Chapter 2 - Nucleic Acids

• What is the function of DNA?

  Holds genetic information which codes for polypeptides.
• What is the function of RNA?

  Transfers genetic information from DNA to ribosomes.
• Name the two molecules form which a ribosome is made.

  RNA and proteins
• Describe the structure of a DNA nucleotide.

  Deoxyribose sugar

  Phosphate group

  Nitrogen containing base (A,G,C,T)

  
• Describe the structure of an RNA nucleotide.


  Ribose sugar

  Phosphate group

  Nitrogen containing base (A,G,C,U)
• Describe the differences between a DNA nucleotide and an RNA nucleotide.

  DNA -

  Deoxyribose sugar

  Thymine base

  
  

  RNA -

  Ribose sugar

  Uracil base
• Describe how nucleotides join together to form bases.

  Condensation reactions, removing water molecules

  Between phosphate group of one nucleotide and deoxyribose/ribose sugar on another.

  Forming phosphodiester bonds.
• Why did many scientist doubt that DNA carried the genetic code?

  The relative simplicity of DNA - chemically simple molecule.
• Describe the structure of DNA

  Polymer of nucleotides

  Each nucleotide formed from deoxyribose sugar, phosphate group and a nitrogen containing organic base.

  Phosphodiester bonds join adjacent nucleotides

  2 polynucleotide chains held by hydrogen bonds.

  Between complementary base pairs (C-G and A-T)

  Double helix
• Describe the structure of mRNA?

  Polymer of nucleotides

  Each nucleotide formed from ribose, phosphate group and nitrogen containing base.

  Bases - uracil, cytosine, guanine and adenine

  Phosphodiester bonds join adjacent nucelotides

  Single helix
• Compare and contract the structure of DNA with mRNA?

  DNA -

  Deoxyribose sugar

  Thymine base

  Double stranded/ helix

  Long

  Hydrogen bonds

  
  

  mRNA -

  Ribose sugar

  Uracil base

  Single stranded

  Shorter

  No hydrogen bonds
• How does the structure of DNA relate to its function?

  Two Strands - act as templates for semi-conservative replication.

  Hydrogen bonds are weak - strands can be separated during replication.

  Complementary base pairing - accurate replication

  Many hydrogen bonds - stable and strong

  Double helix with sugar phosphate backbone - Protects bases

  Long molecule - Stores a lot of genetic information.

  Double helix (coiled) - compact
• Why is semi conservative replication important?

  Ensures genetic continuity between organisms.
• Describe the process of semi conservative DNA replication.

  DNA helicase breaks hydrogen bonds between complementary base pairs, unwinding the double helix.

  Both strands act as a template

  Free DNA nucleotides attracted to exposed bases join by specific complementary base pairing.

  Hydrogen bonds form between adenine - thymine and cytosine - guanine

  DNA polymerase joins adjacent nucleotides on the new strand during condensation reactions.

  Forming phosphodiester bonds
• What is meant by semi-conservative?

  Each new DNA strand consists of one original template strand and one new strand.
• Using your knowledge of enzyme action suggest why DNA polymerase moves in opposite directions along DNA strands.

  DNA has anti parallel strands

  So shapes / arrangements of nucleotides on two ends are different.

  DNA polymerase s an enzyme with a specific shaped active site

  So can only bind to substrate with complementary shape.
• Name the two scientist who proposed the models of chemical structure of DNA and DNA replication

  Watson and Crick
• Describe the work of Meselson and Stahl in validating the Watson-Crick

  model of semi-conservative DNA replication

  1. Bacteria grown in a medium containing heavy nitrogen (nitrogen incorporated in DNA bases)

  - DNA extracted and centrifuged - settles near bottom a all DNA contains heavy strands

  
  

  2. Bacteria transferred to medium containing light nitrogen and allowed to divide once.

  - DNA extracted and centrifuged settles in middle as all DNA contains one light and one heavy strand.

  
  

  3. Bacteria in light nitrogen allowed to divide again.

  - DNA extracted and centrifuged - half settles in middle (one heavy and one light strand) and half settles near top (both light strands).
• What is ATP?


  Adenosine triphosphate
• Describe the structure of ATP?

  Ribose bound to molecule of adenine and 3 phosphate groups.

  Nucleotide derivative.
• Describe the breakdown of ATP?

  ATP (+water) -> ADP + Pi

  Hydrolysis reaction using a water molecule

  Catalysed by ATP hydrolase
• Give two ways in which ATP hydrolysis can be used in cells?

  Coupled to energy requiring reactions within cells (releasing and providing energy)

  e.g active transport and protein synthesis

  
  

  Inorganic Phosphate released can be used to phosphorylate other compounds making them more reactive.
• How is ATP resynthesises in cells?

  ADP + Pi -> ATP + water

  Condensation reaction

  Catalysed by ATP synthase

  During respiration or photosynthesis
• Suggest how the properties of ATP make it a suitable immediate source of energy for cells.


  Releases energy in relatively small amounts/ little energy lsot as heat.

  Single reaction/ one bond hydrolysed to release energy (immediate)

  Cannot pass out of cells
• Explain how hydrogen bonds occur between water molecules.

  Water is a polar molecule

  Slightly negatively charged oxygen attract slightly positive hydrogen atoms of other water molecules.
• Explain how water is a metabolite.

  Used in condensation / hydrolysis/ photosynthesis/ respiration
• Explain how water is a solvent.

  Allows metabolic reactions to occur (faster)

  Allows transport of substance e.g nitrates in the xylem
• Explain how water having high specific heat capacity is beneficial.

  Buffers temperature change

  Can gain/lose heat / energy without changing temperature
• How is water having a high specific heat capacity beneficial to organisms?

  Good habitat for aquatic organisms as temperature is more stable than land

  
  

  Helps organisms to maintain a stable internal body temperature.
• Explain how water having a high latent heat of vaporisation is beneficial.

  Cooling effect through evaporation

  Helps organisms to maintain a constant internal body temperature.
• Explain why water having strong cohesion between molecules is beneficial.

  Supports columns of water e.g transpiration stream

  Provides surface tension supporting small organisms.
• Where are inorganic ions found in the body?

  In solution in the cytoplasm and bodily fluid, some in his concentrations and others in very low concentrations.
• Describe the role of hydrogen ions in the body.

  Maintain pH levels in the body - high conc. = low pH

  Affects enzyme rate of reaction as can cause them to denature.
• Describe the role of iron ions in the body.

  Component of harm group

  allowing oxygen to bind/ associate for transport of oxygen.
• Describe the role of sodium ions in the body.

  Involved in co-transport of glucose /amino acids

  Involved in action potential in neurones

  Affects water potential of cells / osmosis
• Describe the role of phosphate ions in the body.

  Component of nucleotides allowing phosphodiester bonds to form.

  Component of ATP allowing energy release

  Phosphorylates other compounds making them more reactive

  Hydrophilic part allows bilayers to form.