Structure & Base Pairing

Why are only some bases compatible with each other in DNA?

Because of their shapes and hydrogen-bonding patterns — only complementary bases form stable hydrogen bonds (A with T; G with C).

Which base pairs are always found together in DNA?

Adenine (A) always pairs with Thymine (T); Guanine (G) always pairs with Cytosine (C).

What does it mean that the two DNA strands are complementary?

The base sequence on one strand determines the opposite strand’s sequence because A pairs with T and G pairs with C; knowing one strand lets you infer the other.

How many base pairs make one full twist (turn) of the DNA ladder?

Ten base pairs per turn — the ladder makes a twist every ten bases.

What are Chargaff’s rules?

In a given species, the amount of A equals the amount of T, and the amount of G equals the amount of C. Overall base composition (A+T vs G+C) varies between species.

If a DNA sample contains 500 adenosine nucleotides (A), how many thymine (T) nucleotides does it contain? Show the reasoning.

By Chargaff’s rule A = T. If A = 500, then T = 500. (Therefore there are 500 thymine nucleotides.)