Tandem Repeat Concepts

Introduction

This page is about Tandem Repeats.

Definition

A series of repeats that are contiguous

Concepts

Motif Canonical Class

Using the concepts of shifting, the canonical class of a set of motifs can be defined as a equivalence relationship.

  ACG ~ CGA if there exists a shift that allows s(ACG, i) = CGA

This relationship can be show to be reflexive, symmetric and transitive. And a equivalence class can be defined on this.

Example:

This is a distribution of motifs without collapsing

 A
 C
 G
 T
 AA
 CC
 GG 
 TT
 AC
 AG
 AT
 CG
 CT
 GT
 CA
 GA
 

show all permutations show acyclic show shift show reverse complement

Shifting

Consider the motifs ACTT, TACT and TTAC, the following stretches can be observed in the genome

 GGGGGGACTTACTTACTTACTTACTTAGGGGG
 GGGGGGTACTTACTTACTTACTTACTTGGGGG
 GGGGGGTTACTTACTTACTTACTTACTGGGGG

but looking from the right flank, they can easily be CTTA, ACTT and TACT respectively.

The concept of shifting the sequence is useful for grouping such like motifs together.

We define shift as follow

 A shift of a sequence is the sliding of the sequence with the alleles wrapped to the front?

Reverse Complement

Reverse complement is is a common concept in sequence analysis.

 ACCCTCCCCTCT
 AGAGGGGAGGGT

Acyclicity

Motifs are required to be acyclic. For example, a motif ACACAC should just be represented by AC as it is 3 copies of AC.

  A sequence is cyclic if and only if there exists a sub sequence in which it is a multiple copy of.

The definition can be more explicit as follows:

 A sequence is cyclic if and only if there exists a non trivial shift of the sequence that is equivalent to  the sequence.

Take for example, the sequence ACACA, is this a bona fide motif? After it seems like it is 2.5 copies of AC and AC might be more appropriate.

 shift 0: ACACA
 shift 1: CACAA
 shift 2: ACAAC
 shift 3: CAACA
 shift 4: AACAC

So ACACA is a bona fide motif.

Fractional counts

While it is natural to think of a stretch of repeat as a integer, it is useful to consider fractional counts of a repeat especially in large repeat tracts. This is done in Tandem Repeat Finder.

 GGGTTAAGGGTTAAGGGTTAAGGGTTAAGGGTTAAGGG

This is 5.5 copies of the repeat GGGTTAA

TRF Scoring

A nice useful scheme is +2 for matches and -7 for indels and mismatches, and it provides a convenient way of scoring for purity.

Classification

• motif length
• motif basis
• repeat tract lengfth
• purity

Implementation

This is implemented in vt.

Maintained by

This page is maintained by Adrian.