Why is using the definite article “the” before a plural countable noun correct in this case when I have not mentioned the plural noun before?

The rule about using the definite article for a specific noun (whether singular or plural) is not that it has been mentioned before. The rule is:

The definite article (the) is used before a noun to indicate that the identity of the noun is known to the reader. [Butte College Education; emphasis mine]

Very often the identity of the noun is known to the reader because it has been previously mentioned, but it can also be the case that the reader knows the identity of the noun by implication. For example:

• Yesterday's soccer game ended suddenly when the fans started rioting.

It has not been previously mentioned that there were fans at the soccer game, but it can be assumed that the listener is familiar with public sporting events and the fact that they normally involve spectators – the mention of the game implies the existence of fans.

Sometimes the implication is contained within the logical structure of the sentence itself. For example:

• I need to keep our dinner in the oven for at least ten more minutes – the potatoes are still too hard.

One can say this without previously mentioning that the dinner has potatoes in it. However, "dinner" has been previously mentioned, so by using the definite article with "the potatoes," it is implied that the previously mentioned dinner contains potatoes.

This is exactly how "the chromosomes" works in your example – either the reader has knowledge of basic biology and already knows that a nucleus contains chromosomes (they way you know a soccer game has fans in attendance) or they are being informed by this definition that a nucleus contains chromosomes (the way the second example informed the reader that the dinner contains potatoes).