Saturday, June 26, 2010

DNA: strand and coding

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.

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 [antisense is a general term for a sequence of DNA or RNA that is complementary to mRNA] , anticoding strand, template strand, or transcribed strand).

During transcription, RNA polymerase unwinds a short section of the DNA double helix near the start of the gene. This unwound section is known as the transcription bubble. The RNA polymerase, and therefore the transcription bubble, travels along the coding strand in the 5' to 3' direction, and 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 Like how a zipper works, only it unzips it and rezips it without going back and forth.

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