If there are 2000 nucleotides in a strand of dna and 600 contain the nitrogen base cytosine, how many nucleotides contain adenine?

The correct answer is 400.

There is a complementary bounding in each DNA helix between nucleotide bases.  

There are two types of nitrogenous bases :the single-ring (thymine and cytosine) also called pyrimidines, and the double-ring bases (adenine and guanine), also called purines. Two DNA strands are complementary paired: cytosines form three hydrogen bonds with guanines, and adenines form two hydrogen bonds with thymines (C and G is one base pair and A – T is another).

So, if there is 2000 nucleotides in double stranded DNA, C=600 and G=600. That means that A+T=800, A=400 and T=400

the sugar-phosphate backbone forms the structural framework of nucleic acids, including dna and rna. this backbone is composed of alternating sugar and phosphate groups, and defines directionality of the molecule.

the second type of galaxy is called a barred spiral, which has a thing called a bar straddling the nucleus