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Transcription

Transcription is a process of making a copy of genetic information stored in a DNA strand into a complementary strand of RNA (mRNA).
Process of transcription, Transcription
Process of transcription

Transcription requires an enzyme RNA polymerase. The entire or complete enzyme (holoenzyme) consists of a core enzyme and a sigma factor (σ)

The sigma factor recognizes the start signal on DNA and directs the binding of the enzyme to the promoter region. It thus enhances the specificity of transcription.

Eukaryotes have three RNA polymerase.

RNA polymerase I- This enzyme is located in the nucleus and transcribe for rRNAs (28s, 18s, and 5.8s).

RNA polymerase II- This enzyme is localized to the nucleoplasm and transcribe for hnRNAand mRNA.
RNA polymerase III- This enzyme is localized to the nucleus, possibly the nucleolar-nucleoplasm interface and transcribes for tRNA, 5sRNA and snRNAs.

Prokaryotes have only one RNA polymerase which synthesizes all types of RNAs. RNA polymerase odf E.coli has five polypeptide chains-β (binding with sigma factor), β (binding of RNA polymerase to DNA, catalytic), σ (recognition of promoter sequence), two chains of α (structural role).

Process of transcription:-

The segment of DNA that takes part in transcription is called transcription unit

Transcription unit has three components:-

(i) Promoter

(ii) Structural gene

(iii) Terminator


Besides a promoter eukaryotes also require an enhancer.

(i) Promoter has different parts for attachment to various transcription factors. In many cases, the promoter has an AT rich region called TATA box.

(ii) Structural gene is component of that strand of DNA which has 3`- 5` polarity. This strand of DNA is called template strand or master strand or antisense or (-) strand. 

The other strand which has the polarity of 5` – 3` is displaced during transcription this non-template strand, which does not take part in transcription, is also called sense or coding strand or plus (+) strand because genetic code present in this strand is similar to genetic code (present on mRNA) except that uracil is replaced by thymine.

(iii) Terminator, which is called rho factor (ρ) is required for termination of transcription.

Post transcriptional modification: – 

Transcription in eukaryotes is occurs within the nucleus and mRNA moves out of nucleus into the cytoplasm for translation

The mRNA is processed from the primary RNA transcript, a process is called maturation. Initially at the 5` end, a cap (consisting of 7-methyl guanosine or 7 mG) and tail of poly A at the 3` end are added. The cap is a chemically modified molecule of guanosine triphosphate (GTP).

The primary mRNA is longer and localized into the nucleus, where it is also called heterogeneous nuclear RNA (hnRNA) or pre-mRNA. 

The primary mRNAs are made up of two types of segments: 
(i) non-coding or introns and (ii) coding or exons

The introns are removed by a process called RNA splicing. The enzyme ligase joins two adjacent exons to produce mature mRNA. And thus completed the process of transcription.


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