Sequence Alignment

Topics:

• Introduction

• Exact Algorithm

• Alignment Models

• BioPerl functions

Sequence Alignment

•Storing, retrieving and comparing DNA sequences in Databases.

•Comparing two or more sequences for similarities.

•Searching databases for related sequences and subsequences.

•Exploring frequently occurring patterns of nucleotides.

•Finding informative elements in protein and DNA sequences.

•Various experimental applications (reconstruction of DNA, etc.)

Motivation:

Seq.Align. Protein Function

Similar functionMore than 25% sequence identity

?Similar 3D structure

?

Similar sequences produce similar proteins

Gene1 Gene2

?

Alignment - inexact matching

•Substitution - replacing a sequence base by another.•Insertion - an insertion of a base (letter) or

several bases to the sequence. •Deletion - deleting a base (or more) from the

sequence.

(Insertion and deletion are the reverse of one another)

Seq. Align. Score

Commonly used matrices: PAM250, BLOSUM64

Local Alignment

Local AlignmentINPUT: Two sequences S and T .QUESTION: What is the maximum similarity between a

subsequence of S and a subsequence of T ? Find most similar subsequences.

The IDEAs[1…n]t[1…m]

To align s[1...i] with t[1…j] we have three choices:

* align s[1…i-1] with t[1…j-1] and match s[i] with t[j]

* align s[1…i] with t[1…j-1] and match a space with t[j]

* align s[1…i-1] with t[1…j] and match s[i] with a space

s[1…i-1] it[1…j-1] j

s[1… i ] -t[1…j-1] j

s[1…i-1] it[1… j ] -

Local Alignment

Smith-Waterman 1981

*Penalties should be negative*

Local alignment

match 2 mismatch -1

cxde c-de

s: xxxcde

t: abcxdex

xxxcde- - - -- - -

abcxdex

Sequence Alignment

Complexity:

Time O(n*m)

Space O(n*m) (exist algorithm with O(min(n,m)) )

Global Alignment

Global AlignmentINPUT: Two sequences S and T of roughly the same length. QUESTION: What is the maximum similarity between them? Find one of the best alignments.

Global Alignment

Alignment Score: V(n,m)

Needleman-Wunsch 1970

Ends free alignment

Ends free alignment INPUT: Two equences S and T (possibly of different length). QUESTION: Find one of the best alignments between subsequences of S and T when at least one of these subsequences is a prefix of the original sequence and one (not necessarily the other) is a suffix.

or

Ends free alignment

m

n

Gap Alignment

Definition: A gap is the maximal contiguous run of spaces in a single sequence within a given alignment.The length of a gap is the number of indel operations on it. A gap penalty function is a function that measure the cost of a gap as a (nonlinear) function of its length. Gap penalty INPUT: Two sequences S and T (possibly of different length). QUESTION: Find one of the best alignments between the two sequences using the gap penalty function.Affine Gap: Wtotal = Wg + qWs

Wg – weight to open the gap

Ws – weight to extend the gap

BioPerl

“Bioperl is a collection of perl modules that facilitate the development of perl scripts for bio-informatics applications.”

Bioperl is open source software that is still under active development.

www.bioperl.orgTutorialDocumentation

BioPerl

• Accessing sequence data from local and remote databases • Transforming formats of database/ file records • Manipulating individual sequences • Searching for "similar" sequences • Creating and manipulating sequence alignments • Searching for genes and other structures on genomic DNA • Developing machine readable sequence annotations

BioPerl library at TAU

BioPerl is NOT yet installed globally on CS network.In each script you should add the following two lines:

use lib "/a/netapp/vol/vol0/home/silly6/mol/lib/BioPerl/lib";use lib "/a/netapp/vol/vol0/home/silly6/mol/lib/BioPerl/lib/i686-linux";

Sequence Object

Seq – stores sequence, identification labels (id, accession number, molecule type = DNA, RNA, Protein, …), multiple annotations and associated “sequence features”.

Sequence Object

$seq = Bio::Seq->new('-seq'=>'actgtggcgtcaact', '-desc'=>'Sample Bio::Seq

object', '-display_id' => 'something',

'-accession_number' => 'accnum', '-moltype' => 'dna' );

Usually Seq is not created this way.

Sequence Object

$seqobj->display_id(); # the human read-able id of the sequence $seqobj->seq(); # string of sequence $seqobj->subseq(5,10); # part of the sequence as a string $seqobj->accession_number(); # when there, the accession number $seqobj->moltype(); # one of 'dna','rna','protein' $seqobj->primary_id(); # a unique id for this sequence irregardless

# of its display_id or accession number

Accessing Data Base

$gb = new Bio::DB::GenBank(); $seq1 = $gb->get_Seq_by_id('MUSIGHBA1'); $seq2 = $gb->get_Seq_by_acc('AF303112')); $seqio = $gb->get_Stream_by_batch([ qw(J00522 AF303112 2981014)]));

Databases: genbank, genpept, swissprot and gdb.

Seq module

ATGGAGCCCAAGCAAGGATACCTTCTTGTAAAATTGATAGAAGCTCGCAAGCTAGCATCTAAGGATGTGGGCGGAGGGTCAGATCCATAC

MEPKQGYLLVKLIEARKLASKDVGGGSDPY

use Bio::DB::GenBank;

$gb = new Bio::DB::GenBank(); $seq1 = $gb->get_Seq_by_acc('AF303112');

$seq2=$seq1->trunc(1,90); print $seq2->seq(), "\n";

$seq3=$seq2->translate;

print $seq3->seq(), “\n“;

SeqIO object

$seq = $gb->get_Seq_by_acc('AF303112')); $out = Bio::SeqIO->new('-file' => ">f.fasta",

'-format' => 'Fasta');

$out->write_seq($seq);

SeqIO can read/write/transform data in the following formats :

Fasta, EMBL. GenBank, Swissprot, PIR, GCG,SCF, ACE, BSML

Transforming Sequence Files

$in = Bio::SeqIO->new('-file' => “f.fasta", '-format' => 'Fasta');$out = Bio::SeqIO->new('-file' => ">f.embl", '-format' => ‘EMBL');

$out->write_seq($in->next_seq());

#for several sequences while ( my $seq = $in->next_seq() ) {

$out->write_seq($seq); }

#better$in = Bio::SeqIO->new('-file' => “f.fasta",

'-format' => 'Fasta');$out = Bio::SeqIO->new('-file' => ">f.embl",

'-format' => ‘EMBL');# Fasta<->EMBL format converter:

print $out $_ while <$in>;

BioPerl: Pairwise Sequence Alignment

use Bio::Tools::pSW;

$factory = new Bio::Tools::pSW( '-matrix' => 'blosum62.bla',

'-gap' => 12,

'-ext' => 2, );

$factory->align_and_show($seq1, $seq2, STDOUT);

Smith-Waterman Algorithm

Currently works only on protein sequences.

Alignment Objects

SimpleAlign handles multiple alignments of sequences.

#pSW module

$aln = $factory->pairwise_alignment($seq1, $seq2);

foreach $seq ( $aln->eachSeq() ) { print $seq->seq(), "\n"; }

$alnout = Bio::AlignIO->new(-format => 'fasta', -fh => \*STDOUT);

$alnout->write_aln($aln);

Homework

Write a cgi script (using Perl) that performs pairwise Local/Global Alignment for DNA sequences. All I/O is via HTML only.

Input:

1. Choice for Local/Global alignment.

2. Two sequences – text boxes or two accession numbers.

3. Values for match, mismatch, ins/dels.

4. Number of iterations for computing random scores.

Output:

1. Alignment score.

2. z-score value (z= (score-average)/standard deviation.)

Remarks: 1) you are allowed to use only linear space.2)To compute z-score perform random shuffling:srand(time| $$); #init, $$-proc.idint(rand($i)); #returns rand. number between [0,$i].3)Shuffling is done in windows (non-overlapping) of 10 bases length. Number of shuffling for each window is random [0,10].