Finding genome coverage using random reads

[smandape1]

Thank you for looking at my question. I am trying to solve this homework question.

Consider the problem of sequencing genome by random reads. If G is the length of the entire sequence, L is the length of the read and n is the number of reads, then coverage is defined as nL/G . Now, if we want 50% of the original long sequence to be covered by at least one fragment, how much coverage do we need?

I read Lander-Waterman http://www.genetics.wustl.edu/bio548...005/Lander.htm model to understand the concept. But didn't quite get how to solve this problem. I thought to consider the given 50% as probability and y as 1 (the one from Poisson distribution) and calculate lambda (that is the coverage). But I don't think I am on right track. I thought of considering y as 1 because the question says 50% of the original long sequence to be covered by atleast one fragment, which means that those bases are sequenced atleast once.

I may be wrong.

Experts can you guide me please.

Thank you.

[Heisman]

I think you are on the right track, actually. Search for the following sentence in the link you provided and you'll see it is doing what you describe here:

"Given 10x coverage, if we want to calculate the probability that a base is sequenced three times, we just need to substitute 3 for y and 10 for C into the formula:"


Also, you won't get much homework help here as this forum is not designed for that purpose. Your question is actually a very basic statistics question and would be more properly asked at some sort of math forum.

Also, in real life the coverage may not be close to being randomly distributed due to varying GC content affecting coverage to different degrees (depending on the sequencing method and preparation.