Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes. During transcription, a DNA sequence is read by RNA polymerase, which produces a complementary, antiparallel RNA strand. As opposed to DNA replication, transcription results in an RNA complement that includes uracil (U) in all instances where thymine (T) would have occurred in a DNA complement.
Transcription can be explained easily in 4 or 5 steps, each moving like a wave along the DNA.
1. RNA polymerase unwinds/"unzips" the DNA by breaking the hydrogen bonds between complementary nucleotides.
2. RNA nucleotides are paired with complementary DNA bases.
3. RNA sugar-phosphate backbone forms with assistance from RNA polymerase.
4. Hydrogen bonds of the untwisted RNA+DNA helix break, freeing the newly synthesized RNA strand.
5. If the cell has a nucleus, the RNA is further processed (addition of a 3' poly-A tail and a 5' cap) and exits through to the cytoplasm through the nuclear pore complex.

Transcription is the first step leading to gene expression. The stretch of DNA transcribed into an RNA molecule is called a transcription unit and encodes at least one gene. If the gene transcribed encodes a protein, the result of transcription is messenger RNA (mRNA), which will then be used to create that protein via the process of translation. Alternatively, the transcribed gene may encode for either ribosomal RNA (rRNA) or transfer RNA (tRNA), other components of the protein-assembly process, or other ribozymes.

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ORTHOLOGY CLASS Homo sapiens L. (human) [HSA] Mus musculus L. (mouse) [MMU] Caenorhabditis elegans Maupas (nematode) [CEL] Drosophila melanogaster Meigen (fruit fly) [DME] Saccharomyces cerevisiae Meyen ex E.C. Hansen (budding yeast) [SCE] Schizo-saccharomyces pombe Lindner (fission yeast) [SPO] Escherichia coli Migula (bacterium) K-12 MG1655 [ECO] Arabidopsis thaliana (L.) Heynh. (mouse-ear cress) [ATH]
ko:K14721 (DNA-directed RNA polymerase III subunit RPC5) POLR3E Polr3e gi:17555494 Sin RPC37 rpc37 gi:15240503
ko:K03060 (DNA-directed RNA polymerase subunit omega [EC:]) RpoZ
ko:K03046 (DNA-directed RNA polymerase subunit beta' [EC:]) RpoC RpoC1
ko:K03043 (DNA-directed RNA polymerase subunit beta [EC:]) RpoB RpoB
ko:K03040 (DNA-directed RNA polymerase subunit alpha [EC:]) RpoA RpoA
ko:K03027 (DNA-directed RNA polymerases I and III subunit RPAC1) POLR1C Polr1c gi:25155095 gi:20129237 RPC40 rpc40
ko:K03026 (DNA-directed RNA polymerase III subunit RPC4) POLR3D Polr3d gi:28574864 RPC53 rpc53 gi:79486517
ko:K03025 (DNA-directed RNA polymerase III subunit RPC6) POLR3F Polr3f gi:17509641 gi:24648960 RPC34 rpc34 gi:15237811
ko:K03024 (DNA-directed RNA polymerase III subunit RPC7) POLR3G
gi:28574873 RPC31 rpc31 gi:79324969
ko:K03023 (DNA-directed RNA polymerase III subunit RPC3) POLR3C Polr3c gi:17506119 gi:320542649 RPC82 rpc82 gi:18408755
ko:K03022 (DNA-directed RNA polymerase III subunit RPC8) POLR3H Polr3h gi:17566938 gi:24662815 RPC25 rpc25 gi:15222202
ko:K03021 (DNA-directed RNA polymerase III subunit RPC2 [EC:]) POLR3B Polr3b gi:25144348 RpIII128 RET1 rpc2 NRPC2
ko:K03020 (DNA-directed RNA polymerases I and III subunit RPAC2) POLR1D Polr1d gi:17553746 l(2)37Cg RPC19 rpc19 gi:79323435
ko:K03019 (DNA-directed RNA polymerase III subunit RPC10) POLR3K Polr3k gi:17544244 gi:78707272 RPC11 rpc11 gi:15223344
ko:K03018 (DNA-directed RNA polymerase III subunit RPC1 [EC:]) POLR3A Polr3a rpc-1 gi:281360912 RPO31 rpc1 NRPC1
ko:K03017 (DNA-directed RNA polymerase II subunit RPB9) POLR2I Polr2i rpb-9 RpII15 RPB9 rpb9 NRPB9A
ko:K03016 (DNA-directed RNA polymerases I, II, and III subunit RPABC3) POLR2H Polr2h rpb-8 Rpb8 RPB8 rpb8 NRPB8A
ko:K03015 (DNA-directed RNA polymerase II subunit RPB7) POLR2G Polr2g rpb-7 Rpb7 RPB7 rpb7 NRPB7
ko:K03014 (DNA-directed RNA polymerases I, II, and III subunit RPABC2) POLR2F Polr2f rpb-6 RpII18 RPO26 rpb6 NRPB6B
ko:K03013 (DNA-directed RNA polymerases I, II, and III subunit RPABC1) POLR2E Polr2e rpb-5 Rpb5 RPB5 rpb5 NRPB5
ko:K03012 (DNA-directed RNA polymerase II subunit RPB4) POLR2D Polr2d rpb-4 Rpb4 RPB4 rpb4 NRPB4
ko:K03011 (DNA-directed RNA polymerase II subunit RPB3) POLR2C Polr2c rpb-3 RpII33 RPB3 rpb3 NRPB3
ko:K03010 (DNA-directed RNA polymerase II subunit RPB2 [EC:]) POLR2B Polr2b rpb-2 RpII140 RPB2 rpb2 NRPB2
ko:K03009 (DNA-directed RNA polymerases I, II, and III subunit RPABC4) POLR2K gi:149271268 rpb-12 gi:161077092 RPC10 rpc10 NRPB12
ko:K03008 (DNA-directed RNA polymerase II subunit RPB11) POLR2J2
Polr2j rpb-11 Rpb11 RPB11 rpb11 NRPB11
ko:K03007 (DNA-directed RNA polymerases I, II, and III subunit RPABC5) POLR2L RPB10 homolog
rpb-10 Rpb10 RPB10 rpb10 NRPB10
ko:K03006 (DNA-directed RNA polymerase II subunit RPB1 [EC:]) POLR2A Polr2a ama-1 RpII215 RPO21 rpb1 NRPB1
ko:K03005 (DNA-directed RNA polymerase I subunit RPA49) POLR1E Polr1e gi:21357097 RPA49 rpa49 gi:15231416
ko:K03004 (DNA-directed RNA polymerase I subunit RPA43) TWISTNB Twistnb gi:24582356 RPA43 rpa43
ko:K03003 (DNA-directed RNA polymerase I subunit RPA34) RPA34
ko:K03002 (DNA-directed RNA polymerase I subunit RPA2 [EC:]) POLR1B Polr1b gi:17506623 RpI135 RPA135 rpa2 NRPA2
ko:K03001 (DNA-directed RNA polymerase I subunit RPA14) RPA14
ko:K03000 (DNA-directed RNA polymerase I subunit RPA12) ZNRD1 Znrd1 gi:17558010 RpI12 RPA12 rpa12 gi:18404735
ko:K02999 (DNA-directed RNA polymerase I subunit RPA1 [EC:]) POLR1A Polr1a gi:71998295 RpI1 RPA190 nuc1 NRPA1

Last modification date: Oct. 6, 2011