Lesson 1 of 6 / DNA and gene expression
Read the DNA molecule
How do pairing and direction support information storage?
In this lesson: Describe DNA nucleotide structure, double-stranded organisation and information-carrying role.
About 5 min
The key ideaDNA 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
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
Pair the bases
Use A-T and G-C at corresponding positions.
- 2
Label direction
Opposite aligned strands run in opposite directions.
- 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.
Is this true? "Hydrogen bonds join adjacent nucleotides along a DNA strand."
Phosphodiester bonds join the backbone within a strand. Hydrogen bonds link complementary bases between strands.