第四章 序列相似性有哪些信誉好的足球投注网站工具blast.ppt

* * * * * * * * * * * * * * * * * * Some properties of the equation E = Kmn e-lS The value of E decreases exponentially with increasing S (higher S values correspond to better alignments). Very high scores correspond to very low E values. The E value for aligning a pair of random sequences must be negative! Otherwise, long random alignments would acquire great scores For E=1, one match with a similar score is expected to occur by chance. For a very much larger or smaller database, you would expect E to vary accordingly From raw scores to bit scores There are two kinds of scores: raw scores (calculated from a substitution matrix) and bit scores (normalized scores) Bit scores are comparable between different searches because they are normalized to account for the use of different scoring matrices and different database sizes S’ = bit score = (lS - lnK) / ln2 The E value corresponding to a given bit score is: E = mn 2 -S’ Bit scores allow you to compare results between different database searches, even using different scoring matrices. 期望值E是得分大于或等于某个分值S的不同的比对的数目在随机的数据库有哪些信誉好的足球投注网站中发生的可能性。最佳匹配得分是 45比特，E值为le－04(也就是1×10－4，或0.0001)。这表明在使用这些特定有哪些信誉好的足球投注网站参数(包括数据库的规模以及打分矩阵的选择)的情况下，打45分或者更好的情况发生的概率是1/10000。 概率p与E的关系： p = 1 - e-E How to interpret BLAST: E values and p values 比对统计学意义的评价—— E值(E-Value) P值(P-Value)(概率值) BLAST程序中使用了E值而非P值，这主要是从直观和便于理解的角度考虑。比如E值等于5和10，总比P值等于0.993和0.99995更直观。但是当E0.01时，P值与E值接近相同 参数K和λ可分别被简单地视为有哪些信誉好的足球投注网站步长(search spacesize)和计分系统(scoring system)的特征数 Very small E values are very similar to p values. E values of about 1 to 10 are far easier to interpret than corresponding p values. E p 10 05 02 01 00.1 0(about 0.1) 0.05 0(about 0.05) 0.001 0(about 0.001) 0.0001 0.0001000 How to interpret BLAST: E values and p values BLAST 应用实例 多结构域蛋白 (H1N1)  脂质运载蛋白 多结构域蛋白 (H1N1) 的BLAST检索 gi|224983683|pdb|3GBN|B Chain B, Crystal Structure Of Fab Cr6261 In Complex With The 1