Nucleic Acids and their Functions

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Both DNA and RNA are made up of monomers called nucleotides: each nucleotide contains a phosphate group, a nitrogen-containing organic base, and a pentose (5-carbon) sugar: either ribose (RNA) or deoxyribose (DNA). there are two groups of organic bases: pyridimines (single ring) and purines (double ring).

Four main nitrogenous bases found in DNA:

  • Guanine (purine)
  • Cytosine (pyridimine)
  • Adenine (purine)
  • Thymine (pyridimine)

In RNA the pyridimine uracil replaces thymine.


Adenine Triphosphate is also a nucleotide:it has a ribose sugar joined to the adenine base, with three phosphate groups attached.

When the high energy bond between the second and third phosphate group is broken via hydrolysis by the enzyme ATPase, 30.6kj of energy is released for use in the cell, and adenosine diphosphate is formed. This reaction is reversible, requiring energy from respiration of glucose to reform the bond. ATP --> ADP + Pi +30.6KJ energy (Pi = inorganic phosphate) Advantages of ATP:

  • energy is released quickly from a one step reaction involving just one enzyme (hydrolysis of glucose takes many steps).
  • enerygy is released in smal amounts, 30.6KJ where it is needed. By contrast just one molecule of glucose contains 1880KJ which couldn't safely be released all at once.
  • it is the 'universal energy currency', i.e it's a common source of energy for all reactions in all living things.

Roles of ATP in cells:

  • used in many anabolic reactions, e.g DNA and protein synthesis
  • active transport
  • muscle contraction
  • nerve impulse transmission

Structure of DNA

DNA consists of two polynucleotide strands that are arranged into a double helix. First a dinucleotide is formed when a condensation reaction occurs between two nucleotides: the 5th carbon atom of a deoxyribose sugar is joined to the 3rd carbon atom of the deoxyribose sugar of the nucleotide above it, via the phosphate molecule. This continues building a single strand of DNA in the 5'-3' direction.

DNa then forms a double-stranded molecule from two strands: one strand runs in the opposite direction to the other (anti-parallel). Both strands are held together


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