Coding strand

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When referring to DNA transcription (protein biosynthesis), 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.

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 [antisense is a general term for a sequence of DNA or RNA that is complementary to mRNA] [1], 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. 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 (Lewin, pp 235 [2]). Like how a zipper works, only it unzips it and rezips it without going back and forth.

RNA-DNA hybrid

Where the helix is unwound, the coding strand consists of unpaired bases, whilst the template strand consists of an RNA:DNA composite, followed by a number of unpaired bases at the rear. This hybrid consists of the most-recently-added nucleotides of the RNA transcript, complementary base-paired to the template strand. The number of base-pairs in the hybrid is under investigation, but it has been suggested that the hybrid is formed from the last 10 nucleotides added (Griffiths, pp 259-265).

Bibliography

• Griffiths A.J.F. et al. (2005) Introduction to Genetic Analysis (8th edition). W.H. Freeman and Company, New York, USA. ISBN 0-7167-4939-4

• Lewin B. (2000) Genes VII. Oxford University Press Inc., New York, USA. ISBN 0-19-879277-8 (Pbk), ISBN 0-19-879276-X (Hbk)

References

Lewin B. (2007) "Genes IX". Oxford University Press Inc., New York, USA.[3]