Nucleic Acids

• There are two types of nucleic acids - Deoxyribonucleic acids and ribonucleic acids.
• Monomers of nucleic acids are called nucleotides.
• RNA has similarities with DNA but differs in its sugar component (deoxyribose vs ribose), presence of uracil instead of thymine, and single strandedness.
• The structure of DNA consists of four bases (adenine, thymine, cytosine, guanine) arranged into complementary base pairs (A-T, C-G).
• DNA Nucleotides consist of a phosphate group, a pentose sugar called deoxyribose and a nitrogenous base.
• Overall, the DNA nucleotide is negatively charged due to:

  • The negatively charged phosphate group
  • Which is useful when DNA combines with positively charged histones in the nucleus to form new chromosomes.
• Difference between DNA and RNA nucleotides include:

  • In DNA, there is only hydrogen attached at carbon 2 on the carbon ring, making it a deoxyribose sugar.
  • In RNA, there is a hydroxyl (OH) group attached on carbon 2 on the carbon ring, making it a ribose sugar.
  • DNA nucleotides may contain the bases - adenine, guanine, cytosine, thymine.
  • RNA nucleotides do NOT contain thymine, it's replaced with Uracil
• Purine bases are adenine and guanine - they consist of 3 carbon rings.
• Pyramidine bases are thymine, cytosine and uracil - they consist of 2 carbon rings.
• Adenine and thymine form 2 hydrogen bonds.
• Guanine and cytosine form 3 hydrogen bonds.
• Polynuclotides are monomers of nucleotides that join together via condensation reaction, removing a water molecule, joined together through phosphodiester bonds.
• How are polynucleotides formed?

  1. The phosphate group on carbon 5 of the pentose sugar of a nucleotide forms phosphodiester bond with the OH group at carbon 3 on the pentose sugar of another adjacent nucleotide, via condensation reaction, removing a water molecule.
  2. This forms a strong sugar phosphate backbone, with a nitrogenous base attached to each suagar.
• Why are phosphodiester bonds very strong?

  To ensure that the genetic code is not broken down, preventing mutations.
• How do DNA double helixes form?

  • Two anti-parallel strands of polynucleotides
  • Join together via hydrogen bonds between the complementary bases through complementary base pairing.
  • The anti-parallel strands then twist and coil around each other
  • Forming a DNA double helix.
• Sugar phosphate backbone - function

  • It is a strong structure due to the strong phosphodiester bonds
  • This allows DNA to be stable and rigid, to prevent the genetic code from breaking down.
• Double-helix - function

  • Contains sugar phosphate backbone outside with strong phosphodiester bonds
  • Contains weak hydrogen bonds between the nitrogenous bases on the inside
  • The sugar phosphate backbone protects the weak hydrogen bonds on the inside, preventing the genetic code from breaking down.
• Weak hydrogen bonds - function

  • Allows DNA helicase to break the bonds between complementary bases to be used for replication.
• DNA is large and compact - function

  • This means it stores more genetic information
  • Therefore more of it can combine with histones to form more of the stable chromosome.
• DNA has complementary bases - function

  • This allows identical copies of it to be made, with the correct genetic information.
• DNA Purification Step 1 with reasons


  Grind sample with water - to break the cell wall of the plant cell.
• DNA Purification STEP 2 with reasons


  Add the ground sample to a test tube and add detergent - this breaks down the cell membrane and the nuclear envelope, which releases DNA from the nucleus.
• DNA Purification STEP 3 with reasons

  Add the enzyme protease into it - this breaks down the proteins, histones, that are combined with DNA.
• DNA Purification STEP 4 with reasons


  Add drops of ethanol - this causes DNA to precipitate as a white solid
• DNA purification STEP 5 with reasons


  Remove the precipitate of DNA by wrapping it around a glass rod
• Semi-conservative replication STEP 1


  DNA helicase unzips the double helix by breaking the hydrogen bonds between the complementary bases.

  Leaving with two separate strands of DNA with exposed bases.
• Semi-conservative replication STEP 2


  Free DNA nucleotides in the nucleus pair up with the complementary bases by a process of complementary base pairing.

  They form hydrogen bonds between the complementary bases.

  Adenine pairs up with thymine, Guanine pairs up with cytosine.
• Semi-conservative replication STEP 3


  The enzyme DNA polymerase then joins the free nucleotides together via condensation reaction, producing phosphodiester bonds between them, this process is also known as polymerisation (making a polymer).

  A sugar phosphate backbone is now formed, which is very strong and rigid.
• Semi-conservative replication STEP 4


  The strands then twist and coil around each other, forming a double helix structure of DNA.

  
  

  This leaves us with two identical DNA molecules.
• Why is this called Semi-conservative


  This is because in the new DNA molecule, one of the strands is an old/original template, and the other is the new template.