Transcription.

The process of copying genetic informations from one strands of the DNA into RNA is termed as Transcriptions.

     

  

     CONTEXT

1. Introduction  
2. Transcriptionunit
3. Types of RNA polymerase 
4. Differences between transcription in prokaryotes and Eukaryotes.
5. Process of Transcription. 

Introduction 

         In transcription only a segments of DNA and only one of the strands is copied into RNA. Here also, the principle of complementarity governs the process of transcription, except the adenosine now forms base pairs with the Uracil instead of thymine. However, unlike the process of replication, which once set in, the total DNA of an organisms gets duplicated. This necessitates are defining the boundaries that would demarcate the regions and the strands of DNA that would be transcribed. 

Transcription unit

               A transcription unit in DNA is defined primarily by the three region in the DNA:

     I) A Promotor

     II) The structural gene

     III) A terminator 

                      Structural genes is the parts of DNA from which RNA is formed.

It is polycistronic, in prokaryote and is monocistronic, in eukaryote.

Transcription occur with the helps of DNA dependent to RNA polymerase.

Polymerization occurs in 5′ → 3′ direction.

The 3′ → 5′ strands of DNA acts as the templates strand/ anti-sense strands.The 5′ → 3′ strand is the coding strands/ sense strands.

All reference point while defining the transcription units is maded with coding strands.

Lying towards 5′ ends (upstream) of structural genes is the promoter. It is the binding sites for RNA polymerase for transcription to be start

Lying towards 3′ ends (downstream) of structural genes is the terminator, where transcription end.

RNA polymerase:

Single RNA polymerase synthesize all types of RNA in prokaryotes.

RNA polymerase is a holoenzyme made of 5 polypeptide chain :- 2α ( helps in chain initiation and interaction with regulatory proteins ) , β( catalytic centre - chain initiation and elongations ), β′(Dna binding ) & σ factors.Sigma (σ) factor recognize the promoter to which RNA polymerase bind and the ‘σ’ factor is release immediately after initiations of.RNA polymerase without ‘σ’ factor is core polymerase.

Image result for rna polymerase in prokaryotes

3.There are 3 RNA polymerases in eukaryotes:

RNA polymerase I synthesize 28S, 18S & 5.8S rRNAs.

RNA polymerase II synthesize hnRNA (heterogeneous nuclear RNA) that form mRNA.

RNA polymerase III synthesize t RNA, 5S rRNA & small nuclear RNAs.

Difference between prokaryotes and eukaryotes transcription -

A) Coupled transcription-translation that is called attenuation 

   is the rule in case of prokaryotes .

Coupled transcription translation is (Eukaryotes )

not possible.

B) Occurs in the cytoplasm.( Prokaryotes )

Occurs in the nucleus. (Eukaryotes )

C) Initiation of transcription does not need any proteins or initiation factors.( Prokaryotes )

Initiation of transcription requires proteins called transcription factors. These are TFIIA, TFIIB , TFIIF, TFIID, TFIIE AND TFIIH. These recognise TATA BOX.((Eukaryotes )  

C) Transcription and translation nearly simultaneous.(Prokaryotes)

 The process of mRNA, Processing of hnRNA includes:

        Addition of 5’cap(7 methylguanosine)

        Addition of 3’poly A tail. (Eukaryotes )

D) The 23S, 16S and 5S rRNAs are formed from a single primary transcript. (Prokaryotes)

The 28S, 18S, 5.8S and 5S rRNAS are formed from the two primary transcripts.(Eukaryotes)

E) Inhibitors:

(Rifampin: RNA polymerase binds to β subunit.

Actinomycin-Intercalates to interrupt transcription.)- Prokaryotes 

Inhibitors:

α amanitin: Inhibits RNA polymerase the 2 most srongly- Eukaryotes. 

    

5. Process of transcription

    Transcription proceed in the following general steps as follow :-

• RNA polymerase, together with one or more general transcription factor, bind to promoter DNA.
• RNA polymerase generates a transcription bubble, which separate the two strand of the DNA helix. This is done by breaking hydrogen bond between the complementary DNA nucleotides.

C. RNA polymerase adds RNA nucleotides (which are complementary to the nucleotides of one DNA strand).

D. RNA sugar-phosphate backbone forms with assistance from RNA polymerase to form an RNA strand.

E. Hydrogen bonds of the RNA–DNA helix break, freeing the newly synthesized RNA strand.

F. If the cell has a nucleus, the RNA may be further processed. This may include polyadenylation, capping, and splicing.

The RNA may remain in the nucleus or exit to the cytoplasm through the nuclear pore complex.

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