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.