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DNA

Biology: Cambridge International A Level, topic 6.1.5

Structure of DNA

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

DNA structure

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.
Base pairing in DNA

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.
The base complementary rule

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.
Antiparallel

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.

Test yourself on DNA

  • Which of the following statements is true of DNA?

  • Which of the following can be found on the inside of the DNA double helix?

  • A mutation occurs in a double stranded DNA molecule, and cytosine is replaced with adenine. What impact do you think this will have on the DNA structure?

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