The Structure of DNA
- The nucleic acid DNA is a polynucleotide – it is made up of many nucleotides bonded together in a long chain
A DNA nucleotide
- 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 is made up of alternating deoxyribose sugars and phosphate groups bonded together 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
A single DNA polynucleotide strand showing the positioning of the ester bonds
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:
- The purine adenine (A) always pairs with the pyrimidine thymine (T) – two hydrogen bonds are formed between these bases
- The purine guanine (G) always pairs with the pyrimidine 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
A section of DNA – two antiparallel DNA polynucleotide strands held together by hydrogen bonds
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 molecules form a three-dimensional structure known as a DNA double helix
Examiner Tip
Make sure you can name the different components of a DNA molecule (sugar-phosphate backbone, nucleotide, complementary base pairs, phosphodiester bonds, hydrogen bonds) and make sure you are able to locate these on a diagram. Remember that phosphodiester bonds join the nucleotides in the sugar-phosphate backbone, and hydrogen bonds join the bases of the two complementary strands together. Remember that the bases are complementary, so the number of A = T and C = G. You could be asked to determine how many bases are present in a DNA molecule if given the number of just one of the bases. Note that knowledge of purines and pyrimidines is not required
