Transcription

When I learned about the transcription of RNA, I realized it was pretty similar to how DNA replicated, except with a few differences:

• Tanscription of pre-RNA is single stranded instead of double stranded
• Uracil replaces Thymine
• ribose sugars are used instead of deoxyribose 
• pre-RNA uses DNA as a template 

 

When looking at the bigger picture though, pre-RNA transcription also includes three major parts: Initiation, Elongation, and Termination.

Initiation:

• The place where transcription happens is chosen on the DNA, based on a the location of the promoter (also known as the 'TATA' box) which signifies the beginning of transcription and the location of the terminator ('AAAUAAA' box) which signifies the end

• Then, transcription factors recognize the location of the promoter region, and bind to the promoter. Since pre-RNA is single stranded, only one strand of the DNA is used as a template. The other strand, the coding strand, is used to help guide the pre-RNA when it matches the complementary bases.

• The transcription factors are used to attract the RNA polymerase II, the enzyme used assemble the base pairs. Once the RNA polymerase II is tracked down,it begins to unwind the DNA helix.

Elongation:

•  The polymerase II initiates the production at the promoter region, moving towards the terminator (going upstream and away from the promoter, then downstream and towards the terminator) as it synthesizes complementary bases to the DNA template, and matching the coding strand with the exception of T which is replaced with U.

•  Just like DNA, pre-RNA grows in the direction of 5' to 3', attaching on to the 3' OH group, as it is the most reactive. Unlike DNA replication, however, the pre-RNA has no need for Okazaki fragments, since RNA is single stranded.

•  As one RNA polymerase complex begins to move along the DNA, another RNA polymerase can bind to the promoter and begin synthesizing another strand of RNA simultaneously. This indicates that hundreds of RNA from the same DNA at one time.

• Also, the RNA polymerase II seems to be a one-man army, as it does not need polymerase III to proofread its work. This is because RNA is only used to form protein molecules, not the genetic make up. Thus, the production of RNA is much less time time consuming than DNA.

 

Termination:

• As the pre-RNA is being synthesized, the RNA polymerase II is also rewinding the double helix of the DNA at the same time, and the already made RNA detaches itself.

• Once the pre-RNA is synthesize, it is capped at the 5' end with Guanine, as the structure itself is too unstable, while poly-A-tails are added to the 3' with Adenine. 

• Once the pre-RNA is modified, then it is ready to be sent into the cytoplasm.