Monday, July 27, 2020

Molecular Basis of Inheritance - Notes | Class 12 | Part 5: Transcription



-    It is the process of copying genetic information from one strand of the DNA into RNA.

-    Here, adenine pairs with uracil instead of thymine.

-    The DNA- dependent RNA polymerase catalyzes the polymerization only in 5’→3’direction.

-    3’→5’ acts as template strand. RNA is built from this.

-    5’→3’ acts as coding strand. This is copied to RNA.



-    During transcription, both strands are not copied because

   The code for proteins is different in both strands. This complicates the translation.

   If 2 RNA molecules are produced simultaneously, this would be complimentary to each other. It forms a double stranded RNA and prevents translation.

Transcription Unit

-    It is the segment of DNA between the sites of initiation and termination of transcription. It consists of 3 regions:

   A promoter: Binding site for RNA polymerase. Located towards 5'-end (upstream).

   Structural gene: The region between promoter and terminator where transcription takes place.

   A terminator: The site where transcription stops. Located towards 3'-end (downstream).

Transcription unit and gene

Gene is a functional unit of inheritance. It is the DNA sequence coding for an RNA (mRNA, rRNA or tRNA).

Cistron is a segment of DNA coding for a polypeptide during protein synthesis. It is the largest element of a gene.

Structural gene in a transcription unit is 2 types:

} Monocistronic structural genes (split genes): It is seen in eukaryotes. Here, coding sequences (exons or expressed sequences) are interrupted by introns (intervening sequences).

Exons appear in processed mRNA.

Introns do not appear in processed mRNA.

} Polycistronic structural genes: It is seen in prokaryotes. Here, there are no split genes.

Transcription in prokaryotes

In bacteria (Prokaryotes), synthesis of all types of RNA are catalysed by a single RNA polymerase. It has 3 steps:

} Initiation: Here, the enzyme RNA polymerase binds at the promoter site of DNA. This causes the local unwinding of the DNA double helix. An initiation factor (σ factor) present in RNA polymerase initiates the RNA synthesis.

} Elongation: RNA chain is synthesized in 5’-3’ direction. In this process, activated ribonucleoside triphosphates (ATP, GTP, UTP & CTP) are added. This is complementary to the base sequence in the DNA template.

} Termination: A termination factor (ρ factor) binds to the RNA polymerase and terminates the transcription.

Transcription in Bacteria

In bacteria, transcription and translation can be coupled (translation begins before mRNA is fully transcribed) because

·   mRNA requires no processing to become active.

·    Transcription and translation take place in the same compartment (no separation of cytosol and nucleus).

Transcription in eukaryotes

In eukaryotes, there are 2 additional complexities:

1.  There are 3 RNA polymerases:

·   RNA polymerase I: Transcribes rRNAs (28S, 18S & 5.8S).

·   RNA polymerase II: Transcribes the heterogeneous nuclear RNA (hnRNA). It is the precursor of mRNA.

·   RNA polymerase III: Transcribes tRNA, 5S rRNA and snRNAs (small nuclear RNAs).

2.  The primary transcripts (hnRNA) contain exons and introns and are non-functional. Hence introns must be removed. For this, it undergoes the following processes:

·   Splicing: From hnRNA, introns are removed (by the spliceosome) and exons are spliced (joined) together.

·   Capping: Here, a nucleotide methyl guanosine triphosphate (cap) is added to the 5’ end of hnRNA.

·   Tailing (Polyadenylation): Here, adenylate residues (200-300) are added at 3’-end.

Now, it is the fully processed hnRNA, called mRNA.

Transcription & Processing in Eukaryotes

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