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Does anyone know what is the difference between two miRNAs like hsa-miR-33a and hsa-miR-33b? The last letter in them shows what?

Also, if we know that hsa-miR-33a targets gene A, can we say that hsa-miR-33b targets gene A, too? or can we say that it is much probable that hsa-miR-33b targets gene A since hsa-miR-33a targets A?

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They are paralogs i.e. they have high level of sequence homology. They may have the same seed sequence too (see this). The paralogs can have same targets if they have same seed sequence but it is not essential in all cases. However, the extent of homology required for this kind of classification is poorly defined. miRBase still follows the guidelines mentioned in this (quite old) article:

When such a candidate is so similar in sequence to the known miRNA that the probe designed to detect it would surely cross-hybridize with the known miRNA, criterion A need not be satisfied, and the candidate can be annotated as a variant form of the known miRNA, provided that the candidate itself meets criterion C, and there is also very high confidence that one of these paralogs is a confirmed miRNA (i.e., its classification is supported by at least three of the listed criteria). When there are two or more mismatches between the miRNA paralogs such that probes could differentiate between the species on Northern blots, criterion A must be satisfied experimentally. Sometimes partially overlapping cDNA sequences are identified from the same locus; these are assumed to represent differentially processed products from the gene.

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  • $\begingroup$ mind that homology is a 0 or 1 term. I mean two sequences are homolog or not. Its better to use "similarity" instead of "homology". $\endgroup$
    – MySky
    Commented Apr 30, 2017 at 7:54
  • $\begingroup$ @MySky not really. Percent homology is a commonly used term. $\endgroup$
    – WYSIWYG
    Commented Apr 30, 2017 at 8:36
  • $\begingroup$ The term "percent homology" is often used to mean "sequence similarity". The percentage of identical residues (percent identity) or the percentage of residues conserved with similar physicochemical properties (percent similarity), e.g. leucine and isoleucine, is usually used to "quantify the homology". Based on the definition of homology specified above this terminology is incorrect since sequence similarity is the observation, homology is the conclusion. Sequences are either homologous or not. $\endgroup$
    – MySky
    Commented May 3, 2017 at 17:34
  • $\begingroup$ look at this page: en.wikipedia.org/wiki/Sequence_homology $\endgroup$
    – MySky
    Commented May 3, 2017 at 17:34

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