The Structure of DNA (AQA A Level Biology): Revision Note

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