The Structure of DNA (AQA A Level Biology): Revision Note
Exam code: 7402
DNA: structure & function
Function
Deoxyribonucleic acid (DNA) is an important information-carrying molecule
The function of DNA is to hold or store genetic information
DNA is the molecule that contains the instructions for the growth and development of all organisms
Structure
Nucleic acid structure
The nucleic acid DNA is a polynucleotide – it is made up of many nucleotides bonded together in a long chain

DNA molecule structure
DNA molecules are made up of two polynucleotide strands lying side by side, running in opposite directions – the strands are said to be antiparallel
Each DNA polynucleotide strand contains alternating deoxyribose sugars and phosphate groups bonded to form the sugar-phosphate backbone. These bonds are covalent bonds known as phosphodiester bonds
The phosphodiester bonds link the 5-carbon of one deoxyribose sugar molecule to the phosphate group from the same nucleotide, which is itself linked by another phosphodiester bond to the 3-carbon of the deoxyribose sugar molecule of the next nucleotide in the strand
Each DNA polynucleotide strand is said to have a 3’ end and a 5’ end (these numbers relate to which carbon on the pentose sugar could be bonded with another nucleotide)
As the strands run in opposite directions (they are antiparallel), one is known as the 5’ to 3’ strand, and the other is known as the 3’ to 5’ strand
The nitrogenous bases of each nucleotide project out from the backbone towards the interior of the double-stranded DNA molecule

Hydrogen bonding
The two antiparallel DNA polynucleotide strands that make up the DNA molecule are held together by hydrogen bonds between the nitrogenous bases
These hydrogen bonds always occur between the same pairs of bases:
Adenine (A) always pairs with thymine (T) – two hydrogen bonds are formed between these bases
Guanine (G) always pairs with cytosine (C) – three hydrogen bonds are formed between these bases
This is known as complementary base pairing
These pairs are known as DNA base pairs

Double helix
DNA is not two-dimensional as seen in the diagram above
DNA is described as a double helix
This refers to the three-dimensional shape that DNA molecules form

Examiner Tips and Tricks
Be able to identify and label the components of a DNA molecule:
Sugar-phosphate backbone
Nucleotides
Complementary base pairs (A=T, C≡G)
Phosphodiester bonds (between nucleotides)
Hydrogen bonds (between bases)
You may be asked to calculate base numbers using base pairing rules if given the quantity of one base.
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