What is transcription?

Transcription is nothing but the formation of RNA from DNA template using RNA polymerase.

The central dogma proposed by Francis Crick in 1953 :
DNA——— RNA———-
Transcription Translation
The formation of ss RNA molecules from ds DNA using RNA polymerase
Usually 80-85% mRNA is synthesized from DNA & also there are different regions in DNA for rRNA & tRNA’s .
Also the 3 different RNA’s require different RNA polymerases for synthesis.
1. RNA poly.- 1—– Synthesizes rRNA
2.RNA poly.-2—– Synthesizes mRNA
3. RNA poly.-3—– Synthesizes tRNA’s
Transcription process is just similar to & has 3 phases for simplification:
1. Initiation,
2. Elongation,
3. Termination
All the 3 different enzymes have similar initiation , identical elongation but different termination process.
Initiation occurs when polymerase recognizes the promoter sequence.
Promoter sequence is before the actual gene sequence & varies from organism to organism & also between polymerases.
Once it recognizes the sequence it initiates the transcription process .
In initiation process it tries to skip this DNA sequence by producing 8-9 oligonucleotide RNA abortive sequence.
In elongation process , it just typifies the DNA sequence.
The last step termination is done by hairpin loop or protein called rho in prokaryotes.
While in eukaryotes :
RNA poly 1- Terminates at specific 11bp rRNA gene sequnce.
RNA poly 2- There is a sequence upstream AAUAA & downstream GU rich sequence separated by 40-60 nucleotides.
There are 2 factors present on the enzyme , they recognize the sequence & cleaves the RNA now the factor meets the enzyme & terminates.
RNA poly 3- Termination signal is long stretch of T bases which is recognized by poly 3 & terminates by forming dead end complex.

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