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transcription

Transcription is the first step in protein synthesis. It is synthesis of RNA under direction from DNA. In many ways transcription is quite similar to DNA replication, except, in this case, instead of new DNA nucleotides being added, RNA nucleotides are added to the DNA to form a RNA strand known as the primary transcript. The primary transcript eventually goes on to become a messenger RNA (mRNA) strand after some modification. It is mRNA that directs the following step of protein synthesis, translation.

Transcription begins when RNA polymerase breaks the hydrogen bonds between the two sides of a DNA strand and begins adding RNA nucleotides. RNA polymerase first binds to promoter regions, which include the intiaition site and several other nucleotides, on the DNA strand. However, RNA polymerase can not recognize the promoter regions on its own. Thus, transcription factors search along the DNA strand for promoter regions and the RNA polymerase recognizes the transcription factors.

When RNA polymerase sucessfully binds to a promoter region, it begins adding RNA nucleotides. Since RNA polymerase only works in a 5' to 3' direction only one side of the DNA strand is active in transcription. The addition of RNA nucleotides is directed by the DNA. Specific nucleotide sequences on the DNA strand mark initiation and termination sites, when transcription begins and ends. The initiation sequence, termination sequence, and all the intervening nucleotides together are colectively known as a transcription unit.

As in DNA replication, bases that are complementary to those on the original DNA strand are added in a 5' to 3' direction. However, RNA lacks the base thymine (thymine is specific to DNA),which is complementary to adenine. Instead the RNA has the base uracil. Thus, when adding complementary bases during transciption the complementary base pairings are Cytosine for Guanine, as in DNA replication, and Adenine for Uracil. These nucleotides are added to the DNA strand by RNA polymerase. RNA polymerase unwinds one turn of the DNA helix at a time and adds nucleotides. When one turn of the helix has been completed and the next is about to begin unwinding, the newly added RNA nucleotides seperate from the DNA strand and the DNA strand rewinds. Nucleotide addition progresses at a rate of about 60 nucleotides per second.

Transcription continues until RNA polymerase reaches a termination site on the DNA strand. This site has a sequence of nucleotides, the most common of which being AATAAA in eukaryotic cells, that tell RNA polymerase to stop adding nucleotides. Thus the formation of the primary transcript has been completed.

In eukaryotic cells, the primary transcript undergoes modification before it becomes messenger RNA (mRNA) and is ready to leave to nucleus and go into the cytoplasm. The 5' end of the molecule, where transcription began, is capped off with a modified form of guanine. This cap serves to protect the mRNA molecule from degredation in the cytoplasm. Also, it serves as a signal to ribosomal subunits in the cytoplasm. At the 3' end of the molecule a poly-A-tail is added, which consists of 30-200 adenine nucleotides. It also protects the mRNA molecule from degredation and helps in the export of the mRNA from the nucleus to the cytoplasm. Also, the molecule is shortened in a process known as RNA splicing. The noncoding regions of the molecule, known as introns, are cut out by splicesomes. This leaves only the functional coding regions, exons, in the mRNA molecule.

Thus transcription is completed and a mRNA molecule is formed. Messenger RNA is now able to leave to nucleus and go to the cytoplasm and perform its function in the next step in protein synthesis, translation.

Proceed to translation

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