ASBiologyAQARevision NotesBiological MoleculesNucleic Acids: Structure & DNA ReplicationCalculating the Frequency of Nucleotide Bases

Calculating the Frequency of Nucleotide Bases (AQA AS Biology): Revision Note

Exam code: 7401

Lára Marie McIvor

Written by: Lára Marie McIvor

Reviewed by: Cara Head

Updated on

Frequency of bases

  • All DNA nucleotides contain the same phosphate group and deoxyribose sugar but 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

  • Therefore

    • the frequency or number of adenine is equal to the frequency or number of thymine

    • the frequency or number of cytosine is equal to the frequency or number of guanine

  • The frequency of bases can be calculated by knowing the number or percentage of a specific base

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

Examiner Tips and Tricks

This is a fairly common exam question so make sure you are comfortable with this calculation.

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Previous:The Process of Semi-Conservative ReplicationNext:The Watson Crick Model
Author
Reviewer
Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Biology, Psychology & Sociology Subject Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

Cara Head

Reviewer: Cara Head

Expertise: Biology & Psychology Content Creator

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding

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