Nucleic acids

• Nucleic acid


  Consists of a pentose sugar and a nitrogenous base
• ATP


  • Formed in an endergonic reaction (in respiration)
  • 30.6kJ of energy is stored in this bond and released when it is hydrolysed into ADP and an inorganic phosphate (Pi)
  • Called the 'universal energy currency' as it is used to provide energy for all biochemical reactions in all living organisms
  • Releases energy in one hydrolysis reaction controlled by one enzyme
  • Releases energy in small, usable amounts
  • Travels easily to where it may be used for secretion, muscle contraction, nerve transmission or active transport
• RNA


  • Phosphate
  • Ribose
  • Nitrogenous base (A,U,C,G)
  • Linked together in a single-stranded polynucleotide
  • 3 different types with different functions: Messenger RNA (mRNA), Ribosomal RNA (rRNA), Transfer RNA (tRNA)
• DNA


  • Phosphate
  • Deoxyribose
  • Nitrogenous base (A,T,C,G)
  • Made from one strand of nucleotides linked by hydrogen bonds between the bases to another strand that runs antiparallel to the first
  • Purines: Adenine and Guanine
  • Pyrimidines: Cytosine and Thymine
  • A pairs with T, C pairs with G
• Differences between RNA and DNA

  • RNA: Ribose sugar, Single stranded, A,U,C,G bases, Short polynucleotides
  • DNA: Deoxyribose sugar, Double stranded, A,T,C,G bases, Long polynucleotides
• DNA replication


  1. DNA helicase breaks the hydrogen bonds between the bases in the double helix

  2. This unwinds the DNA and exposes unpaired bases

  3. Free nucleotides in the nucleoplasm are bound to their complementary bases on the unzipped strand

  4. DNA polymerase joins the nucleotides together by condensation reactions between sugar and phosphate groups of adjacent nucleotides

  5. Eventually, 2 new DNA molecules are formed from 1 new and 1 old strand of the DNA (semi-conservative replication)
• Meselson and Stahl's experiment disproves alternative theories of DNA replication
• Conservative replication - direct copying of the nucleotide sequence onto a new double stranded molecule which would give one light and one heavy molecule in generation 1
• Dispersive replication - where half the nucleotides are placed randomly in the DNA being replicated to make new molecules which would give successively lighter molecules and therefore a band between hybrid and light in generation 2
• During post-transcriptional modification, exons can be spliced (joined) together in different orders leading to formation of different polypeptides. It may be that one gene-one polypeptide theory is not entirely correct
• As there are 64 possible triplet codes for the four bases, some amino acids have more than one code. This is why the code is termed degenerate. A change in code does not always mean a change in amino acid
• Transcription


  1. DNA helicase unzips a section (gene) of the DNA by breaking the hydrogen bonds between complementary base pairs

  2. RNA polymerase links to the template (coding) strand of DNA and attaches mRNA nucleotides to their complementary base pairs

  3. This copying stops at a stop sequence / codon

  4. The newly made premRNA then leaves the DNA

  5. Post-transcriptional modification of the pre-mRNA takes place to remove the non-coding introns, leaving only the coding sections (exons) in the mature mRNA
• Translation


  1. mRNA is a linear chain of three base codons. There are complementary anticodons on tRNA molecules

  2. When the mRNA leaves the nucleus, it attaches to the small subunit of a ribosome

  3. The large subunits of a ribosome have 2 attachment sites for tRNA. The ribosome holds the mRNA and the tRNA (which have attached amino acids) in position for the amino acids to form peptide bonds and create a polypeptide chain

  4. The codon on the mRNA (3 base code) determines the tRNA as the tRNA which attaches must have a complementary 3 base code

  5. The tRNA that matches the codon on the mRNA has a specific amino acid attached to the 3' end of the tRNA molecule. The ribosome moves along the mRNA holding each tRNA in place until the amino acid attaches. The tRNA then leaves, the ribosome moves along and the next tRNA attaches to the next codon
• One gene = one polypeptide
• Modification of new polypeptides

  Polypeptides can be modified by the addition of carbohydrates, lipids or phosphate or can be combined together, e.g. haemoglobin