利用DNA建造工厂以及电脑

As  if  the  blueprint  for  life  wasn't  busy  enough,  nanotech  researchers  are  putting  DNA  to  work  in  tiny  mechanical  devices  and  as  templates  for  electronic  circuits.

Recent  DNA  constructions  include  microscopic  patterns,  tiny  gears  and  a  molecular  assembly  line.  Although  still  mostly  at  the  demonstration  level,  DNA  nanotech  is  a  rapidly  growing  field.

The  first  person  to  see  DNA's  potential  beyond  biology  was  Naiman  Seeman,  a  chemist  at  New  York  University.  More  than  twenty  years  ago,  he  began  imagining  how  the  genetic  information  in  DNA  might  be  engineered  to  perform  useful  tasks.

"DNA  structures  are  programmable  by  sequence,  and  so  are  their  intermolecular  interactions,"  Seeman  says.  "That  makes  them  unique."

Whereas  nature  alone  dictates  how  most  molecules  interact,  DNA  comes  with  a  built-in  code  that  researchers  can  re-formulate  to  control  which  DNA  molecules  bond  with  each  other.  The  goal  of  this  DNA  tinkering  is  microscopic  factories  that  can  produce  made-to-order  molecules,  as  well  as  electronic  components  10  times  smaller  than  current  limits.

"Nanofabrication  is  where  we  are  going,"  Seeman  told  LiveScience.  "It  will  happen  soon."

Smart  glue

A  single  strand  of  DNA  is  essentially  a  long  sequence  made  up  of  the  chemical  bases  adenine  (A),  thymine  (T),  cytosine  (C)  and  guanine  (G).  Every  living  thing  carries  a  unique  genetic  code  in  its  cells  written  out  in  these  "letters."

Two  strands  of  DNA  can  fuse  together  and  form  the  famous  double  helix,  discovered  by  Crick  and  Watson  in  1953.  But  this  twisted  ladder  arrangement  can  only  happen  if  all  the  bases  on  the  two  strands  match  up,  so  that  A's  bond  with  T's  and  C's  bond  with  G's.

Scientists  use  this  selective  adhesive  to  build  and  control  DNA  machines.

"The  bonds  are  like  smart  glue  that  know  which  pieces  go  together,"  explains  Thomas  LaBean  of  Duke  University.

LaBean  and  others  typically  begin  with  a  design  for  a  structure  that  has  several  DNA  pieces.  A  computer  program  writes  out  the  code  for  the  different  strands,  which  are  then  synthesized  using  standard  biological  methods.  Mixed  together  in  a  water-based  solution,  the  pieces  with  matching  codes  will  link  up  to  form  several  copies  of  the  desired  structure.

It's  like  an  airplane  model  kit,  except  all  you  have  to  do  is  shake  the  box  and  all  the  little  parts  automatically  find  each  other  and  glue  together.

Puzzle  pieces

DNA  in  nature  is  often  just  one  long  continuous  chain,  but  researchers  would  prefer  to  have  other  shapes  at  their  disposal.

More  than  three  decades  ago,  biologists  discovered  that  cells  create  cross-shaped  DNA  molecules  during  replication  and  repair.  The  side-arms,  or  branches,  grow  out  of  a  genetic  code  whose  letters  read  the  same  forwards  and  backwards,  like  the  palindromes  "racecar"  and  "rotator."

Seeman  and  others  have  modified  the  sequence  of  palindromic  DNA  to  make  a  stable  4-armed  molecule.  They  have  also  coaxed  DNA  to  branch  with  3,  5  and  6  arms.

These  two-dimensional  stick  figures  are  only  a  few  nanometers  across,  where  a  nanometer  is  one  billionth  of  a  meter.  Researchers  design  them  with  "sticky  ends"—single  DNA  strands  that  act  as  latches  between  molecules.  Whole  arrays  of  these  connecting  figures  can  be  put  together  like  pieces  in  a  puzzle.

Earlier  this  year,  LaBean  and  his  collaborators  built  4x4  lattices  with  16  cross-shaped  DNA  pieces.  By  attaching  a  type  of  protein  to  specific  "pixels"  on  these  grids,  the  team  spelled  out  "DNA."

The  ability  to  attach  particles  to  DNA  pieces  is  a  step  towards  fabricating  nano-electronics.  Scientists  can  hitch  functional  materials  like  metals,  semiconductors  and  insulators  to  specific  DNA  molecules,  which  can  then  carry  their  cargo  to  pre-specified  positions.  Already  this  technique  has  been  used  to  make  a  simple  transistor,  as  well  as  metallic  wires.   (责任编辑：泉水)

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利用DNA建造工厂以及电脑