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Thursday, 29 July 2021

Transcription l Molecular basis of inheritance l Genetics l NEET l NTA l NCERT l NOTES

Transcription

·         Transcription is also governed by the complementarity of bases as in DNA. (Except that uracil in place of thymine is complementary to adenine).

·         Only one of the strands of the DNA acts as the template for RNA synthesis for the following reasons:

(I)        If both the strands code for RNA, two different (complementary) RNA molecules and two different proteins would be formed; hence the genetic information-transfer machinery would become complicated.

(II)       Since the two RNA molecules produced would be complementary to each other, they would wind together to form a double-stranded RNA without carrying out translation; that means the process of transcription would become futile.

 

·         A transcription unit in DNA has three regions:

(I)        A Promoter    

(II)       Structural and           

(III)     A terminator

 

 


 Schematic structure of transcriptional unit

 

·         The process is catalysed by DNA-dependent RNA-polymerase, which catalyses the polymerisation of nucleotides only in 5’-3’ direction.

·         The DNA strand with 3’-5’ polarity is called ‘template strand’, while the other strand with 5’-3’ polarity is called ‘coding strand’.

·         The coding strand is displaced and does not code for RNA, but reference points regarding transcription are made in relation to it.

·         The promoter refers to a particular sequence of DNA located towards the 5’ end (upstream) of the coding strand, where the RNA polymerase becomes bound for transcription.

·         The terminator is a sequence of DNA located towards the 3’ end (downstream) of the coding strand, where the process of transcription would stop.

·         There are additional regulatory sequences that may be present upstream or downstream to the promoter.

(A)       Transcription in prokaryotes

¾    In prokaryotes, the structural genes are polycistronic and continuous.

¾    In prokaryotes, there is a single DNA-dependent RNA polymerase, that catalyses the transcription of all the three types of RNA (mRNA, tRNA, rRNA).

¾    RNA polymerase binds to the promoter and initiates the process along with certain initiation factors sigma.

¾    It uses ribonucleoside triphosphates (also called ribonucleotides) for polymerisation on a DNA template following complementarity of bases.

¾    The enzyme facilitates the opening of the DNA-helix and elongation continues.

¾    Once the RNA polymerase reaches the terminator, the nascent RNA falls off and the RNA polymerase also separates; it is called termination of transcription and is facilitated by certain termination factors Rho.

¾    In prokaryotes, the mRNA synthesised does not required any processing to become active and both transcription and translation occur in the same cytosol; translation can start much before the mRNA is fully transcribed, i.e., transcription and translation can be  coupled.


                                           

                                                    Process of transcription in Bacteria

 

(B)       Transcription in Eukaryotes

¾    In eukaryotes, the structural genes are monocistronic and ‘split’.

¾    They have coding sequences called exons that form part of mRNA and non-coding sequences, called introns, that do not form part of the mRNA and are removed during splicing.

¾    In eukaryotes, there are at least three different RNA polymerases in the nucleus, apart from the RNA polymerase in the organelles, which function as follows:

                                             Process of  transcription in Eukaryotes

 

Þ    RNA polymerase I transcribes  rRNAs (26S, 18S and 5.8S),

Þ    RNA polymerase II transcribes the precursor of mRNA (called as heterogenous nuclear RNA (hnRNA) and

Þ    RNA-polymerase III catalyses  transcription of  tRNA

¾    The primary transcript contains both exons and introns and it is subjected to a process, called splicing, where the introns are removed and the exons are joined in a definite order to form  mRNA

¾    The hnRNA undergoes two additional processes called ‘capping’ and ‘tailing’.

¾    In capping, methyl guanosine triphosphate is added to the 5’ end of  hnRNA.

¾    In tailing, adenylate residues (about 200-300) are added at the 3’-  end of  hnRNA

¾    The fully processed hnRNA is called mRNA and is released from the nucleus into the cytoplasm

 


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