AQA A Level Biology復(fù)習(xí)筆記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:
- 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
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
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