January 15, 2022
John Doe
There are about 3 billion of them in each cell, making a code that is unique – resulting in you, a unique individual! The code pairs up so that G-C and A-T always combine to make the double-helix that you see in the picture, such that GATTACA would be paired with CTAATGT (the complement). The code is read in a specific direction so GATTACA on one strand would be TGTAATC on the other (the reverse complement). One of the good things about BLAST is that it doesn’t care which version you give it, it will still point you to the correct gene.
One other thing to note at this point is probabilities. You did this at school with tossing a coin (where the code is H for heads or T for tails). What would be the probability of HHHH (1 in 2^4 = 1/16). The same applies for TTTT. The same applies for THTH, or any specific sequence of coin tosses. Try it yourself if you don’t believe me (predict the sequence first and then see how many times you have to run it). Genetic code is essentially a “four sided coin”. So for any run of a specific sequence (e.g. GATC) the probability of getting that EXACT sequence is 1 in 4^4, or for any number n of nucleotides (nt or bases) the chance is 1 in 4^n (this is simplified because in some situations the probability of the next base being X depends on the surrounding bases).
BLAST has two sections – nucleotide (BLASTn) and protein (BLASTp). BLASTp deals with amino acid sequences, in just the same way as nucleotide sequences. But there is a big difference because there are 20 amino acids (rather than 4 nucleotides) and therefore even short runs (e.g. QTNS = Glu-Thr-Asn-Ser) would carry a probability of somewhere around 1 in 20^4 (simplified), which is 1 in 160,000. The probability of a specific 5-amino acid sequence arising by random chance on the same basis jumps to 1 in 3.2million!
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#DNA