Answer:
Nitrogen Base
Explanation:
the square is where the bases connect to get different patterns inside your DNA
Transcription is the process of transcribing DNA into mRNA, this process is catalyzed by RNA polymerase. mRNA is produced from DNA in transcription
It is definitely not a primary source: this would be a book itself.
Now, is it a tertiary source? I don't think so: the author of the review should have read the book and should only be referring to this book in the review.
I think it's a secondary source.
