Transcription in prokaryotes and eukaryotes

Transcription in prokaryotes:-

Introduction:-

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

•   Here, adenine pairs with uracil instead of thymine.

•   Both strands are not copied during transcription, 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, hence form a double stranded RNA. This 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 (Transcription start site):- Binding site for RNA polymerase.

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

-   A terminator:- The site where transcription stops.

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

•   3’→5’ acts as template strand and 5’→3’ acts as coding strand.

3’-ATGCATGCATGCATGCATGCATGC-5’ template strand.

5’-TACGTACGTACGTACGTACGTACG-3’ coding strand.

Transcription unit and gene:-

•   Gene:- Functional unit of inheritance. It is the DNA sequence coding for RNA molecule.

•  Cistron:- A segment of DNA coding for a polypeptide.

•  Structural gene in a transcription unit is of 2 types:-

i. Monocistronic structural genes (split genes):- It is seen in eukaryotes. Here, the coding sequences  exons are interrupted by introns.

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

Reverse Transcription:- It is the process in cells by which an enzyme makes a copy of DNA from RNA. The enzyme that makes the DNA copy is called reverse transcriptase and is found in retroviruses, such as the human immunodeficiency virus (HIV).

Steps of transcription in prokaryotes:-

i. 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.

ii. Elongation:- The RNA chain is synthesized in the 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.

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

NOTE:- In bacteria (Prokaryotes) transcription and translation can be coupled (Translation can begin 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).

Polysome or Polyribosome or Ergosome:- It is a group of ribosomes bound to an mRNA molecule like “beads” on a “thread”. It consists of a complex of an mRNA molecule and two or more ribosomes that act to translate mRNA instructions into polypeptides.

Transcription in eukaryotes:- 

In eukaryotes, there are 2 additional complexities-

i. There are 3 RNA polymerases:-

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

•  RNA polymerase II:- Transcribes mRNA.

•  RNA polymerase III:- Transcribes tRNA, 5S rRNA and snRNAs.

ii. The primary transcripts (hnRNA):- It contain both the exons and introns and is non-functional. Hence introns have to be removed. For this, it undergoes splicing process.

Heterogenous RNA Processing:- The hnRNA is the collective term for the unprocessed mRNA (pre-mRNA) molecules in the nucleus. It contain both the exons and introns and is non-functional. Hence introns have to be removed. For this, it undergoes the following processes-

i. Splicing:- From hnRNA introns are removed by the spliceosome and exons are joined together.

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

iii. Tailing (Polyadenylation):- Here, adenylate residues (200-300) are added at 3’-end. It is the fully processed hnRNA, now called mRNA.