Nucleic Acids

Nucleic Acids

1. Structure of RNA and DNA

   • Nucleotide Structure
     • made up of 3 components
       • a pentose sugar
       • a phosphate group
       • a nitrogen containing organic base
         • A/T/C/G/U
           • A is complementary to T and C is complementary to G
     • the pentose sugar, organic base and phosphate group are joined by a condensation reaction
       • forms a mononucleotide
       • bond between two mononucleotides is a phosphodiester bond
         • dinucleotide
   • RNA - ribonucleic acid
     • polymer made up of nucleotides
     • single relatively short polynucleotide chain
     • pentose sugar is always ribose. Organic bases are A,G,C,U.
     • one type of RNA transfers genetic info. Another is involved in protein synthesis. Ribosomes are made of another type
   • DNA structure
     • pentose sugar is deoxyribose. Organic bases are A,T,C,G
     • DNA is made of 2 strands of nucleotides
       • they are both very long and joined by hydrogen bonds.
     • double helix forms a structural backbone
   • DNA stability
     • the phosphodiester backbone protects the more chemically reactive bases
     • hydrogen bonds link the bases
       • C-G = 3 A-T = 2
   • DNA function
     • hereditary material responsible for passing genetic information from cell to cell and generation to generation
     • almost infinite variety of sequences of bases
       • leading to increased genetic variation
     • rarely mutates and very stable
     • two strands joined by hydrogen bonds meaning it can separate during replication
     • large molecule and carries lots of genetic information
     • genetic information is protected from chemical and physical forces becuause it is located within the phosphodiester backbone

2. DNA replication

   • cell division occurs in two ways
     • nuclear division = nucleus divides (mitosis/meiosis)
     • cytokinesis = follows nuclear division and the whole cell divides
   • semi-conservative replication
     • needs... the nucleotide bases, both strands of DNA to act as a template, DNA polymerase, source of chemical energy
     • 1. DNA helicase breaks the hydrogen bonds
       • 2. helix unwinds and separates into two strands
         • 3. each exposed polynucleotide chain acts as a template
           • 4. nucleotides are joined together in condensation reactions by DNA polymerse
             • 5. each new DNA molecule has one original strand and one new one

3. Energy and ATP

   • Structure = adenine, ribose and phosphates
   • ATP has 3 phosphate groups
     • the bonds between them are unstable and lave low activation energy
       • they are easily broken and release lots of energy when they do
         • usually only the terminal phosphate is removed
           • ATP + H2O -> ADP + Pi + E
             • water is used to convert AP to ADP, this is known as hydrolysis. The reaction is catalysed by ATP hydrolase
               • this is a reversible reaction. That is a condensation reaction and is catalysed by ATP synthatse
                 • this can occur in three ways
                   • photophosphorylation
                   • oxidative phosphorylation
                   • substrate-level phosphorylation
   • ATP is a good immediate energy source because it is instable
     • ATP is a better immediate energy source than glucose because the amount of energy released is more manageable and the hydrolysis of ADP to ATP is a single reaction
   • ATP is involved in energy-requiring processes in cells
     • metabolic processes - provides the energy needed to build up macromolecules from their basic sub-units
     • movement - provides energy for muscle contraction and the energy for filaments to slide past one another
     • active transport - energy to change the shape of carrier proteins
     • secretion - needed to form lysosomes
     • activation of molecules - Pi released during hydrolysis can be used to phosphorylate other compounds to make them more reactive

4. Water

   • Dipolar
     • oxygen atoms are slightly negative and hydrogen slightly positive, so the molecule has positive and negative poles
   • Hydrogen bonding
     • opposite poles attract and the attractive forces between them is a hydrogen bond
     • individually the bonds are weak
   • SHC
     • water molecules bond together meaning it takes more energy to break the bonds
   • latent heat of vaporisation
     • hydrogen bonding between water molecules means that it requires a lot of energy to evaporate 1 gram of water
   • cohesion and surface tension
     • the tendency of molecules to stick together is known as cohesion.
     • with its hydrogen bonding, water has large cohesive forces and these allow it to be pulled through a tube, such as a xylem vessel
   • water in metabolism
     • used to break down complex molecules by hydrolysis
     • produced in condensation reactions
     • chemical reactions take place in aqueous medium
     • major raw material in photosynthesis
   • solvent
     • water readily dissolves other substances such as gases, wastes, inorganic ions and enzymes
   • important features
     • evaporation cools organisms
       • not easily compressed so provides support
         • transparent so aquatic plants can photosynthesise
     • inorganic ions (iron, phosphate ions, hydrogen ions, sodium ions)

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