. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 272 R. F. DOOLITTLE HEMOGLOBIN PRESENT -200 -400 -600 -800 -1000. Primates Bony Fishes Jawless Fishes Invertebrates Ol o c in s_ s o. o. <r Plants FIGURE 1. Gene duplication event that led to the existence of hemoglobin « and fi chains. The timescale is based on how fast the amino acid sequences are changing as determined by species comparisons (from Doolittle, 1984). to an internal repeat may have taken place. Although this is a very useful method, it is often subject to a certain amount of misinterpretation, and it is w
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 272 R. F. DOOLITTLE HEMOGLOBIN PRESENT -200 -400 -600 -800 -1000. Primates Bony Fishes Jawless Fishes Invertebrates Ol o c in s_ s o. o. <r Plants FIGURE 1. Gene duplication event that led to the existence of hemoglobin « and fi chains. The timescale is based on how fast the amino acid sequences are changing as determined by species comparisons (from Doolittle, 1984). to an internal repeat may have taken place. Although this is a very useful method, it is often subject to a certain amount of misinterpretation, and it is worth our while to remind ourselves how it works. First, it is a fact that the sequences of various proteins change at different but characteristic rates. For many proteins, as we shall see, the rate of change is really quite constant. In some situations, nonetheless, the rate of change of a particular type of protein may speed up or slow down, and we must be on the watch for a certain amount of quirkiness in molecular clocks. When they run smoothly they'll be in accord with what we find by the "occurrence ; Consider a well known example. The a and ft chains of vertebrate hemoglobin have been sequenced from many different species in all five classes. By consulting the divergence times in Table II, which are based on the fossil record, and quantitatively comparing the sequences from members of each group, we can estimate that the two chains are each changing at a rate of about 11 amino acid replacements per 100 residues per 100 million years. Today the two sequences in most vertebrates are 55% different (45% identical). As it happens, sequence change follows an exponential course (because of back mutations and multiple changes at the same site), and a 55% difference is actually equivalent to 90 actual changes for every 100 residues. At a rate of 11 per 100 million years, then, the time since the duplication that gave rise to divergence must be about 400 million
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
