Bioinformatics - Similarity, identity, and homology
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Related things are often similar and similar things are often related.
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Relationships
are defined by common ancestry: siblings are related to and through their
parents, nieces and nephews through their grandparents. On the same token
different globin molecules are related: they all relate back
to an ancestral heme-incorporating globin-like protein. Relationships among
family members are determined by similarities: biologically through physical
similarities, socially/legally by identical last names. Relationships of
proteins and genes are determined by the degree of similarity among their
sequences. |
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Everybody
has their own, unique DNA sequence, just as everybody has their own,
unique set of fingerprints. Any organism's DNA sequence has been provided
by its forebears; parents for humans, and progenitor cells for bacteria
and other single-cell organisms. Changes in DNA are introduced by two mechanisms:
spontaneous mutations (infrequent) and recombination of paternal and maternal
DNA during meiosis (frequent). Thus, a person's genome does usually not
resemble either her father's or her mother's genome, it is combined of parts
of both of these. The genomes of cells which are derived from mitotic procreation
however, are almost always identical to the genome of the progenitor cell. |
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Homology
is a term used for genes or proteins which are derived from the same ancestor.
Homology cannot be expressed as fraction, either two sequences are derived
from a common ancestor, i.e. homologous, or they are not. Scientists infer
homology from sequence similarity. |
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Similarity
is a measure for relatedness. 100% similarity would be identity. In order
to find out whether things are similar, we frequently compare them side-by-side.
Such as the two images below. Or we find some way to describe them. Such
as the formula that is being used to describe the loops and whorls in fingerprints.
Then, searching for matches does not require to compare images but formulas
can be used to query databases. |
Below are the images of regular hemoglobin and
an artistically "mutated" hemoglobin molecule. Compare the two images and try to find the three defects in
the right one, that would render a person's blood unable to transport oxygen if this molecule really existed.
- Click left image.
- To preserve the large molecule in a new browser window, hold down 'ctrl'-key and press 'n'.
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- Then click the right image.
- Re-size screens that they fit next to each other.
- Compare.
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