Assume that a 100 base pair DNA ladder contains 45 cytosines. How many guanines are there?

45 guanines

Explanation:

In a DNA strand, each nitrogen base only pairs with one other specific base.

Adenine pairs with thymine, and cytosine pairs with guanine.

This means that the amount of adenine is always equal to the amount of thymine, and the amount of cytosine is always equal to the amount of guanine.

So, if the DNA ladder has 45 cytosines, there will be 45 guanines.

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