What Is The Rational Explanation For The Letter "N" In The Dna Nucleotide Sequence?

Question

[arulthasan83cyr]

I browsed the NCBI website to observe the nucleotide sequences of the chromosome 1. In the sequence, there are lot of N among the other 4 nucleotide bases. It is well understood that N represents the any one of the 4 basic nucleotide sequence. But, my doubt is that why we could not able to reveal the exact type nucleotide present in that position of sequence. (Please excuse me, If question does not have any sense)

Answer 1

[Devon Ryan]

Some parts of pretty much all the references still contain gaps, often due to having highly repetitive underlying sequence (e.g., around centromeres) or due to high heterogeneity between samples (i.e., multiple samples were used to generate the reads and they simply don't agree on a region, likely because the samples differ there). Assembling a genome turns out to be a rather difficult process.

Answer 2

[Noushin N]

Could it possibly be that you are looking at regions of unresolved sequence or assembly gaps? Regions around telomeres, and centromeres tend to be highly enriched in Ns.