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Number of substitutions between two protein-coding genes. Dan Graur. Handout. Computing the number of substitutions between two protein-coding sequences is more complicated, because a distinction should be made between synonymous and nonsynonymous substitutions.
Computing the number of substitutions between two protein-coding sequences is more complicated, because a distinction should be made between synonymous and nonsynonymous substitutions.
Aims:1. Compute two numerators: The numbers of synonymous and nonsynonymous substitutions.2. Computetwo denominators: The numbers of synonymous and nonsynonymous sites.
Difficulties with denominator:
1. The classification of a site changes with time: For example, the third position of CGG (Arg) is synonymous. However, if the first position changes to T, then the third position of the resulting codon, TGG (Trp), becomes nonsynonymous.
2. Many sites are neither completely synonymous nor completely nonsynonymous. For example, a transition in the third position of GAT (Asp) will be synonymous, while a transversion to GAG or GAA will alter the amino acid.
Difficulties with numerator:1.When two homologous codons differ from each other by two substitutions or more the order of the substitutions must be known in order to classify substitutions into synonymous and nonsynonymous.
Example: CCC in sequence 1 and CAA in sequence 2.
CCC (Pro) CCA (Pro) CAA (Gln)
1 synonymous and 1 nonsynonymous
CCC (Pro) CAC (His) CAA (Gln)
Difficulties with numerator:2. Transitions occur with different frequencies than transversions. 3. The type of substitution depends on the mutation. Transitions result more frequently in synonymous substitutions than transversions.
1. Classification of sites. Consider a particular position in a codon. Let i be the number of possible synonymous changes at this site. Then this site is counted as i/3 synonymous and (3 – i)/3 nonsynonymous.
In TTT (Phe), the first two positions are nonsynonymous, because no synonymous change can occur in them, and the third position is 1/3 synonymous and 2/3 nonsynonymous because one of the three possible changes is synonymous.
2. Count the number of synonymous and nonsynonymous sites in each sequence and compute the averages between the two sequences. The average number of synonymous sites is NS and that of nonsynonymous sites is NA.
Pathway (1) requires one synonymous and one nonsynonymous change, whereas pathway (2) requires two nonsynonymous changes.
The unweighted method:Average the numbers of the different types of substitutions for all the possible scenarios. For example, if we assume that the two pathways are equally likely, then the number of nonsynonymous differences is (1 + 2)/2 = 1.5, and the number of synonymous differences is (1 + 0)/2 = 0.5.
The weighted method. Employ a priori criteria to assign the probability of each pathway. For instance, if the weight of pathway 1 is 0.9, and the weight for pathway 2 is 0.1, then the number of nonsynonymous differences between the two codons is (0.9 1) + (0.1 2) = 1.1, and the number of synonymous differences is 0.9.
The number of synonymous differences per synonymous site is pS = MS/NS The number of nonsynonymous differences per nonsynonymous site is pA = MA/NA
p = n /L
The Poisson model is used to convert p into the number of amino replacements between two sequences (d ):
d = – ln(1 – p)
The variance of d is estimated as
V(d) = p/L (1 – p)