DNA: Structure & Function (AQA A Level Biology): Revision Note

DNA: structure & function

• Deoxyribonucleic acid (DNA) is a type of nucleic acid and forms a polynucleotide – it is made up of many nucleotides linked together in a chain

Nucleic acids

• DNA is a polynucleotide, this means it is made up of many nucleotides bonded together in a long chain

DNA molecule structure

• DNA consists of two polynucleotide strands that run in opposite directions — this arrangement is called antiparallel

• Each strand has a sugar-phosphate backbone, formed by alternating deoxyribose sugars and phosphate groups joined by phosphodiester bonds

• Phosphodiester bonds connect as follows:

  • The 5th carbon of one deoxyribose sugar bonds to the phosphate group

  • Then that phosphate group bonds to the 3rd carbon of the next sugar in the chain

• Each strand has a 3’ end and a 5’ end, based on which carbon on the sugar is free for bonding

• One DNA strand runs from 5’ to 3’, and the other runs from 3’ to 5’, making the two strands antiparallel

• 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

DNA 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