2 Nucleic Acids + 8.1 + 9.1

• DNA holds genetic information
• RNA transfers genetic information from DNA to ribosomes, which are formed from RNA and proteins.
• Nucleotides contain a pentose sugar, a nitrogenous base and a phosphate group
• Nucleotides make DNA or RNA which are nucleic acids
• DNA or RNA nucleotides are joined together by condensation reactions forming phosphodiester bonds.
• Structure of DNA: 2 strands joined in anti-parallel, held together by hydrogen bonds between specific complementary base pairs, twisting into a double helix
• RNA is single-stranded
• Differences between DNA and RNA nucleotides:

  1. DNA nucleotides have the pentose sugar deoxyribose, whereas RNA nucleotides have the pentose sugar ribose
  2. DNA nucleotides can have the nitrogenous base thymine, whereas RNA nucleotides have uracil instead.
• Differences between DNA and RNA molecules:

  1. DNA molecules are double stranded, whereas RNA molecules are single-stranded.
  2. DNA is longer whereas RNA is shorter
• Structure of DNA related to its functions:

  1. Double stranded so both strands can act as templates for semi-conservative replication
  2. Weak hydrogen bonds between bases so can be unzipped for replication
  3. Complementary base pairing so provides accurate replication
  4. Double helix with sugar phosphate backbone so protects bases/genetic information
  5. Long molecule so can store lots of genetic information
  6. Coiled double helix so compact
• Process of DNA replication:

  1. DNA helicase breaks hydrogen bonds between bases, unwinds double helix
  2. Two strands which both act as templates
  3. Free floating DNA nucleotides are attracted to exposed bases via specific complementary base pairing, hydrogen bonds form
  4. DNA polymerase joins adjacent nucleotides on new strand by condensation, forming phosphodiester bonds
  5. Replication is semi-conservative so each new strand formed contains one original/template strand and one new strand
  6. Ensures genetic continuity between generations of cells
• Adenine and Thymine have 2 hydrogen bonds between them whereas Cytosine and Guanine have 3 hydrogen bonds between them.
• Adenine and Guanine are purines (2 rings) whereas Thymine, Cytosine and Uracil are pyrimidines (1 ring)
• Why does DNA polymerase move in opposite directions along the DNA strands?

  • DNA has antiparallel strands
  • DNA polymerase is an enzyme with a specific shaped active site which can only bind to substrate with a complementary shape
  • It can only bind to and add nucleotides to the phosphate (3') end of the developing strands so it works in a 5' to 3' direction
• ATP stands for adenosine triphosphate
• Structure of ATP:

  1. Ribose sugar, a molecule of adenine and 3 phosphate groups
  2. It is a nucleotide derivative so a modified form of a nucleotide
• ADP stands for adenosine diphosphate, which has one less phosphate than ATP
• ATP hydrolysis: $ATP -> ADP +$$Pi$
• ATP hydrolysis is catalysed by the enzyme ATP hydrolase
• Watson and Crick discovered the structure of DNA
• In ATP hydrolysis, energy is released when bonds between inorganic phosphate groups are broken
• In ATP hydrolysis, the inorganic phosphate released can be used to phosphorylate other compounds e.g. glucose, often making them more reactive (i.e. lowers activation energy)
• ATP condensation: $ADP +$$Pi -> ATP$
• ATP condensation is catalysed by the enzyme ATP synthase
• ATP condensation happens during respiration or photosynthesis
• ATP cannot be stored
• Why is ATP a suitable immediate source of energy?

  • ATP releases energy is small, manageable amounts so no energy is wasted
  • Only one bond is hydrolysed (single reaction) to release energy which is why energy release is immediate
• How does hydrogen bonding occur between water molecules?

  1. Water is a polar molecule (Oxygen has a partial negative charge and hydrogen has a partial positive charge)
  2. Slightly negatively charged oxygen atoms attract slightly positively charged hydrogen atoms of other water molecules
  3. So hydrogen bonds (weak attractive force) form between water molecules
• Water has a high specific heat capacity because it is polar so many hydrogen bonds form between water molecules, which allow water to absorb a relatively large amount of heat energy before its temperature changes.
• Importance of high specific heat capacity in biology:

  1. Good habitat for aquatic organisms e.g. lakes as temperature more stable than land
  2. Organisms mostly made up of water so helps maintain a constant internal body temperature so enzyme activity is not affected
• Water has a high latent heat of evaporation because it is polar so many hydrogen bonds form between water molecules, which can absorb a lot of energy before breaking, when water evaporates.
• Importance of high latent heat of evaporation in biology:

  1. Evaporation of small amount of water e.g. sweat is an efficient cooling mechanism
  2. Helping organisms maintain a constant body temperature
• Water is cohesive because it is polar so many hydrogen bonds form between the water molecules so they tend to stick together.
• Importance of cohesion in biology:

  1. Column of water does not break when pulled up a narrow tube e.g. xylem during transpiration
  2. Produces surface tension at an air-water surface so invertebrates can walk on water
• Water is a solvent because it is polar so it can separate (dissolve) ionic compounds e.g. NaCl as the positive end is attracted to the negative ion (Cl-) and the negative end is attracted to the positive ion (Na+)
• Importance of water as a solvent in biology:

  1. Acts as a medium for metabolic reactions
  2. Acts as a transport medium e.g. in xylem to transport nitrates which are needed to make amino acids
• Water can dissolve other substances e.g. inorganic ions, enzymes and urea
• Water is a metabolite because it is reactive
• Inorganic ions occur in solution in the cytoplasm and body fluids of organisms
• The chemical symbol for phosphate is PO4 3-.