The Structure of DNA
DNA is a large polymer molecule that carries an organism's genetic instructions. Understanding its structure, from the nucleotide building blocks to the double helix, is fundamental to biology and often tested with simple percentage calculations.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- DNA is a polymer, a long chain made of repeating monomer units called nucleotides.
- Each nucleotide has three parts: a phosphate group, a deoxyribose sugar, and a nitrogenous base.
- The phosphate and sugar groups of adjacent nucleotides link together to form a strong sugar-phosphate backbone.
- There are four different bases in DNA: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).
- Double-stranded DNA consists of two polynucleotide strands coiled into a double helix.
- The two strands are joined by hydrogen bonds between specific complementary base pairs: Adenine always pairs with Thymine (A-T), and Guanine always pairs with Cytosine (C-G).
- The sequence of these bases along the DNA strand constitutes the genetic code, which dictates protein synthesis.
Formulae
%A = %T and %C = %G To determine the percentage of one base when the percentage of its complementary partner is known in double-stranded DNA.
%A + %T + %C + %G = 100% The total percentage of all four bases in a DNA sample must sum to 100. This is the basis for calculating unknown base percentages.
Definitions
- Nucleotide
- The monomer unit of DNA, consisting of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (A, T, C, or G).
- Double Helix
- The characteristic three-dimensional shape of a double-stranded DNA molecule, resembling a twisted ladder.
- Complementary Base Pairing
- The specific pairing rule between bases in double-stranded DNA: Adenine pairs with Thymine, and Cytosine pairs with Guanine.
Worked example
A sample of double-stranded DNA is found to contain 22% guanine. What percentage of the nucleotides in this sample contain thymine?
- 1
Recall the complementary base pairing rule:
Guanine (G) pairs with Cytosine (C).
- 2
This means the percentage of G equals the percentage of C.
So, %C = 22% - 3
Calculate the total percentage of G and C combined:
22% + 22% = 44% - 4
The remaining percentage must be made up of Adenine (A) and Thymine (T).
Calculate this remainder:
100% - 44% = 56% - 5
The other base pairing rule is that Adenine (A) pairs with Thymine (T), so their percentages are equal.
- 6
Divide the remaining percentage by two to find the percentage of thymine:
56% / 2 = 28%
Answer: 28%
Common mistakes
- ×Forgetting to divide the remaining percentage by two in base composition problems. For example, finding that A+T = 56% and incorrectly stating that T = 56%.
- ×Mixing up the base pairs, for instance, pairing A with G or C with T.
- ×Misinterpreting diagrams of DNA structure, for example, being unable to distinguish the sugar-phosphate backbone from the nitrogenous bases.
- ×Making simple arithmetic errors under pressure, such as in the subtraction (100 - X) or division (/ 2) steps of a base percentage calculation.
No-calculator tips
- ✓Base percentage questions use simple math. Double the percentage you're given, subtract from 100, then halve the result. Practice this sequence to do it quickly and accurately.
- ✓If percentages are tricky, think in whole numbers. Imagine you have 100 nucleotides. If 22 are G, then 22 must be C. That's 44 used up (22+22). You have 56 left (100-44). These 56 must be split evenly between A and T, so 28 are T.
- ✓The two pairs of bases must always add up to 100%. If you know one pair (e.g. G+C = 44%), you instantly know the other (A+T = 56%).