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Lesson 1 of 6 / DNA and gene expression

Read the DNA molecule

How do pairing and direction support information storage?

I can describe DNA nucleotide structure, double-stranded organisation and information-carrying role.

  • I can describe DNA and RNA structure and the roles of mRNA, tRNA and rRNA.Syllabus

    Syllabus 8876, Core 2(a). Describe DNA and RNA structure and the roles of mRNA, tRNA and rRNA.

About 5 min

Make a guess. You are not marked.

What holds the nucleotides of one DNA strand together in a chain?

  1. Phosphodiester bonds in the backbone.
  2. Hydrogen bonds between neighbouring bases.
  3. Peptide bonds, as in proteins.
Show the answer

Phosphodiester bonds in the backbone.

Covalent phosphodiester bonds link sugar to phosphate along a strand. Hydrogen bonds join the two strands.

The key idea

DNA has two complementary, antiparallel nucleotide strands. The base sequence carries information; the covalent backbone and base pairing maintain the molecule.

Explore the idea

Zoom from one unit to paired strands

PSBaseP: phosphateS: deoxyribose sugar

Adjacent nucleotides join through phosphodiester bonds in the sugar-phosphate backbone. The variable base carries sequence information.

Explanation

A DNA nucleotide contains deoxyribose sugar, phosphate and a nitrogenous base: adenine, thymine, guanine or cytosine. Covalent phosphodiester bonds join nucleotides within each sugar-phosphate backbone. The base order varies and carries genetic information.

Two strands form a double helix with bases facing inwards. Adenine pairs with thymine through two hydrogen bonds; guanine pairs with cytosine through three. Complementary pairing gives each strand information about the other.

The strands are antiparallel: one runs 5' to 3' while its partner runs 3' to 5'. These direction labels refer to positions in the sugar backbone. Writing a complementary sequence directly underneath a given strand differs from rewriting that complement in the 5'-to-3' direction.

A gene is a DNA sequence whose information contributes to a functional RNA or polypeptide product. In the protein-synthesis examples here, the base sequence specifies a polypeptide through an RNA intermediate. DNA is the stored information, not material converted directly into protein.

Step by step
  1. 1

    Pair the bases

    Use A-T and G-C at corresponding positions.

  2. 2

    Label direction

    Opposite aligned strands run in opposite directions.

  3. 3

    Check what is requested

    If an answer must be written 5' to 3', reverse the written order of a complement initially aligned 3' to 5'.

Worked example

Complement and direction

A strand is 5'-A G T C-3'. Write its aligned complement and then that complement in the 5'-to-3' direction.

One way to explain it

Aligned: 3'-T C A G-5'. Rewritten 5' to 3': 5'-G A C T-3'. The paired positions remain complementary even though the written order changes.

Why this answer works
  • Pair each displayed base.
  • Use antiparallel direction labels.
  • Reverse the aligned complement only when the requested writing direction requires it.

Watch out for this

A student says: "Hydrogen bonds join adjacent nucleotides along a DNA strand." What is wrong with this?

Show the answer

Phosphodiester bonds join the backbone within a strand. Hydrogen bonds link complementary bases between strands.

Check your understanding

What is the aligned complement of 5'-A C G-3'?

  1. 5'-T G C-3'.
  2. 3'-T G C-5'.
  3. 3'-A C G-5'.
Show the answer

3'-T G C-5'.

Complementary bases occupy matching positions on an antiparallel strand.

Quick recall

Card 1 of 3. Answer in your head, then check.

How do pairing and direction support information storage?

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