DNA Computing on Surfaces by bxk16778

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									DNA Computing on Surfaces


Anne Condon, Computer Science, UBC
Robert Corn, Chemistry, U. Wisconsin
Max Lagally, Materials Science, U. Wisconsin
Lloyd Smith, Chemistry, U. Wisconsin
                Goals
      (Adleman, Science 266:1994)

• Encode information in DNA strands

• Compute on many strands in parallel:
  chemical manipulations = logical
  operations
“…the number of of operations per second …
 would exceed that of current supercomputers
 by a thousandfold…remarkable energy
 efficiency… information density a dramatic
 improvement over existing storage media


       Len Adleman, Science 266:1994
“for certain intrinsically complex problems…where
existing electronic computers are very inefficient and
where massively parallel searches can be organized to
take advantage of the operations that molecular biology
 currently provides, molecular computation might
 compete with electronic computation in the near term”
                     Outline
Background
  • What is computation? What is DNA?
  • DNA computation
      • in the biotech industry
       • in the solution of combinatorial problems
  •Research on DNA computation

DNA Computing on Surfaces
  •Models
  •Experiments

Conclusions
       What is Computation?
        (very simple view)
• Input: string over finite alphabet
• Process: determine if input satisfies
           some property
• Output: yes or no
 Satisfy a Property: Binary Inputs
                                     Output: 1
• set the output of                        and
 a circuit to 1                       1
                                or
                            0          1
                          and         not

             Input:   1    0           0         1
            Satisfy a Property:
            Non-binary Inputs
                                  Output: C
• Set the output of a
 generalized circuit to            G
 a given value
                              T    G



                          A   G     C     G
  Simple Parallel Computation

• Input: set of strings
• Process: independently for each input,
           determine if it satisfies a
           common circuit
• Output: indicate whether there exists an
            input satisfying the circuit
What is DNA?
“DNA Computation:” Affymetrix Arrays
 • Input: strings over {A,C,G,T},
   (represented as the corresponding
   single-stranded DNA)
   Photolithography used to synthesize
   and array DNA strands on a planar
   surface
“DNA Computation:” Affymetrix Arrays

 • Process: e.g. for each input, test if it
   approximately matches a given string
   (i.e. hybridizes to Watson-Crick complement
   of given string)
   “DNA Computation:”
    Affymetrix Arrays


• Output: fluorescence detection
        Adleman’s Hamiltonian Path
               Experiment
                                   S
• Input: generate random paths
• Process:                                 2       1

                                       3
    • select paths from S to T
    • select paths with 7 nodes                5

    • select paths entering all    4
      nodes at least once
                                                   T
• Output: “yes” iff path remains
           Generate Random Paths

• Associate DNA strands with nodes and edges

      2           3           4           5

• Join edge strands in test tube to form double-
  stranded “paths” (hybridization, ligation)

• Wash to form single-stranded paths
       Adleman’s Experiment:
    Select Paths That Enter Node 2
• Attach strand associated with node 2 to
  beads and introduce to test tube
• The paths that enter node 2 hybridize to
  strands on the beads
• Remove beads; wash and detach desired
  paths
Biomolecular Computation Research

• “Classical” DNA/RNA computation
   (e.g. search-and-prune)
• O(1)-biostep computation
   (e.g. self-assembly of 3-D DNA molecules)
Biomolecular Computation
Research
• Splicing-based computation
• Non-computational applications
  (e.g. exquisite detection, DNA2DNA computation,
  DNA nanotechnology, DNA tags)
DNA Computing on Surfaces
      DNA Computing on Surfaces
• Advantages over “solution phase” chemistry:
    •Facile purification steps
    •Reduced interference between strands
    •Easily automated
• Disadvantages:
    •Loss of information density (2D)
    •Lower surface hybridization efficiency
    •Slower surface enzyme kinetics
   DNA Surface Model: Input
DNA strands representing the set {0,1}^n are
synthesized and subsequently immobilized on a
surface in a non-addressed fashion
      Encoding of Binary Information
             in DNA Strands
Word Bit
           1
           2    A strand is comprised of
  1        3
           4
           1
                words. Each word is a
           2
  2        3
           4
                short DNA strand (16mer)
       A
       C
           .
           .    representing one or more
  3    C   .
       T
       .
       .
                bits.
       .
  4
       DNA Word Design Problem
• Requirements of a “DNA code”:
  – Success in specific hybridization between a DNA
    code word and its Watson-crick complement
  – Few false positive signals
• Virtually all designs enforce combinatorial
  constraints on the code words
• Applications:
  – Information storage, retrieval for DNA computing
  – Molecular bar codes for chemical libraries
What combinatorial constraints
 are placed on DNA Codes?
• Hamming: distance between two code
  words should be large
• Reverse complement: distance between a
  word and the reverse complement of
  another word should be large
• Also: frame shift, distinct sub-words,
  forbidden sub-words, …
    Work on DNA code design
• Seeman (1990): de novo design of
  sequences for nucleic acid structural
  engineering
• Brenner (1997): sorting polynucleotides
  using DNA tags
• Shoemaker et al. (1996): analysis of
  yeast deletion mutants using a parallel
  molecular bar-coding strategy
• Many other examples in DNA computing
Word Design Example
   DNA Surface Model: Process
•MARK strands in which bit j = 0 (or 1):
  hybridize with Watson-Crick complements of word
  containing bit j, followed by polymerization
•DESTROY
•UNMARK
  DNA Surface Model: Process

•MARK strands in which bit j = 0 (or 1)
•DESTROY unmarked strands:
  exonuclease degradation
•UNMARK
   DNA Surface Model: Process
MARK strands in which bit j = 0 (or 1):
  hybridize with Watson-Crick complements of word
 containing bit j, followed by polymerization
  DNA Surface Model: Process
•MARK strands in which bit j = 0 (or 1)
•DESTROY unmarked strands
•UNMARK strands:
  wash in distilled water
 DNA Surface Model: Output

• Detect remaining strands (if any)
  by detaching strands from surface and
  amplifying using PCR (polymerase chain
  reaction).
        Computational Power of
         DNA Surface Model
Theorem: Any CNFSAT formula of size m
 can be computed using O(m) mark, unmark
 and destroy operations.

Theorem: Any circuit of size m can be
 computed using O(m) mark, unmark,
 destroy, and append operations.
    Surface DNA Computation:
     the Satisfiability Problem
•Input: 16 strands
•Process: MARK if bit z = 1                      and
            MARK if bit w = 1
            MARK if bit y = 0
            DESTROY                   or       or           or   or
            UNMARK

            MARK if bit w = 0
                                           not         not       not
            MARK if bit y = 0
            DESTROY
            UNMARK                z        w            y        x

            …
•Output: exactly those strands that satisfy
       the circuit remain on the surface.
 DNA Computing on Surfaces:
       Experiments

Students: Tony Frutos, Susan Gillmor,
          Zhen Guo, Qinghua Liu,
          Andy Thiel, Liman Wang
MARK Operation: 4-Base Mismatch Word Design
Repeated MARK, DESTROY, UNMARK Operations
         Append (DNA Ligase)

A. Hybridize with Cb
B. Hybridize with Cab, Wb
C. Ligate; Wash;
   Hybridize with Cb.
 Two-Word Mark and Destroy
A. Mark C1a, C1b, C2b
B. Ligate; Melt single words
C. Destroy; Unmark; Mark C1a, C1b, C2b.
Surface Attachment Chemistry
           Word Readout Strategy

•PCR amplify words
remaining on surface

•Detect PCR products on
single word readout arrays
        4-Variable SAT Demo
•Synthesize; Attach
•Mark
•Destroy     Cycle
•Umark
•Readout
               Conclusions
• DNA computing has expanded the notion of what
  is computation
• Solid-phase chemistry is a promising approach to
  DNA computing
• DNA computing will require greatly improved
  DNA surface attachment chemistries and control
  of chemical and enzymatic processes
• New research problems in combinatorics,
  complexity theory and algorithms
           Open Problem:
        DNA Strand Engineering
Given a DNA strand, there are polynomial-
time algorithms that predict the secondary
structure of the strand.
Inverse Problem: find an
efficient algorithm that,
given a desired secondary
structure, generates a strand
with that structure.

								
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