Lecture 2: Describing Microbial Diversity: the ... - MCD Biology

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B. The best way is by direct comparison of sequences of nucleic acids or proteins. This provides “precise”
numbers for defining relationships between molecules -- and, ideally, organisms.
6. For “orthologous” (of common ancestry and function) nucleic acid (or protein) sequences:
A. Consider:
--Organism X • • • AGCUGCCAGU • • •
X XX
--Organism Y • • • AACCCCCAGU • • •
"DNA OR RNA?
Sequence X is 70% identical to Sequence Y,
Fractional identity is 0.7
Fractional difference is 0.3 (1-0.7)
1) Note the terms: “homologous” = of common ancestry; “orthologous” = of common ancestry and function.
2) Note that the term “homology” is commonly used incorrectly when “identity” is meant. Note that
nucleotide sequences are not “##% similar”, they are “##% identical”; protein seqs, on the other hand, can
be “similar”. (How is that?)