Nucleic acids

• What makes up a nucleotide?

  A pentose sugar, a phosphate group (which is inorganic and negatively charged) gives the sugar-phosphate backbone.

  Nitrogenous base
• How are polynucleotides made?

  Mononucleotides join together by condensation reactions to make dinucleotides and then large chains are polynucleotides. A phosphodiester bond forms between the phosphate group and the sugar.
• What is RNA and what does it do?

  It is a polymer of nucleotides. It has a ribose sugar and the bases adenine, cytosine, guanine and uracil. It is a relatively short polynucleotide chain (only one so no hydrogen bonds). It transfers genetic information from DNA to the ribosomes. Ribosomes are formed from RNA and proteins. After protein synthesis, RNA molecules degrade in the cytoplasm so that they can be reused.
• What is DNA and what does it do?

  It is a polymer of nucleotides. It has a deoxyribose sugar (which has one less oxygen atom than ribose) and the bases adenine, thymine, cytosine and guanine. It holds genetic information.
• How is DNA structured?

  2 extremely long chains of polynucleotides are held together by hydrogen bonds. They are then twisted to form a double helix. The 2 molecules that form a double helix are anti-parallel (meaning they run in opposite directions).
• What is complementary base pairing?

  When bases pair to another base in a specific way. Adenine is complementary to thymine (forming 2 hydrogen bonds) or uracil. Cytosine is complementary to guanine (forming 3 hydrogen bonds). This is because adenine and guanine are pyrimidines (small) whereas thymine, cytosine and uracil are purines (large).
• DNA and RNA are large molecules that store + transfer genetic information and synthesise polypeptides.
• The 'simplicity' of DNA led many scientists to doubt that it carried the genetic code.
• Semi-conservative DNA replication - 4 steps

  1. DNA helicase breaks the hydrogen bonds between the base pairs of DNA ('unzipping'). The DNA helix unwinds and separates into its two strands.
  2. Free DNA nucleotides base pair
  3. Free nucleotides then polymerise, forming phosphodiester bonds. Catalysed by the enzyme DNA polymerase.
  4. Each DNA molecule now has one old (original) strand and one new strand.
• Watson and Crick evidence for DNA replication

  Conservative model = one of the DNA molecules is original, the other DNA molecule is new

  Semi-conservative model = both of the 2 new DNA molecules are made up of one original strand and one new strand.
• Supporting evidence for Watson and Crick - Meselsohn and Stahl

  Bacteria grown on the 14N medium would be lighter than that of 15N (as bacteria incorporates nitrogen from their medium into any new DNA they make).

  1. Original DNA of the bacteria is grown on a medium of 15N. Transferred to a medium of 14N, for a single generation
  2. Centrifuged the extracted DNA (for 1-3 generations)

  Lighter DNA = top of centrifuge tube

  Heavier DNA = bottom of centrifuge tube
• What makes DNA stable?

  Complementary base pairing. Base sequences are protected (from damage) by the sugar-phosphate backbone and the double helix structure.
• Where does the phosphodiester bond form in DNA?

  Between the 3rd carbon on a sugar of a nucleotide and the 5th carbon of another sugar on an adjacent nucleotide (this other sugar just so happens to have a phosphate group attached to it in the same position).
• What is the structure of DNA?

  Nitrogenous bases are hydrophobic - they face inward to protect themselves, which means they can form a double helix.

  Sugar-phosphate backbone is hydrophilic

  Weak interactions form H+ bonds between specific complementary base pairs. These break and form easily.
• What are the 5 points for the function of DNA?

  1. Stable structure - does not degrade or mutate easily
  2. Hydrogen bonds are weak and easily broken - 2 strands of DNA can be easily separated for DNA replication + protein synthesis
  3. Large - can carry huge amounts of genetic information
  4. DNA replication + transfer of genetic info via mRNA is ensured (due to complementary base pairing)
  5. Codes (set of codons e.g. TGC) are specific + contain genetic info, in the form of genes, for protein (structural or functional) synthesis.