# AQA A Level Biology复习笔记1.5.9 Calculating the Frequency of Nucleotide Bases

### Maths Skill: Frequency of Bases

• 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:
• 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

#### Worked Example

In a section of DNA, 28% of the bases were cytosine. Calculate the frequencies 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 