• All DNA nucleotides contain the same phosphate group and deoxyribose sugar. The nitrogenous?base?attached to the sugar can vary between nucleotides
• There are?four?different bases:
  • Adenine (A)
  • Cytosine (C)
  • Thymine (T)
  • Guanine (G)

   

• The bases on each strand?pair up?with each other, holding the two strands of DNA in a double helix
• The bases always pair up in the same way:
  • Adenine always pairs with Thymine (A-T)
  • Cytosine always pairs with Guanine (C-G)

   

• The bases in each of these pairs are said to be?complementary?to one another
• This means that the frequency or number of?adenine?is?equal to?the frequency or number of?thymine?and the frequency or number of?cytosine?is?equal to?the frequency or number of?guanine
• In an exam, you could be given the frequency of just?one?of the bases in a DNA molecule and asked to determine the frequency of the?other three?bases

Step 1: Find the number of guanine bases

C = G, therefore 28% of the bases must be guanine

In total, therefore, cytosine and guanine make up 56% of the total bases

Step 2: Find the number of adenine and thymine bases

This leaves 44% of the bases as adenine and thymine

A = T, therefore 22% of the bases must be adenine and 22% of the bases must be thymine