1.5.3

Polynucleotides

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Structure of DNA and RNA

A DNA molecule is a double helix with two polynucleotide chains held together by hydrogen bonds between specific complementary base pairs. RNA, on the other hand, is made up of only one polynucleotide.

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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.
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RNA structure

  • An RNA molecule is a relatively short polynucleotide chain.
  • RNA only has one polynucleotide chain.
  • RNA is comprised of ribonucleotides that are linked by phosphodiester bonds.

Specific Base Pairing

Only certain types of base pairing are allowed.

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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.
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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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Types of bonding in DNA and RNA

  • Base pairing holds the two DNA polynucleotides together by hydrogen bonding.
  • The single polynucleotide chain is held together by phosphodiester bonds between nucleotides.
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Antiparallel

  • The two polynucleotide strands are antiparallel because they run in opposite directions.
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Uracil

  • The RNA base sequence is complementary to the DNA's coding sequence from which it has been copied.
  • If the DNA strand has a sequence AATTGCGC, the sequence of the complementary RNA is UUAACGCG.

Jump to other topics

1Biological Molecules

2Cells

3Substance Exchange

4Genetic Information & Variation

5Energy Transfers (A2 only)

6Responding to Change (A2 only)

7Genetics & Ecosystems (A2 only)

8The Control of Gene Expression (A2 only)

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