Everything You Need To Know About Nucleic Acids

These are my AS Biology revision notes on nucleic acids – it’s everything you need to know if you’re following OCR’s syllabus 🙂

  • DNA is a polynucleotide, usually double-stranded, made up of nucleotides containing the bases adenine, thymine, cytosine and guanine. It’s stable and acts as an information store as the bases act as a coded sequence.
  • RNA is a polynucleotide, usually single-stranded, made up of nucleotides containing the bases adenine, uracil, cytosine and guanine.
  • Almost all DNA in a eukaryotic cell is found in the nucleus where it acts as an information store. RNA is found in three different forms needed to read and translate the information to produce the various proteins in an organism.
  • The monomer of all nucleic acids is called a nucleotide. Each nucleotide is made from one phosphate group, one sugar molecule and one organic nitrogenous base. These three subunits are joined by condensation reactions resulting in covalent bonds.

  • The phosphate group is always the same. The sugar molecule is a five carbon sugar – either ribose or deoxyribose.
  • A condensation reaction between the phosphate group of one nucleotide and the sugar of another nucleotide, forming a long chain of nucleotides. This repeating sugar-phosphate chain is the ‘backbone’ of the molecule and the organic bases project out from this backbone.
  • Chains of nucleotides are called nucleic acids. Only nucleotides carrying the same sugar bind together so if the sugar is ribose it’s RNA, if it’s deoxyribose then it’s DNA.
  • The organic bases are either purines (larger) or pyrimidines (smaller).The purines are adenine and guanine. The pyrimidines are thymine, cytosine and uracil.
  • Uric acid is produced when excess purines are broken down in the liver. It’s insoluble at low temperatures and forms crystals that are deposited in joints at the extremities causing gout.
  • The chain of nucleotide monomers is called a polynucleotide. When two polynucleotides come together a DNA molecule is formed. Hydrogen bonds between the base pairs in opposite chains strengthening the molecule. This is vital as it carries the instructions to make an organism.

  • The two DNA strands run parallel to each other as the space between is taken up by the nitrogenous bases projecting inwards. The term antiparallel is used as the strands run in opposite directions to each other – the sugars are pointing in opposite directions.
  • The chains are the same distances apart as the bases pair up in a specific way. If there is a purine on one chain, opposite there will be a pyrimidines on the other chain. Adenine always pairs with Thymine (or uracil in RNA) and Guanine always pairs with Cytosine. Hydrogen bonds form between the bases. Base pairing rules apply due to the different structure of the bases. The base pairing is described as complementary, so A is complementary to T, and G is complementary to C.
  • In a complete DNA molecule, the antiparallel chains twist to form the final structure called a double helix.
  • When a cell divides, the new cell must receive a full copy of the DNA for that organism and this must occur precisely. DNA replication takes place in the interphase of the cell cycle and is the process that creates identical sister chromatids.
  • In order to make a new copy of a DNA molecule, the double helix is untwisted and the hydrogen bonds between the bases are broken apart, exposing the bases.  Free DNA nucleotides are hydrogen-bonded onto the exposed bases according to the base-pairing rules. Covalent bonds are formed between the phosphate of one nucleotide and the sugar of the next to seal the backbone.  This continues along the whole molecule producing two new DNA molecules, both an exact copy of the original due to the base pairing rules.
  • This process of DNA replication is described as semi-conservative replication as each new DNA molecule consists of one conserved strand and one newly built strand
  • The sequence of bases is information storage – it’s in the form of a code to build proteins. As the molecules are long a lot of information can be stored. The base pairing means complementary strands of information can be replicated. The double helix gives the molecule stability. Hydrogen bonds allow easy ‘unzipping’ for copying and reading information.
  • RNA is structurally different from DNA as the sugar molecule in the nucleotides is ribose instead of deoxyribose. The nitrogenous base uracil is found instead of the organic base thymine.  The polynucleotide chain is usually single-stranded. There are three forms of the RNA molecule.
  • Base-pairing rules mean molecules of RNA can be made so they are complementary to molecules of DNA. This is because exposed nucleotides can have free RNA nucleotides hydrogen-bonded to them and then the sugar-phosphate backbone is sealed to form a chain of RNA nucleotides.

  • Copying the genetic code of the DNA base sequence is called transcription.
  • Messenger RNA (mRNA): made as a strand complementary to one strand of a DNA molecule (template molecule) making it a copy of the other DNA strand (coding strand) of the double-helix.
  • Ribosomal RNA (rRNA): found in ribosomes.
  • Transfer RNA (tRNA): carries amino acids to the ribosomes where they are bonded together to form polynucleotides.


  • A gene is a length of DNA that codes for one (or more) polypeptides. Each gene occupies a specific place (locus) on a chromosome. Different versions of the same gene are called alleles.
  • The sequence of bases in DNA code for the sequence of amino acids for a particular protein molecule. This gene can be exposed by splitting the hydrogen bonds that hold the double helix together in that region. RNA nucleotides form the complementary strand mRNA which is a copy of the gene. The mRNA peels away from the DNA and leaves the nucleus through a nuclear pore before attaching to a ribosome. tRNA molecules bring amino acids to the ribosome in the correct order according to the base sequence on the mRNA. The amino acids are joined together by peptide bonds to give a protein with a specific primary structure (which gives rise to the secondary and tertiary structures).


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