Introduction to Bioinformatics Algorithms Chapter 5 Greedy Algorithms and Genome Rearrangements By: Hasnaa Imad

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Outline What are GREEDY Algorithms? What are Genome Rearrangements? Cabbage vs. Turnip Human vs. mice What is the relation between them? Sorting by Reversals Approximation Algorithms

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info What are GREEDY Algorithms? Most algorithms are iterative procedures. GREEDY algorithms choose the “most attractive” alternative at each iteration. GAs often return suboptimal results but take very short time. Few GAs find optimal solutions.

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Why are cabbages and turnips look and taste different even they share a recent common ancestor? What are Genome Rearrangements? Cabbage vs. Turnip

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info What are Genome Rearrangements? In 1980s Jeffrey Palmer studied evolution of plant organelles by comparing mitochondrial genomes of the cabbage and turnip Cabbage vs. Turnip

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info 99% similarity between genes These surprisingly identical gene sequences differed in gene order This study helped pave the way to analyzing genome rearrangements in molecular evolution What are Genome Rearrangements? Cabbage vs. Turnip

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Gene order comparison: What are Genome Rearrangements? Cabbage vs. Turnip

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Gene order comparison: What are Genome Rearrangements? Cabbage vs. Turnip

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Gene order comparison: What are Genome Rearrangements? Cabbage vs. Turnip

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Gene order comparison: What are Genome Rearrangements? Cabbage vs. Turnip

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Gene order comparison: Before After Evolution is manifested as the divergence in gene order What are Genome Rearrangements? Cabbage vs. Turnip

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info What are Genome Rearrangements? Human vs. mice Mice and humans share virtually the same set of genes Mice are well-established experimental model. Genes easily found in mice genome sequences Test experimentally the function of those genes.

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Waardenburg ’ s Syndrome: Mouse Provides Insight into Human Genetic Disorder Waardenburg’s syndrome is characterized by pigmentary dysphasia Gene implicated in the disease was linked to human chromosome 2 but it was not clear where exactly it is located on chromosome 2

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Waardenburg ’ s syndrome and splotch mice A breed of mice (with splotch gene) had similar symptoms caused by the same type of gene as in humans Scientists succeeded in identifying location of gene responsible for disorder in mice Finding the gene in mice gives clues to where the same gene is located in humans

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info So… What is the relation between Greedy algorithms and Genome Rearrangements?

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Biologists are interested in evolutionary scenario.. Scenario involves finding the smallest number of reversals (flips). Not easy to find them. Relation is.. Mouse X Chromosome Order: Human X Chromosome Order:

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Two Solutions 1- Sorting by reversals 2- Approximation Algorithms

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Reversals and Gene Orders Gene order is represented by a permutation   1   i-1  i  i  j-1  j  j  n   1   i-1  j  j  i+1  i  j  n Reversal  ( i, j ) reverses (flips) the elements from i to j in  ,j)

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Reversals: Example  =  (3,5) 

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Reversals: Example  =  (3,5)  (5,6)

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Reversal Distance Problem Goal: Given two permutations, find the shortest series of reversals that transforms one into another Input: Permutations  and  Output: A series of reversals  1,…  t transforming  into  such that t is minimum t - reversal distance between  and  d( ,  ) - smallest possible value of t, given  and  Please Try Exercise 1

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Sorting By Reversals Problem Goal: Given a permutation, find a shortest series of reversals that transforms it into the identity permutation (1 2 … n ) Input: Permutation  Output: A series of reversals  1, …  t transforming  into the identity permutation such that t is minimum

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Sorting By Reversals: Example t =d(  ) - reversal distance of  Example :  = So d(  ) = 3

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Signed Permutations Up to this point, all permutations to sort were unsigned But genes have directions… so we should consider signed permutations 5’5’ 3’3’  =

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Sorting by reversals: 5 steps

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Sorting by reversals: 4 steps

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Sorting by reversals: 4 steps What is the reversal distance for this permutation? Can it be sorted in 3 steps?

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Pancake Flipping Problem The chef is sloppy; he prepares an unordered stack of pancakes of different sizes The waiter wants to rearrange them (so that the smallest winds up on top, and so on, down to the largest at the bottom) He does it by flipping over several from the top, repeating this as many times as necessary Christos Papadimitrou and Bill Gates flip pancakes

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Pancake Flipping Problem: Greedy Algorithm Greedy approach: 2 prefix reversals at most to place a pancake in its right position, 2n – 2 steps total at most William Gates and Christos Papadimitriou showed in the mid-1970s that this problem can be solved by at most 5/3 (n + 1) prefix reversals This Problem remains unsolved.

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Pancake Flipping Problem: Formulation Goal: Given a stack of n pancakes, what is the minimum number of flips to rearrange them into perfect stack? Input: Permutation  Output: A series of prefix reversals  1, …  t transforming  into the identity permutation such that t is minimum

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Sorting By Reversals: A Greedy Algorithm If sorting permutation  = , the first three elements are already in order so it does not make any sense to break them. The length of the already sorted prefix of  is denoted prefix(  ) prefix(  ) = 3 This results in an idea for a greedy algorithm: increase prefix(  ) at every step

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Doing so,  can be sorted Number of steps to sort permutation of length n is at most (n – 1) Greedy Algorithm: An Example

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Greedy Algorithm: Pseudocode SimpleReversalSort(  ) 1 for i  1 to n – 1 2 j  position of element i in  (i.e.,  j = i) 3 if j ≠i 4    *  (i, j) 5 output  6 if  is the identity permutation 7 return

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Analyzing SimpleReversalSort SimpleReversalSort does not guarantee the smallest number of reversals and takes five steps on  = : Step 1: Step 2: Step 3: Step 4: Step 5:

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info But it can be sorted in two steps:  = Step 1: Step 2: So, SimpleReversalSort(  ) is not optimal Optimal algorithms are unknown for many problems; approximation algorithms are used Analyzing SimpleReversalSort (cont ’ d)

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Approximation Algorithms These algorithms find approximate solutions rather than optimal solutions The approximation ratio of an algorithm A on input  is: A(  ) / OPT(  ) where A(  ) -solution produced by algorithm A OPT(  ) - optimal solution of the problem

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Approximation Ratio/Performance Guarantee Approximation ratio (performance guarantee) of algorithm A: max approximation ratio of all inputs of size n For algorithm A that minimizes objective function (minimization algorithm): max |  | = n A(  ) / OPT(  )

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Approximation Ratio/Performance Guarantee Approximation ratio (performance guarantee) of algorithm A: max approximation ratio of all inputs of size n For algorithm A that minimizes objective function (minimization algorithm): max |  | = n A(  ) / OPT(  ) ex: 6/8 For maximization algorithm: min |  | = n A(  ) / OPT(  ) ex: 10/8

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info  =    2  3 …  n-1  n A pair of elements  i and  i + 1 are adjacent if  i+1 =  i + 1 For example:  = (3, 4) or (7, 8) and (6,5) are adjacent pairs Adjacencies and Breakpoints

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info There is a breakpoint between any adjacent element that are non-consecutive:  = Pairs (1,9), (9,3), (4,7), (8,2) and (2,5) form breakpoints of permutation  b(  ) - # breakpoints in permutation   Breakpoints: An Example

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Adjacency & Breakpoints An adjacency - a pair of adjacent elements that are consecutive A breakpoint - a pair of adjacent elements that are not consecutive π = adjacencies breakpoints Extend π with π 0 = 0 and π 7 = 7

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info We put two elements  0 =0 and  n + 1 =n+1 at the ends of  Example: Extending with 0 and 10 Note: A new breakpoint was created after extending Extending Permutations  =  = Please Try Exercise 2: a,b

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info  Each reversal eliminates at most 2 breakpoints.  = b(  ) = b(  ) = b(  ) = b(  ) = 0 Reversal Distance and Breakpoints

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info  Each reversal eliminates at most 2 breakpoints.  This implies: reversal distance ≥ #breakpoints / 2  = b(  ) = b(  ) = b(  ) = b(  ) = 0 Reversal Distance and Breakpoints

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Sorting By Reversals: A Better Greedy Algorithm BreakPointReversalSort(  ) 1 while b(  ) > 0 2 Among all possible reversals, choose reversal  minimizing b(   ) 3     (i, j) 4 output  5 return

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Sorting By Reversals: A Better Greedy Algorithm BreakPointReversalSort(  ) 1 while b(  ) > 0 2 Among all possible reversals, choose reversal  minimizing b(   ) 3     (i, j) 4 output  5 return Problem: this algorithm may work forever

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Strips Strip: an interval between two consecutive breakpoints in a permutation Decreasing strip: strip of elements in decreasing order (e.g. 6 5 and 3 2 ). Increasing strip: strip of elements in increasing order (e.g. 7 8) A single-element strip can be declared either increasing or decreasing. We will choose to declare them as decreasing with exception of the strips with 0 and n+1 Please Try Exercise 2: c

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Reducing the Number of Breakpoints Theorem 1: If permutation  contains at least one decreasing strip, then there exists a reversal  which decreases the number of breakpoints (i.e. b(   ) < b(  ) )

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Things To Consider For  = b(  ) = 5 Choose decreasing strip with the smallest element k in  ( k = 2 in this case)

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Things To Consider (cont’d) For  = b(  ) = 5 Choose decreasing strip with the smallest element k in  ( k = 2 in this case)

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Things To Consider (cont’d) For  = b(  ) = 5 Choose decreasing strip with the smallest element k in  ( k = 2 in this case) Find k – 1 in the permutation

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Things To Consider (cont’d) For  = b(  ) = 5 Choose decreasing strip with the smallest element k in  ( k = 2 in this case) Find k – 1 in the permutation Reverse the segment between k and k-1: b(  ) = b(  ) = 4

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Reducing the Number of Breakpoints Again If there is no decreasing strip, there may be no reversal  that reduces the number of breakpoints (i.e. b(   ) ≥ b(  ) for any reversal  ). By reversing an increasing strip ( # of breakpoints stay unchanged ), we will create a decreasing strip at the next step. Then the number of breakpoints will be reduced in the next step (theorem 1).

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Things To Consider (cont’d) There are no decreasing strips in , for:  = b(  ) = 3   (6,7) = b(  ) = 3  (6,7) does not change the # of breakpoints  (6,7) creates a decreasing strip thus guaranteeing that the next step will decrease the # of breakpoints.

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info ImprovedBreakpointReversalSort ImprovedBreakpointReversalSort(  ) 1 while b(  ) > 0 2 if  has a decreasing strip 3 Among all possible reversals, choose reversal  that minimizes b(   ) 4 else 5 Choose a reversal  that flips an increasing strip in  6     7 output  8 return

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info ImprovedBreakPointReversalSort is an approximation algorithm with a performance guarantee of at most 4 It eliminates at least one breakpoint in every two steps; at most 2b(  ) steps Approximation ratio: 2b(  ) / d(  ) Optimal algorithm eliminates at most 2 breakpoints in every step: d(  )  b(  ) / 2 Performance guarantee: ( 2b(  ) / d(  ) )  [ 2b(  ) / (b(  ) / 2) ] = 4 ImprovedBreakpointReversalSort: Performance Guarantee

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info Please Note I DID NOT cover Section 5.5

An Introduction to Bioinformatics Algorithmswww.bioalgorithms.info References An Introduction to Bioinformatics Algorithms - Neil C.Jones and Pavel A.Pevzner Ch5http://bix.ucsd.edu/bioalgorithms/slides.php# Ch5