· In transcription, the process relies on the complementarity of bases, similar to DNA, but with uracil (U) replacing thymine (T) as the complementary base to adenine (A).
- Only one strand of DNA serves as the template for RNA synthesis. This is because:
- If both DNA strands coded for RNA, it would result in two different (complementary) RNA molecules, leading to the production of two different proteins. This would complicate the genetic information-transfer process.
- The two RNA molecules produced would be complementary to each other and could wind together to form a double-stranded RNA. This would prevent translation from occurring, rendering transcription ineffective.
· A transcription unit in DNA has three regions:
(I) A Promoter
(II) Structural and
(III) A terminator
Schematic structure of transcriptional unit
· The transcription process in eukaryotes is catalyzed by DNA-dependent RNA polymerase, which facilitates the polymerization of nucleotides in the 5’-3’ direction.
· Template and Coding Strands: The DNA strand with 3’-5’ polarity is known as the template strand, while the strand with 5’-3’ polarity is called the coding strand. The coding strand does not code for RNA but serves as a reference point for transcription.
· Promoter: The promoter is a specific DNA sequence located towards the 5’ end (upstream) of the coding strand. It is the site where RNA polymerase binds to initiate transcription.
· Terminator: The terminator is a DNA sequence located towards the 3’ end (downstream) of the coding strand, signaling the end of transcription.
Regulatory Sequences: Additional regulatory sequences may be found upstream or downstream of the promoter, playing a role in the regulation of transcription(A) Transcription in prokaryotes
¾In prokaryotes, transcription involves several unique features and processes:
- Polycistronic and Continuous Genes: Prokaryotic structural genes are polycistronic, meaning they contain multiple genes in a single mRNA molecule, and they are continuous without introns.
- Single RNA Polymerase: Prokaryotes use a single DNA-dependent RNA polymerase to catalyze the transcription of all three types of RNA—mRNA, tRNA, and rRNA.
- Initiation of Transcription: The RNA polymerase binds to a specific DNA sequence known as the promoter. This process is aided by initiation factors, notably the sigma factor.
- Polymerization Process: The RNA polymerase uses ribonucleoside triphosphates (ribonucleotides) as substrates to synthesize RNA, following the base-pairing rules on the DNA template.
- Elongation and Termination: The enzyme facilitates the unwinding of the DNA helix and elongates the RNA strand. When the RNA polymerase reaches the terminator sequence, the newly synthesized RNA strand is released, and the RNA polymerase detaches. This process, known as termination, is assisted by termination factors like Rho.
- No RNA Processing Required: In prokaryotes, the mRNA synthesized during transcription is immediately functional and does not require further processing. Transcription and translation occur in the same cytosol, allowing translation to begin even before transcription is complete, effectively coupling the two processes.
Process of transcription in Bacteria
(B) Transcription in Eukaryotes
¾
Process of transcription in Eukaryotes
Þ In eukaryotes, transcription involves several complex and distinct processes:
- Monocistronic and Split Genes: Eukaryotic structural genes are monocistronic, meaning each mRNA molecule codes for a single protein, and they are 'split' into coding (exons) and non-coding sequences (introns).
- Exons and Introns: Exons are coding sequences that become part of the mRNA, while introns are non-coding sequences that are removed during splicing.
- Multiple RNA Polymerases: Eukaryotes have at least three different RNA polymerases in the nucleus, in addition to RNA polymerases in organelles:
- RNA Polymerase I: Transcribes rRNA (26S, 18S, and 5.8S).
- RNA Polymerase II: Transcribes the precursor of mRNA, known as heterogeneous nuclear RNA (hnRNA).
- RNA Polymerase III: Catalyzes the transcription of tRNA.
- Splicing: The primary transcript, which contains both exons and introns, undergoes splicing to remove introns and join exons in a specific order to form mRNA.
- Capping and Tailing: hnRNA undergoes additional processing:
- Capping: Addition of methyl guanosine triphosphate to the 5’ end of hnRNA.
- Tailing: Addition of 200-300 adenylate residues to the 3’ end of hnRNA.
- Formation of mRNA: The fully processed hnRNA is called mRNA and is released from the nucleus into the cytoplasm for translation.
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