Transcription:

The process of copying genetic information from one strand of the DNA into RNA is termed as transcription. The principle of complementarity governs the process, except that adenosine now base pairs with uracil instead of thymine. Here only a single fragment of DNA gets copied into RNA.

Transcription unit

 

The transcription unit of DNA contains three regions in the DNA:

(i) The promoter: It is the binding site for RNA polymerase for initiation of transcription. The promoter is located towards 5’ –end (upstream) of coding strands. It provides the binding site for RNA polymerase and also start transcription.

(ii) The structural gene: It codes for enzyme or protein for structural functions.

(iii) The terminator: It is the region where transcription ends. The terminator is located towards the 3’ –end (downstream) of the coding strand .Here, the process of transcription would stop.

• The DNA-dependent RNA polymerase helps in DNA replication by catalyzing the polymerization in only one direction, i.e., 5’ ⟶ 3’.
• The DNA strand that has the polarity 3’ ⟶ 5’ acts as a template and is also referred to as template strand. The strand which does not get transcripted is called coding strand and has the polarity 5’ ⟶ 3’. Its sequence is same as RNA.

Transcription Unit and the Gene

• The segment of DNA coding for a polypeptide is called cistron.
• In eukaryotes, the transcription unit possess a structural gene specific only for a single polypeptide. Thus it is called monocistronic.
• In prokaryotes, the transcription unit possessing the structural genes for many polypeptides Which are part of single metabolic pathway are called polycistronic.
• The gene in eukaryotes are split into the coding sequence of DNA called exon, and non-coding sequence of DNA called intron. mRNA contains only exon but no intron.

Transcription in Prokaryotes

In prokaryotes, the structural gene is polycistronic . In bacteria, the transcription of all the three types of RNA (mRNA, tRNA and rRNA) is catalysed by single DNA-dependent enzyme, called the RNA polymerase. The transcription is completed in three steps: initiation, elongation and termination.

1. Initiation: σ (sigma) factor recognizes the start signal and promotor region on DNA which then along with RNA polymerase binds to the promoter to initiate transcription.
2. Elongation:  The RNA polymerase after initiation of RNA transcription loses the σ factor but continues the polymerization of ribonucleotides to form RNA.
3. Termination: Once the RNA polymerase reaches the termination region of DNA, the RNA polymerase is separated from DNA-RNA hybrid and then RNA separates. This process is called termination which is facilitated by a termination factor ρ (rho).

In prokaryotes, both transcription and translation occur in the cytosol. It can be said that transcription and translation are coupled together.

Transcription in Eukaryotes

1. The structural genes are monocistronic in eukaryotes. It takes place in the nucleus.
2. In eukaryotes, three types of RNA polymerases are found in the nucleus:

RNA polymerase I transcribes rRNAs (28S, 18S, and 5.8S).

RNA polymerases II transcribes the precursor of mRNa (called heterogeneous nuclear RNA or hnRNA).

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

Post-transcriptional modifications

1. Coding gene sequence called exons form the part mRNA and non-coding sequence called introns are removed during  by RNA called splicing.
2. The hnRNA undergoes two additional process called capping and tailing.
3. In capping, an unusual nucleotide, methyl guanosine triphosphate, is added to the 5’ –end of hnRNA.
4. In tailing, adenylate residues (about 200-300) are added at 3’ –end.
5. Now this is fully processed hnRNA.