The DNA molecule is read in different directions depending on what process you mean, but the key convention is that nucleotide sequences are written and “read” 5′ to 3′, while the template strand used by polymerases is read 3′ to 5′.

Basic idea

  • Each DNA strand has directionality, labeled 5′ (five-prime) and 3′ (three-prime) ends.
  • By convention, when a DNA sequence is written in textbooks or databases, it is shown and read from 5′ → 3′.

During replication and transcription

  • DNA and RNA polymerases synthesize new nucleic acid only in the 5′ → 3′ direction.
  • To do that, they must read the template DNA strand in the opposite direction, 3′ → 5′.

“Reading” DNA in genetics questions

  • In exam or forum questions that ask “in which direction is the DNA molecule read?”, they usually mean the direction a polymerase reads the template, which is 3′ → 5′.
  • When people talk about “reading a DNA sequence” as in looking at the gene on paper or in a database, they almost always mean 5′ → 3′ along the strand shown.

Quick summary

  • Sequence notation: 5′ → 3′.
  • Polymerase reading of template DNA: 3′ → 5′.
  • Polymerase synthesis of new strand (DNA or RNA): 5′ → 3′.

Information gathered from public forums or data available on the internet and portrayed here.