Question

Bioinformatics:

What could be some of the reasons why your query sequence did not exactly match the sequence in the database (think of sampling, sequencing, and biological reasons).

Answer 1

Query sequences are not exactly matches to the database sequences because each life form is different from other. All life forms have same basic DNA sequences; A, T, G and C but the arrangements are so unique that in every form of life different combination of A, T, G, and C occur. Therefore, sequences show similarity but not exactly. Suppose in case of humans, as we all belong to the common ancestral, we must share sequence similarity with them but not exactly due to course of time (evolution).

The most common reason for similarity is DNA sequences, rather than protein sequences are compared.

Moreover, homologous sequences not always show significant similarity because thousands of homologous protein alignments are not significant, but are clearly homologous based on statistically significant structural similarity. Thus, when a similarity search finds a statistically significant match, we can confidently infer that the two sequences are homologous; but if no statistically significant match is found in a database, we cannot be certain that no homologs are present.