Coding strand

When referring to DNA transcription, the coding strand is the DNA strand which has the same base sequence as the RNA transcript produced (although with thymine replaced by uracil). It is this strand which contains codons, while the non-coding strand contains anti-codons. During transcription, RNA Pol II binds the non-coding strand, reads the anti-codons, and transcribes their sequence to synthesize an RNA transcript with complementary bases. By convention, the coding strand is the strand used when displaying a DNA sequence. It is presented in the 5' to 3' direction.

Alternative terms for strands

Wherever a gene exists on a DNA molecule, one strand is the coding strand (or sense strand or non-template strand), and the other is the noncoding strand (also called the antisense strand, anticoding strand, template strand, or transcribed strand).

Strands in transcription bubble

During transcription, RNA polymerase unwinds a short section of the DNA double helix near the start of the gene (the transcription start site). This unwound section is known as the transcription bubble. The RNA polymerase, and with it the transcription bubble, travels along the noncoding strand in the opposite, 3' to 5', direction, as well as polymerizing a newly synthesized strand in 5' to 3' or downstream direction. The DNA double helix is rewound by RNA polymerase at the rear of the transcription bubble (Lewin, pp 235 ). Like how two adjacent zippers work, when pulled together, they unzip and rezip as they proceed in a particular direction.