Hm. So it appears that, two years ago, I wrote a post on calculating the average gene length in prokaryotes. I found a half-draft of the second part and decided to finish it off.
In part 1 we defined mGenes (“maybe-genes”) as the pieces you get after breaking the genome at each stop codon, and predicted that the probability of finding an mGene of length L is given by the following equation:
By plotting this function it is clear that the probability of a set of codons being an mGene plummets quickly, so that there is nearly a 0 probility of finding an mGene of 100 codons (300 bases) in a random sequence (black line in Fig. 1). I confirmed this with a 1,000,000 base synthetic genome (all code is at the end of this entry), resulting in the red circles in Fig. 1 that perfectly overlap with the prediction line.
So now we know what to expect from a completely random genome: Nearly all mGenes will be less than 100 codons (300 bases) in length. This is much shorter than your typical gene, and so I would expect there to be a large number of mGenes larger than this size in a real genome. So let’s check it out, using a fully sequenced prokaryotic genome!
One of my side research projects involves processing large numbers of genomes (specifically, all fully-sequenced prokaryotic genomes). Since I’m playing with the data anyway, sometimes I end up with random questions that can be answered with what I already have on hand. One such question is this: “What is the average length of a prokaryotic gene?” We could figure this out fairly directly, but it’s always best to have a prediction in hand first. After all, if we have no idea what kind of values to expect, how can we trust the accuracy of a more direct (and experimental) method?
So what do we know? There are 4 possible bases (A, G, C, and T) and three such bases make up a codon. This means that each position of the codon can be any of 4 bases, so there are 4*4*4 = 64 possible codons. Of these, 3 are stop codons (meaning that they mark the end of a gene). We generally think of there being only 1 start codon (ATG, coding for methionine), but it turns out that prokaryotes often use other codons instead. Plus, if there are multiple ATG’s in the same stretch of DNA, how do we know which is the actual start?
This sequence has two potential start sites (in bold) and two stop codons (in bold italics). We can unambiguously choose the first stop codon, but we have no way of knowing without more evidence which start codon is the real one.
To get around this, let’s take a conservative approach in calling sequences a “gene”. Instead of anything beginning with a start codon and ending with a stop, let’s take the entire genome and blast it to bits by cutting at every stop codon.