RNA Polymerase

 



RNA  Polymerase: 

RNA polymerase, abbreviated as RNA pol or RNAP, is an enzyme in molecular biologist that synthesises RNA from a DNA template. The process of transcription, RNA polymerase copies the sequence of DNA into an RNA polymerase copies the sequence of DNA into an RN sequence with the help of the enzyme helicase that opens up the wounded DNA strands.Transcription requires DNA_ dependent RNA_ polymerase. 

The RNA pol not only transcribes DN but also facilitates the process of attachment and elongation of nucleotides,proofreads the transcribed RN and takes part in the recognition of terminator regions.The RNA produced by RNAP are functional mRNAs that encode protein (translation) or produce non_ coding functional RNAs, such as tRNA, rRNA,and miRNA. RNA polymerase is an important enzyme  that is found in prokaryotes, eukaryotes, as well as viruses. The size and number of subunits in the RNAP complex are variable depending upon the type of organism. 


Bacteria and archaea have a single RNA pol, eukaryotes have five types of enzyme, each with their designated functions.


What is the Structure of the RNA polymerase?

 RNA polymerase in a prokaryotes have _ only one RNA polymerase which synthesizes  of RNAs . RNA polymerase of E.coil has a m.wt of about 490,000 and has 6 polypeptides, two of these are identical hence the enzyme contains 5 distinct polypeptides subunits:an alpha (œ) subunit,a beta( ß) subunit, a beta prome(ß) subunit, an omega , and a sigma  (ó) subunit.ó factor is involved in initiation of transcription and has no catalytic functions. Holoenzyme contains 2œ, ß,ß ,omega and ó polypeptides. Ó factor recognizes the start signal or promoter site on the DNA. A factor is required for termination of transcription .A number of other factors are also required for unwinding DNA strands, base pairing, separation and processing of transcribed RNA.


RNA polymerse


 The function of  RNA polymerase?

1): Activation of Ribonucleotides: Prior to transcription ribonucleotides are activated through phosphorylation. Enzyme phosphorylase is required along with energy. Activated ribonucleotides taking part in transcription are ATP,GTP,UTP and CTP.



2): DNA template__ On specific segments of DNA corresponding to one or more cistrons become derepressed and ready to transcribe.Each DNA segment has a promoter region and a terminator region.Transcription starts at the promoter region and terminates at terminator regions. RNA polymerase recognition site and binding site are present at promoter region. Chain opens in the region occupied by TA TA AG nucleotides in most of the prokaryotes. Unwindases and gyrases separate the chain.Termination region has poly  A base sequence or pallindromic sequence ( identical base sequence running in opposite directions in the two DNA chains). RNA polymerase binds to the promoter region. The two DNA strands uncoil progressively from the site of polymerase  binding. One of the two strands of DNA functions as template for transcription of RNA.It is called master or sense strand. Transcription 

Proceeds in 5' ➡️3' direction. 

3): Base pairing: Ribonucleoside triphosphates present in the medium come and lie opposite the nitrogen bases of the DNA template. They form complementary pairs_ U opposite A, A opposite T,C opposite G and G opposite C. Two extra phosphates are removed by the activity of pyrophosphatase and energy is released in this process.

4): Chain formation _ RNA chain is formed over DNA template by the action of RA polymerase on the adjacent ribonucleotides.With the initiation of chain formation, ó factor of the RNA polymerase separates andthr core enzyme moves along the DNA template causing elongation of RNA chain at the rate of 30 nucleotide/ second. The polymerase reaches the terminator region, RNA synthesis stops by Rho factor.Terminator region has 5' UUUUU A_ 3'sequence suggesting that 3' _ AAAAAA T _ 5' in the sense strand of DNA is atleast part of transcription terminator sequence. 

5): Separation of RNA: Termination or Rho factor has ATPase activity ( Roberts). It helps in releasing completed RNA chain which is called primary transcript.

6): Duplex formation: The two DNA strand form linkages amongst complementary base pairs after the release of primary transcript. Gyrases, unwindases and helix destabilizing proteins are released .Double helical structure of the DNA is resumed.


7): Post transcription processing _ RNA strand formed after transcription is larger, than the functional RNAs Post transcription processing in necessary to form functional RNA It is of 4 types: (i) cleavage (ii)Splicing 
(iii) Terminal additions.(iv) Nueotide modifications 
(i) Cleavage : Larger RNA precursors are cleaved form smaller RNAs. Primary transcript of r__ RNA is 45 S in eukaryotes.primary _ transcript is cleared by ribonuclease p( RNA enzyme) to form 5_ 7 t_ RNA precursors. 


(ii) Splicing : Eukaryotic transcripts possess extra segments ( introns or intervening  sequences). These are removed by nucleases. Ribozome is a self splicing intron involved in some of these reactions as well as catalysing polymerisation.


(iii) Terminal additions: Additional nucleotides are added to the end of RNAs for specific functions e.g.CCA segments in t_ RNA, cap nucleotides at 5' end of _ m_ RNA or poly _ A segments at 3' end of m_ RNA.


(iv) Nucleotide modifications: These occur commonly in t _ RNA. It is methylation. ( e.g methyl cytosine, methyl guansone), deamina_ tion ( e.g. inosine from adenine), dihydrouracil, pseudouracil etc.

In vitro synthesis of RNA was first performed by Ochoa ( 1967).

Ribonucleic acid or RNA : RNA or ribonucleic acid is a single chain poly_ ribonucleotide which functions as carrier of coded genetic or hereditary information from DNA to cytoplasm for taking part in translation and enzyme synthesis. It has 7p^ 12,00p ribonucleotides joined end to end. Backbone is formed by the alternating phosphate and ribose sugar. Phosphate combines with carbon 5' of its sugar and carbon 3' of next sugar similar to the arrangement found in DNA strand. Nitrogen bases are attached to sugars at carbon 1 of the latter. RNA  has uracil instead of thymine, rest of the nitrogenous bases are common in  DNA and RNA. These bases can  be arranged in any sequence which is complementary to the base sequence of DNA_ e.g. ATACTG sequence of DNA will have UAUGAC over RNA. There are 6 types of RNA s_ r_ RNA ,m_ RNA, t_  RNA, genetic ( genomic), small nuclear and small cytoplasm. 




Cytoplasm, Composition Cytoplasmic Streaming (Cyclosis) ,Role Of Cytoplasm



















































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