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Nucleic Acids
Structure of DNA and RNA:
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Deoxyribonucleic acid and ribonucleic acid are important information-carrying molecules
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DNA and RNA are both polymers of nucleotides:
Phosphoric acid joins the nucleoside (sugar and
base) forming a Phosphoester bond in a
condensation reaction
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Phosphoester Bond
Pyrimidine bases: thymine/uracil,
cytosine (single ring structure)
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C & G join by 3 hydrogen bonds
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 A condensation reaction between 2 nucleotides forms a phosphodiester
bond, join between sugar of one and phosphate of another
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 Double helix = antiparallel chains twisted
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 The simplisity of DNA’s structure = doubt that it carries the genetic code
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o Hydrogen bonds are easily broken = easy replication
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o Sugar-phosphate backbone acts as protection to the bases
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Meselson-Stahl experiment is evidence (E
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The 2 strands of DNA unwind, leaving what
will be 2 template strands
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Complementary nucleotides join by DNA
polymerase
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ATP:
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ATP is used as the main (immediate) energy source to carry out processes within cells
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The bonds between the phosphate groups are unstable
and so have low activation energy and are easily
broken
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The hydrolysis of ATP can be coupled to energy-requiring reactions within cells
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ATP is resynthesized by the condensation of ADP and Pi
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Synthesis of ATP from ADP occurs in three ways:
o In chlorophyll-containing plant cells during photosynthesis (photophosphorylation)
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o In plant and animal cells when phosphate groups are transferred from donor molecules to ADP
(substrate-level phosphorylation)
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ATP is used in energy-requiring processes in cells including:
o Metabolic processes – provides energy needed to build up macromolecules from their basic units
e
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making starch from glucose or polypeptides from amino acids
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o Active Transport – provides energy to change the shape of carrier proteins in plasma membranes
allowing molecules or ions to be moved against a concentration gradient
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o Activation of molecules – the inorganic phosphate released during hydrolysis of ATP can be used to
phosphorylate other compounds in order to make them more reactive, lowering the activation
energy in enzyme-catalysed reactions
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g
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Water:
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Water is a major component of cells, it is a polar molecule, joined by hydrogen bonds (an electrostatic
attraction between a positive hydrogen atom and a negatively charged oxygen molecule  cohesion
It has several properties that are important in biology:
o Is a metabolite (product of metabolism) in many metabolic reactions including hydrolysis and
condensation reactions
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o Has a relatively high heat capacity, buffering changes in temperature
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E
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sweating
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Inorganic ions:
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Occur in solution in the cytoplasm and body fluids of organisms, in different concentrations
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Phosphate ions form a structural role in DNA molecules and a role in storing energy in ATP molecules
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Sodium ions are important in the transport of glucose and amino acids across plasma membranes

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