Nucleic Acids

• Deoxyribonucleic acid (DNA): stores genetic information

  Ribonucleic acid (RNA): transfers the genetic code found in DNA out of the nucleus to the ribosomes in the cytoplasm
• The components of a DNA nucleotide are:

  • a deoxyribose sugar
  • a phosphate group
  • a nitrogenous base: adenine, thymine, cytosine or guanine
• The components of a RNA nucleotide are:

  • a ribose sugar
  • a phosphate group
  • a nitrogenous base: adenine, uracil, cytosine or guanine
• Polynucleotides are made up of multiple nucleotides joined together by condensation reactions, which occur between the phosphate group of one nucleotide and the pentose sugar of another to form a phosphodiester bond.
• The chain of alternating phosphate groups and pentose sugars produced is known as the sugar-phosphate backbone.
• DNA molecules are made up of 2 antiparallel polynucleotide strands, held together by hydrogen bonds between the nitrogenous bases. They coil to form a double helix structure.
• Complementary base pairing:

  • purine adenine always pairs with pyrimidine thymine/uracil - 2 hydrogen bonds are formed between these bases
  • purine guanine always pairs with pyrimidine cytosine - 3 hydrogen bonds are formed between these bases
• RNA molecules are made up of one polynucleotide strand and are shorter than DNA.

  Examples of RNA molecules are messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).
• DNA is copied via semi-conservative replication, where each DNA molecule consists of one original strand and one newly synthesised strand. This ensures genetic continuity between generations of cells.
• Semi-conservative replication:

  1. DNA helicase unwinds the DNA double helix by breaking hydrogen bonds between base pairs on the two DNA polynucleotide strands.
  2. Each original strand acts as a template for a new strand. Free floating nucleotides are attracted to their complementary exposed bases.
  3. DNA polymerase joins the adjacent nucleotides of the new strand together in condensation reactions, forming phosphodiester bonds.
  4. Hydrogen bonds form between bases of the original and new strand, forming a new DNA molecule.
• DNA was first observed in the 1800s, but scientists doubted it could carry the genetic code as it has a relatively simple chemical composition. Some argued it is carried by proteins which are more chemically varied.
• DNA was determined to be the carrier of the genetic code in 1953. In the same year the double helix structure was determined by Watson and Crick.
• Watson and Crick came up with the theory of semi-conservative DNA replication. But it wasn't until Meselson and Stahl's experiment later that this theory was validated.
• Meselson and Stahl's experiment:

  1. 2 samples of bacteria were grown - one in a nutrient broth containing light nitrogen (14N), one containing heavy nitrogen (15N).
  2. A sample of DNA was taken from each batch of bacteria and spun in a centrifuge - DNA from heavy nitrogen bacteria settled lower down the centrifuge than the light one.
  3. Heavy nitrogen bacteria replicate in light nitrogen broth. DNA sample was taken and spun in the centrifuge.
  4. If replication was conservative: 2 strands - one at the top and one at the bottom; semi-conservative: 1 strand in the middle
• Features of DNA and their importance in replication

  • weak hydrogen bonds: easily broken to allow the 2 strands to separate
  • complementary base pairing: allows accurate DNA replication
  • double stranded: both strands can act as templates
  • sugar-phosphate backbone: protects bases, allowing accurate DNA replication
• Role of enzymes in replication

  • DNA helicase: breaks hydrogen bonds between DNA strands
  • DNA polymerase: joins adjacent nucleotides, forms phosphodiester bonds
• Nucleotides can only be added in a 5' to 3' direction

  • DNA has antiparallel strands, where nucleotides are aligned differently
  • DNA polymerase has an active site with a specific shape
  • it is only complementary to the 5' end