA</span> <span word="molecule">molecule</span> <span word="goes">goes</span> <span word="on">on</span> <span word="to">to</span> <span word="make">make</span> <span word="a">a</span> <span word="specific">specific</span> <span word="protein">protein</span> . <span word="In">In</span> <span word="some">some</span> <span word="cases">cases</span> <span word="this">this</span> <span word="protein">protein</span> <span word="will">will</span> <span word="be">be</span> <span word="structural">structural</span>, <span word="and">and</span> <span word="will">will</span> <span word="accumulate">accumulate</span> <span word="at">at</span> <span word="the">the</span> <span word="cell">cell</span>-<span word="wall">wall</span> <span word="or">or</span> <span word="within">within</span> <span word="the">the</span> <span word="cell">cell</span> <span word="to">to</span> <span word="give">give</span> <span word="it">it</span> <span word="particular">particular</span> <span word="structural">structural</span> <span word="properties">properties</span>. <span word="In">In</span> <span word="other">other</span> <span word="cases">cases</span> <span word="the">the</span> <span word="protein">protein</span> <span word="will">will</span> <span word="be">be</span> <span word="an">an</span> <span word="enzyme">enzyme</span>; <span word="a">a</span> <span word="micro">micro</span>-<span word="machine">machine</span> <span word="that">that</span> <span word="catalyses">catalyses</span> <span word="a">a</span> <span word="certain">certain</span> <span word="reaction">reaction</span>, <span word="such">such</span> <span word="as">as</span> <span word="the">the</span> <span word="breakdown">breakdown</span> <span word="of">of</span> <span word="a">a</span> <span word="food">food</span> <span word="source">source</span> <span word="or">or</span> <span word="toxin">toxin</span>. <span word="Some">Some</span> <span word="proteins">proteins</span> <span word="though">though</span> <span word="serve">serve</span> <span word="only">only</span> <span word="to">to</span> <span word="activate">activate</span> <span word="other">other</span> <span word="genes">genes</span>, <span word="and">and</span> <span word="these">these</span> <span word="are">are</span> <span word="the">the</span> <span word="transcription">transcription</span> <span word="factors">factors</span> <span word="that">that</span> <span word="are">are</span> <span word="the">the</span> <span word="main">main</span> <span word="players">players</span> <span word="in">in</span> <span word="regulatory">regulatory</span> <span word="networks">networks</span> <span word="or">or</span> <span word="cascades">cascades</span>. <span word="By">By</span> <span word="binding">binding</span> <span word="to">to</span> <span word="the">the</span> <span word="promoter">promoter</span> <span word="region">region</span> <span word="at">at</span> <span word="the">the</span> <span word="start">start</span> <span word="of">of</span> <span word="other">other</span> <span word="genes">genes</span> <span word="they">they</span> <span word="turn">turn</span> <span word="them">them</span> <span word="on">on</span>, <span word="initiating">initiating</span> <span word="the">the</span> <span word="production">production</span> <span word="of">of</span> <span word="another">another</span> <span word="protein">protein</span>, <span word="and">and</span> <span word="so">so</span> <span word="on">on</span>. <span word="Some">Some</span> <span word="transcription">transcription</span> <span word="factors">factors</span> <span word="are">are</span> <span word="inhibitory">inhibitory</span>.</p><p> <span word="In">In</span> <span word="single">single</span>-<span word="celled">celled</span> <span word="organisms">organisms</span> <span word="regulatory">regulatory</span> <span word="networks">networks</span> <span word="respond">respond</span> <span word="to">to</span> <span word="the">the</span> <span word="external">external</span> <span word="environment">environment</span>, <span word="optimising">optimising</span> <span word="the">the</span> <span word="cell">cell</span> <span word="at">at</span> <span word="a">a</span> <span word="given">given</span> <span word="time">time</span> <span word="for">for</span> <span word="survival">survival</span> <span word="in">in</span> <span word="this">this</span> <span word="environment">environment</span>. <span word="Thus">Thus</span> <span word="a">a</span> <span word="yeast">yeast</span> <span word="cell">cell</span>, <span word="finding">finding</span> <span word="itself">itself</span> <span word="in">in</span> <span word="a">a</span> <span word="sugar">sugar</span> <span word="solution">solution</span>, <span word="will">will</span> <span word="turn">turn</span> <span word="on">on</span> <span word="genes">genes</span> <span word="to">to</span> <span word="make">make</span> <span word="enzymes">enzymes</span> <span word="that">that</span> <span word="process">process</span> <span word="the">the</span> <span word="sugar">sugar</span> <span word="to">to</span> <span word="alcohol">alcohol</span>. <span word="This">This</span> <span word="process">process</span>, <span word="which">which</span> <span word="we">we</span> <span word="associate">associate</span> <span word="with">with</span> <span word="wine">wine</span>-<span word="making">making</span>, <span word="is">is</span> <span word="how">how</span> <span word="the">the</span> <span word="yeast">yeast</span> <span word="cell">cell</span> <span word="makes">makes</span> <span word="its">its</span> <span word="living">living</span>, <span word="gaining">gaining</span> <span word="energy">energy</span> <span word="to">to</span> <span word="multiply">multiply</span>, <span word="which">which</span> <span word="under">under</span> <span word="normal">normal</span> <span word="circumstances">circumstances</span> <span word="would">would</span> <span word="enhance">enhance</span> <span word="its">its</span> <span word="survival">survival</span> <span word="prospects">prospects</span>.</p><p> <span word="In">In</span> <span word="multicellular">multicellular</span> <span word="animals">animals</span> <span word="the">the</span> <span word="same">same</span> <span word="principle">principle</span> <span word="has">has</span> <span word="been">been</span> <span word="put">put</span> <span word="in">in</span> <span word="the">the</span> <span word="service">service</span> <span word="of">of</span> <span word="gene">gene</span> <span word="cascades">cascades</span> <span word="that">that</span> <span word="control">control</span> <span word="body">body</span>-<span word="shape">shape</span>. <span word="Each">Each</span> <span word="time">time</span> <span word="a">a</span> <span word="cell">cell</span> <span word="divides">divides</span>, <span word="two">two</span> <span word="cells">cells</span> <span word="result">result</span> <span word="which">which</span>, <span word="although">although</span> <span word="they">they</span> <span word="contain">contain</span> <span word="the">the</span> <span word="same">same</span> <span word="genome">genome</span> <span word="in">in</span> <span word="full">full</span>, <span word="can">can</span> <span word="differ">differ</span> <span word="in">in</span> <span word="which">which</span> <span word="genes">genes</span> <span word="are">are</span> <span word="turned">turned</span> <span word="on">on</span> <span word="and">and</span> <span word="making">making</span> <span word="proteins">proteins</span>. <span word="Sometimes">Sometimes</span> <span word="a">a</span> <span word="self">self</span>-<span word="sustaining">sustaining</span> <span word="feedback">feedback</span> <span word="loop">loop</span> <span word="ensures">ensures</span> <span word="that">that</span> <span word="a">a</span> <span word="cell">cell</span> <span word="maintains">maintains</span> <span word="its">its</span> <span word="identity">identity</span> <span word="and">and</span> <span word="passes">passes</span> <span word="it">it</span> <span word="on">on</span>. <span word="Less">Less</span> <span word="understood">understood</span> <span word="is">is</span> <span word="the">the</span> <span word="mechanism">mechanism</span> <span word="of">of</span> <span word="epigenetics">epigenetics</span> <span word="by">by</span> <span word="which">which</span> <span word="chromatin">chromatin</span> <span word="modification">modification</span> <span word="may">may</span> <span word="provide">provide</span> <span word="cellular">cellular</span> <span word="memory">memory</span> <span word="by">by</span> <span word="blocking">blocking</span> <span word="or">or</span> <span word="allowing">allowing</span> <span word="transcription">transcription</span>. <span word="A">A</span> <span word="major">major</span> <span word="feature">feature</span> <span word="of">of</span> <span word="multicellular">multicellular</span> <span word="animals">animals</span> <span word="is">is</span> <span word="the">the</span> <span word="use">use</span> <span word="of">of</span> <span word="morphogen">morphogen</span> <span word="gradients">gradients</span>, <span word="which">which</span> <span word="in">in</span> <span word="effect">effect</span> <span word="provide">provide</span> <span word="a">a</span> <span word="positioning">positioning</span> <span word="system">system</span> <span word="that">that</span> <span word="tells">tells</span> <span word="a">a</span> <span word="cell">cell</span> <span word="where">where</span> <span word="in">in</span> <span word="the">the</span> <span word="body">body</span> <span word="it">it</span> <span word="is">is</span>, <span word="and">and</span> <span word="hence">hence</span> <span word="what">what</span> <span word="sort">sort</span> <span word="of">of</span> <span word="cell">cell</span> <span word="to">to</span> <span word="become">become</span>. <span word="A">A</span> <span word="gene">gene</span> <span word="that">that</span> <span word="is">is</span> <span word="turned">turned</span> <span word="on">on</span> <span word="in">in</span> <span word="one">one</span> <span word="cell">cell</span> <span word="may">may</span> <span word="make">make</span> <span word="a">a</span> <span word="product">product</span> <span word="that">that</span> <span word="leaves">leaves</span> <span word="the">the</span> <span word="cell">cell</span> <span word="and">and</span> <span word="diffuses">diffuses</span> <span word="through">through</span> <span word="adjacent">adjacent</span> <span word="cells">cells</span>, <span word="entering">entering</span> <span word="them">them</span> <span word="and">and</span> <span word="turning">turning</span> <span word="on">on</span> <span word="genes">genes</span> <span word="only">only</span> <span word="when">when</span> <span word="it">it</span> <span word="is">is</span> <span word="present">present</span> <span word="above">above</span> <span word="a">a</span> <span word="certain">certain</span> <span word="threshold">threshold</span> <span word="level">level</span>. <span word="These">These</span> <span word="cells">cells</span> <span word="are">are</span> <span word="thus">thus</span> <span word="induced">induced</span> <span word="into">into</span> <span word="a">a</span> <span word="new">new</span> <span word="fate">fate</span>, <span word="and">and</span> <span word="may">may</span> <span word="even">even</span> <span word="generate">generate</span> <span word="other">other</span> <span word="morphogens">morphogens</span> <span word="that">that</span> <span word="signal">signal</span> <span word="back">back</span> <span word="to">to</span> <span word="the">the</span> <span word="original">original</span> <span word="cell">cell</span>. <span word="Over">Over</span> <span word="longer">longer</span> <span word="distances">distances</span> <span word="morphogens">morphogens</span> <span word="may">may</span> <span word="use">use</span> <span word="the">the</span> <span word="active">active</span> <span word="process">process</span> <span word="of">of</span> <span word="signal">signal</span> <span word="transduction">transduction</span>. <span word="Such">Such</span> <span word="signalling">signalling</span> <span word="controls">controls</span> <span word="embryogenesis">embryogenesis</span>, <span word="the">the</span> <span word="building">building</span> <span word="of">of</span> <span word="a">a</span> <span word="body">body</span> <span word="plan">plan</span> <span word="from">from</span> <span word="scratch">scratch</span> <span word="through">through</span> <span word="a">a</span> <span word="series">series</span> <span word="of">of</span> <span word="sequential">sequential</span> <span word="steps">steps</span>. <span word="They">They</span> <span word="also">also</span> <span word="control">control</span> <span word="maintain">maintain</span> <span word="adult">adult</span> <span word="bodies">bodies</span> <span word="through">through</span> <span word="feedback">feedback</span> <span word="processes">processes</span>, <span word="and">and</span> <span word="the">the</span> <span word="loss">loss</span> <span word="of">of</span> <span word="such">such</span> <span word="feedback">feedback</span> <span word="because">because</span> <span word="of">of</span> <span word="a">a</span> <span word="mutation">mutation</span> <span word="can">can</span> <span word="be">be</span> <span word="responsible">responsible</span> <span word="for">for</span> <span word="the">the</span> <span word="cell">cell</span> <span word="proliferation">proliferation</span> <span word="that">that</span> <span word="is">is</span> <span word="seen">seen</span> <span word="in">in</span> <span word="cancer">cancer</span>. <span word="In">In</span> <span word="parallel">parallel</span> <span word="with">with</span> <span word="this">this</span> <span word="process">process</span> <span word="of">of</span> <span word="building">building</span> <span word="structure">structure</span>, <span word="the">the</span> <span word="gene">gene</span> <span word="cascade">cascade</span> <span word="turns">turns</span> <span word="on">on</span> <span word="genes">genes</span> <span word="that">that</span> <span word="make">make</span> <span word="structural">structural</span> <span word="proteins">proteins</span> <span word="that">that</span> <span word="give">give</span> <span word="each">each</span> <span word="cell">cell</span> <span word="the">the</span> <span word="physical">physical</span> <span word="properties">properties</span> <span word="it">it</span> <span word="needs">needs</span>. <span word="It">It</span> <span word="has">has</span> <span word="been">been</span> <span word="suggested">suggested</span> <span word="that">that</span>, <span word="because">because</span> <span word="biological">biological</span> <span word="molecular">molecular</span> <span word="interactions">interactions</span> <span word="are">are</span> <span word="intrinsically">intrinsically</span> <span word="stochastic">stochastic</span>, <span word="gene">gene</span> <span word="networks">networks</span> <span word="are">are</span> <span word="the">the</span> <span word="result">result</span> <span word="of">of</span> <span word="cellular">cellular</span> <span word="processes">processes</span> <span word="and">and</span> <span word="not">not</span> <span word="their">their</span> <span word="cause">cause</span>. <span word="However">However</span>, <span word="recent">recent</span> <span word="experimental">experimental</span> <span word="evidence">evidence</span> <span word="has">has</span> <span word="favored">favored</span> <span word="the">the</span> <span word="attractor">attractor</span> <span word="view">view</span> <span word="of">of</span> <span word="cell">cell</span> <span word="fates">fates</span>.</p>
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