Assessment of sequence alignment
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Assessment of sequence alignment. Lecture 10. Introduction. The Dot plot Matrix visualisation matching tool: Basics of Dot plot Examples of Dot plot matching sequences Tandems repeats self matching Inverted repeats: genetic palindromes . Sequence alignment Analysis.

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  • The Dot plot Matrix visualisation matching tool:

    • Basics of Dot plot

    • Examples of Dot plot matching sequences

    • Tandems repeats self matching

    • Inverted repeats: genetic palindromes

Sequence alignment analysis
Sequence alignment Analysis

  • In order to measure the degree of similarity between sequences they must first be aligned to maximise the matching score (refer to lecture 11):

  • Example 1

  • I am from Cork

  • I am not from Cork

  • ****

  • (4 matches out of 18; based on length of bottom string)

  • Example 2

  • I am ---- from Cork

  • I am not from Cork

  • **** **********

  • (14 matches out of 18; based on length of bottom string)

The dot plot
The Dot plot

  • A “better” way of doing this is to represent each sequence as a table or matrix, where one sequence represents the rows and the other the columns. The Dot plot Matrix is a visual way of seeing the alignment between two sequences:

    • The first sequence (query sequence) represents the rows and the other sequence (subject sequence) represents the columns.

    • All elements (row/column) are checked for a match and if there the cell is marked.

    • This will show all areas of both sequences where matches occur.

Dot plot
Dot plot

  • Consider the following:

    • Diagonal lines represent a alignments (match)

    • Horizontal lines between aligned sequences indicate gaps are required (where the gaps indicate a deletion/insertion)

  • This has four “potential” aligned sequences:

    • D->Y;

    • H->N

    • R->0

    • 0->H

  • Longest sequence of alignments are:

    • “THIS” ; and “SEQUENCE“;

    • “IS” would be considered as gaps

  • The pink dots: they can represent noise (spurious alignments)

adapted from understanding bioinformatics p. 77

Dot plot matrix purpose
Dot plot Matrix: purpose

  • This allows us to visualise areas of “local alignment” as opposed to global alignment.

  • One of the main purpose to find domains / motifs that match . This could be useful for many reasons; e.g. promoter factor binding site, finding exons….

  • For visualisation of pair-wise alignment you have one query on the x-axis and the other on the y-axis.

Dot plot noise
Dot Plot noise

This shows the effect of noise (blue line has be been inserted to highlight alignment if interest. The figure on the left represents SH2 sequence (sample files ) plotted against inself. The one on the right has been filter; in this case an alignment must be at least 10 residues long with a score of 3. adapted from understanding bioinformatics p. 77

Dot plot matrix imperfect match
Dot plot Matrix: imperfect match

  • Some alignments require gaps to increase the matching score; the gaps are used represent inclusion/deletion mutations

  • The diagram shows that most of the 2 sequences are aligned. Where there are gaps indicates areas of non-alignment or mismatches: gaps or substitutions

Adapted from: dotplot example

Dot plot example 1

Refer to saved web page

Dot plot: example 1

Dot plot for tandem repeats
Dot plot for Tandem Repeats

  • The human genome has many tandem repeats small sequences of nucleic acids (bases)/ Amino acids that are repeated and are ubiquitous in genomes and can compromise 50% of genome. (Richard 2008)

  • They can be used as genealogical markers

  • To determine specific regions of interest; e.g. introns

  • Play a significant part in evolution Gemayel 2010

  • An example of a protein with multiple repeats is human mucin (Baxevanis 2005 p. 297)

Tandem repeat dot plot
Tandem repeat dot plot

  • To determine if there is tandem repeats the sequence is compared with itself (refer table 1)

  • The more diagonals the more repeats

  • The diagonals at the bottom left compare the start with the finish

  • The fact the main diagonal means the both sequences are the same .

  • The lines are symmetrical around the main diagonal:

Tandem repeats example
Tandem repeats (Example)

  • BRCA2 gene has a number of BRC repeats (39 residues long. The diagram shows two plots: one with noise (unfiltered) and the other showing two repeating sequences. Adapted from Figure 4.3 understanding bioinformatics

Genetic palindromes
Genetic “Palindromes”

  • A palindrome is a word that is spelt the same from right to left as well as from left to write: This will give an “X” shaped dot-plot. (try; eye, navan; never odd or even …..)

  • Remember left to right is (5’ to 3’) on primary strand and right to left is (5’ to 3’) on the complimentary strand. Alternatively it means a match between a strand and its reverse compliment.

  • 2 possible types of “Genetic Palindromes” [the difference being that the left to right, read, is on one strand while the right to left, read, is on its complimentary strand]:

    • Restrictive enzymes such as EcoR1:

      • 5’ GAATTC 3’

      • 3’ CTTAAG 5’

    • Inverted repeats

      • On different segments; each repeat read the same (GTGAG) but in opposite directions. An example is promoter region for the CAP protein in the lac operon :

        • 5‘ GTGAGnnnCTCAC 3'3' CACTCnnnGAGTG 5’

  • What will the dot plot for the above 2 sequences look like.

Supplementary reading
Supplementary reading

  • The following provides links to further reading on DOT PLOTS.

    • introduction to dotplot (figure 6 gives a more indepth view of different types of plots referred to above: alignment, alignment with gaps, tandem repeats, palindromes…..

    • Inverted repeats and dotplot. (more advanced analysis of plots for inverted repeats)

Exam question
Exam Question

  • Describe, using a suitable example, how to construct a dot plot matrix for the alignment of DNA/AA sequences. (10 marks)

  • Describe the significance of two types of repeating sequences found in DNA sequences (6 marks)

  • Explain, using suitable examples, how the DOT plot matrix can find the two types of repeating regions [what is plotted against what and what will the DOT PLOT look like] (14 marks)


  • Baxevanis A.D. 2005 Bioinformatics: a practical guide to the analysis of genes and proteins chapter 11; Wiley

  • Klug, W. S. (2010); the essentials of genetics; 7th ed Pearson Education

  • Gemayel, R. et al 2010 Variable tandem repeats accelerate evolution of coding and regulatory sequences. Annu Rev genet 44: 445-477

  • Richard, G.F. (2008) Comparative genomics and molecular dynamics of DNA repeats in eukaryotes. Microbiol Mol biol rev 2008 Dec;72(4):686-727