that does not form part of gene
s, that is which is never transcribe
d into protein
s. The average beastie has an inordinate amount of DNA that seems to serve no purpose. A genome
can be 90% junk
It's not that it's some mysterious stretch of DNA whose function is at present unknown. Some gene whose effect is yet to be discovered. Junk DNA can be a single base repeated hundreds or thousands of times, or a short group of bases. It's impossible that this can contain any information in the genetic code.
Possibilities: it just bulks up the chromosome physically, for some gross chemical purpose. Or it breaks it up logically, so that there is a greater likelihood that an arbitrary crossover point will be between genes rather than within them. But given that crossover within a gene is typically the same as crossover at one end, since alleles are identical to each other almost everywhere unless you happen to get on the wrong side of a SNP, it's a bit dubious how natural selection could favour increasing lengths of junk just to avoid splitting two SNPs. But maybe it does, I don't know.
Some of it is docking sites adajacent to genes, so that the transcription mechanism can identify a gene accurately and get a grip on the beginning. However, these regions may have well-defined marker sequences, so it wouldn't be right to call this junk. In any case, how much of the neighbouring DNA do you really need to identify the boundaries of a gene? Not 90%+.
Some of it might be selfish DNA, sequences that get transcribed and duplicated by RNA without contributing proteins, but that can't explain the untranscribable lengths of CCCCCCC....