Learn Swedish: Forskaren analyserar genen. Det är nämligen en viktig molekyl. - The researcher is analyzing the gene. It is an important molecule, you see.

Nämligen is a sentence adverb. In a Swedish main clause, sentence adverbs belong in the mid-clause position, which is strictly after the finite (first) verb. In this case, it must go immediately after är. You cannot just tack it onto the end of the sentence like we do with "you see" in English.

How does nämligen connect these two separate sentences?

Nämligen provides the reason or explanation for the statement that just happened. It functions similarly to "because," but grammatically it isn't a conjunction. If you used eftersom (because), you would have to link them into one sentence: Forskaren analyserar genen eftersom det är en viktig molekyl. Using nämligen lets you write two independent sentences while still showing that the second explains the first.

Could I start the second sentence with Nämligen to emphasize the explanation?

No, that would sound extremely unnatural to a Swede. Unlike English, where you might start a sentence with "Namely...", Swedish strongly restricts nämligen to the mid-clause position.

Why does the second sentence start with Det är? Shouldn't it be Den är, since it refers to genen (an en-word)?