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Polynucleotides

Biology: Cambridge International A Level, topic 6.1.3

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.

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.
RNA structure

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.

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.
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.
Types of bonding in DNA and RNA

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.
Antiparallel

Antiparallel

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

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.

Test yourself on Polynucleotides

  • Which of the following statements is true of DNA?

  • Which of these statements correctly describes an RNA molecule?

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

  • Which of the following statements is true about base pairing?

  • If the sequence of one strand of DNA is AATTGGCC, what would the sequence of the complementary strand be?

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