Structure of DNA

A DNA molecule is a double helix with two polynucleotide chains held together by hydrogen bonds between specific complementary base pairs.

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DNA structure

  • DNA is made up of two polynucleotide chains arranged in a double helix structure.
    • The sugar and phosphate lie on the outside of the helix and form the DNA's backbone.
    • The nitrogenous bases are stacked in the inside of the helix, like a pair of staircase steps.
    • Hydrogen bonds hold the base pairs together.
      • There are two hydrogen bonds between A and T.
      • There are three hydrogen bonds between C and G.
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Base pairing in DNA

  • Adenine (A) can pair with thymine (T).
  • Guanine (G) can pair with cytosine (C).
    • This means that there is always an identical number of adenine and thymine bases and of guanine and cytosine bases in DNA.
  • Structurally, thymine and cytosine are known as pyrimidine nucleotides.
  • Structurally, adenine and guanine are known as purine nucleotides.
    • So purine nucleotides pair with pyrimidine nucleotides.
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The base complementary rule

  • The base complementary rule states that the DNA strands are complementary to each other.
    • If the sequence of one strand is AATTGGCC, the complementary strand would have the sequence TTAACCGG.
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Antiparallel

  • The two polynucleotide strands are antiparallel because they run in opposite directions.
    • This means that the 5′ carbon end of one strand will face the 3′ carbon end of its matching strand.
  • We call the strand that runs in the 5'-3' direction the "lagging strand".
  • We call the strand that runs in the 3'-5' direction the "leading strand".
    • This antiparallel orientation is important to DNA replication and in many other nucleic acid interactions.
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Jump to other topics

1Cell Structure

2Biological Molecules

3Enzymes

4Cell Membranes & Transport

5The Mitotic Cell Cycle

6Nucleic Acids & Protein Synthesis

7Transport in Plants

8Transport in Mammals

9Gas Exchange

10Infectious Diseases

11Immunity

12Energy & Respiration (A2 Only)

13Photosynthesis (A2 Only)

14Homeostasis (A2 Only)

15Control & Coordination (A2 Only)

16Inherited Change (A2 Only)

17Selection & Evolution (A2 Only)

18Classification & Conservation (A2 Only)

19Genetic Technology (A2 Only)

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