Transcription is the process through which a DNA sequence is enzymatically copied by an RNA
polymerase to produce a complementary RNA.

In the case of protein-encoding DNA, transcription is the beginning of the process that ultimately leads to the translation of the genetic code (via the mRNA intermediate) into a functional peptide or protein.

Like DNA replication, transcription proceeds in the 5' → 3' direction (ie the old polymer is read in the 3' → 5' direction and the new, complementary fragments are generated in the 5' → 3' direction).

Transcription is divided into 3 stages: initiation, elongation and termination.

Transcription in eukaryotes differs from that in prokaryotes in several areas:

• Translation in eukaryotes is separated in time and space from transcription, in prokaryotes they can occur simultaneously.
  .
• Eukaryotes use three different RNA polymerase complexes, compared to a single enzyme in prokaryotes.
• Eukaryotic RNA polymerase does not use a sigma factor, and the promoter consensus is different from prokaryotes.
• Transcription in eukaryotes is affected mainly by binding of transcription factors and activator proteins to enhancer sequences.
• mRNA in eukaryotes is modified by the addition of a 5' cap and a 3' poly(A) tail.
• Precursor mRNA in eukaryotes has introns and exons, the introns are removed by splicing.

There are three different RNA polymerases in eukaryotes

• RNA polymerase I transcribes ribosomal RNA in the nucleolus.
• RNA polymerase II transcribes messenger RNA in the nucleoplasm.
• RNA polymerase III transcribes transfer RNA (and one rRNA species) in the nucleoplasm.

Eukaryotic promoters have a TATA box around -25 and often have a CAAT box and sometimes a GC box somewhere between -40 and -110.

RNA polymerase requires a complex of transcription factors to initiate transcription. A TATA box-binding component of one transcription factor recognizes the promoter region, and a series of other transcription factors and RNA polymerase bind to the site.

Transcription can be stimulated by enhancer sequences located far upstream, downstream, or in the middle of the gene. These sequences bind activator proteins which are responsible for the stimulation. These signals are species- and tissue-specific, and account for the regulation of specific genes in certain tissues and the host range of viruses that have usurped these sequences.

Eukaryotic mRNA contains a 5' cap. A guanine residue is added in a 5'-5' linkage at the end, and this molecule is methylated at the N-7 position. Additional methylations of the 2'-OH groups of the 2nd and 3rd residues can also occur. This cap protects the 5' end from nucleolytic activity.

Eukaryotic mRNA contains a 3' poly(A) tail. Termination may occur through a rho-independent method. The 3' end is cleaved by a specific endonuclease, and approximately 250 adenine residues are added from ATP.

Precursors of eukaryotic mRNA contain introns, intervening sequeces which are removed by splicing. Splice sites are distinguished by consensus sequences which bind to complexes of RNA and proteins to form a spliceosome. The RNA in the spliceosome complex base-pairs with the consensus sequence, recognizing and aligning the appropriate sites for the reaction to occur.

Splicing proceeds through a lariat intermediate. An adenine residue at the branch point forms a third phosphodiester bond with its 2'-OH, cleaving the 3' end of the upstream exon from the rest of the molecule. This end then attacks the 5' end of the downstream exon, joining the two exons and releasing the intron in the lariat form.



to make the subject more easier i added anation at the limk
http://207.207.4.198/pub/flash/26/transmenu_s.swf