The Structure of DNA (Cambridge International A-Level Biology)

DNA Structure

Spec point — describe the structure of a DNA molecule as a double helix, including: • the importance of complementary base pairing between the ′ to ′ strand and the ′ to ′ strand (antiparallel strands) • differences in hydrogen bonding between C–G and A–T base pairs • linking of nucleotides by phosphodiester bonds

DNA structure

• 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
  • The phosphate group is 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

Exam Hint Make sure you can:

• Name the different components of a DNA molecule (sugar-phosphate backbone, nucleotide, complementary base pairs, phosphodiester bonds, hydrogen bonds)
• Locate these on a diagram

You must know how many hydrogen bonds occur between the different base pairs.

Remember that the bases are complementary so the number of A = the number of T and the number of C = the number of G.

You could be asked to determine how many bases are present in a DNA molecule if given the number of one of the bases.