SBI4U - Transcription (Protein synthesis preparation)


INITIATION: Transcription begins with initiation, which requires a promoter, a special sequence of DNA to which the RNA polymerase binds very tightly Eukaryotic genes generally have one promoter each, while in prokaryotes and viruses, several genes often share one promoter.

Promoters are important control sequences that “tell” the RNA polymerase two things:

• Where to start transcription

• Which strand of DNA to transcribe


ELONGATION: Once RNA polymerase has bound to the promoter, it begins the process of elongation . RNA polymerase unwinds the DNA about 10 base pairs at a time and reads the template strand in the 3′-to-5′ direction. Like DNA polymerase, RNA polymerase adds new nucleotides to the 3' end of the growing strand, but does not require a primer to get this process started. The new RNA elongates from the first base, which forms its 5′ end, to its 3′ end. The RNA transcript is thus antiparallel to the DNA template strand. Because RNA polymerases do not proofread, transcription errors occur at a rate of one for every 104 to 105 bases. Because many copies of RNA are made, however, and because they often have only a relatively short life span, these errors are not as potentially harmful as mutations in DNA.


TERMINATION: Just as initiation sites in the DNA template strand specify the starting point for transcription, particular base sequences specify its termination The mechanisms of termination are complex and of more than one kind. For some genes, the newly formed transcript falls away from the DNA template and the RNA polymerase. For others, a helper protein pulls the transcript away.


The fallowing vide also explains the next stage after transcription. (Translation)



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