Nucleic Acids

Nucleic Acids

  • The monomer units in nucleic acids are called nucleotides.
  • They consist of a pentose sugar (deoxyribose/ribose) a phosphate group and a nitrogenous base that is either a pyrimidine or a purine.
  • These three components are joined in condensation reactions by covalent bonds.

Nucleotide derivatives

ATP and ADP are nucleotide derivatives and are often present in co-enzymes such as NADP – nicotinamide adenine dinucleotide phosphate which is used in photosynthesis.






DNA structure

  • DNA is made of two polynucleotide strands, antiparallel to each other.
  • The monomers in DNA include deoxyribose sugar, a phosphate group and the bases cytosine, thymine, adenine and guanine.
  • The monomers are bonded by covalent phosphodiester bonds.
  • The bases are bonded with hydrogen bonds in complementary base pairing. These hydrogen bonds are weak but together are stronger as there is a large number of them.
  • It stores genetic information and hereditary material in the sequence of its bases.
  • DNA coils into a double helix shape.
  • DNA is a macromolecule and is very long to store all the information.
  • Purines and pyrimidines always pair together to provide equal sized rungs on the ladder like structure of DNA. A pairs with T, C pairs with G and vice versa.
  • The 5’ end is where the phosphate is attached to the fifth carbon and the 3’ end is where the phosphate is attached to the third carbon.
  • In eukaryotes, DNA is stored in the nucleus and is wound around histone proteins to form chromosomes. Each chromosome is one molecule of DNA. There is also a loop inside mitochondria and chloroplasts.
  • In prokaryotes, DNA is in a loop floating free within the cytoplasm and is not wound around histone proteins. It is therefore naked.


DNA Replication

DNA uses semi-conservative replication as both of the original strands are used in the two daughter strands.

DNA is a self-replicating molecule and its replication takes place during interphase in cell division.

The DNA in mitochondria and chloroplasts also replicates each time these organelles divide.


  • DNA unwinds (catalysed by gyrase enzyme) and unwinds (catalysed by DNA helicase)
  • Free DNA nucleotides form hydrogen bonds to the exposed bases (using complementary base pairing) this is catalysed by DNA polymerase and occurs in the 5’ – 3’ direction ONLY
  • Discontinuous okazaki fragments are made on the lagging strand of DNA that flows from 3’ – 5’ which are then connected up by DNA ligase
  • The nucleotides join together to form a new molecule of DNA

Mutations may occur in replicating the genetic code. These mutations occur when DNA is not accurately copied and when the wrong nucleotide is inserted into the strand. Some mutations are harmful, such as achondroplasia (a form of dwarfism), Marfan syndrome (a connective tissue disorder), and Huntington disease (a degenerative disease of the nervous system). Some are not, such as red hair.

Mutations can be passed on and inherited from parents or can occur spontaneously.



Protein synthesis


  • In the nucleus, DNA unwinds and unzips

  • Hydrogen bonds between the complementary nucleotides break
  • RNA polymerase catalyses the pairing of free RNA nucleotides to the template strand of the DNA
  • The RNA strand is identical to the other coding strand of DNA
  • The mRNA chain passes out of the nucleus through a nuclear pore
  • Exons are regions of the RNA strand that code for the amino acids of the protein being synthesized whereas introns are segments that do not code for the protein and are not useful
  • Introns are cut out and the exon regions are spliced together to form a long chain of codons
  • The mRNA chain progresses out of the nucleus and over onto the rough endoplasmic reticulum that has the ribosomes on it
  • Ribosomes are made up of two subunits

  • Transfer RNA molecules bring amino acids to the ribosome and pair up their anticodons with the codons in the mRNA molecule.
  • Peptide bonds form between the adjacent amino acids forming the polypeptide
  • ATP is required for translation
  • When the polypeptide has been assembled the RNA chain breaks down to be recycled again
  • The newly synthesized polypeptide is folded into its shape to form the protein