Скачать или смотреть Mechanism of Transcription: DNA 🧬to RNA – The Process of Genetic Expression

Transcription is the biological process by which genetic information stored in DNA is copied into a complementary RNA strand. This is the first step in gene expression, leading to the synthesis of proteins. Transcription occurs in three main stages: Initiation, Elongation, and Termination.

1. Initiation
Transcription begins when RNA polymerase binds to the promoter region of the gene, with the help of transcription factors.

In prokaryotes, RNA polymerase directly recognizes the promoter, while in eukaryotes, transcription factors help recruit RNA polymerase II to the promoter.

The DNA double helix unwinds, creating a transcription bubble, exposing the template strand.

2. Elongation
RNA polymerase moves along the template strand (antisense strand) in the 3' to 5' direction, synthesizing a complementary mRNA strand in the 5' to 3' direction.

Free ribonucleotides (ATP, UTP, GTP, CTP) are incorporated, forming a growing RNA strand based on complementary base pairing (A-U, G-C).

The RNA strand detaches as it is synthesized, and the DNA helix reforms behind the polymerase.

3. Termination
Transcription stops when RNA polymerase encounters a termination signal.

In prokaryotes, termination occurs by:

Rho-independent termination: Formation of a GC-rich hairpin loop followed by a poly-U tail in RNA, causing detachment.

Rho-dependent termination: The Rho protein binds to the RNA and disrupts transcription.

In eukaryotes, termination is coupled with RNA processing, where the RNA polymerase transcribes a polyadenylation signal, leading to cleavage of the transcript.

Post-Transcriptional Modifications (Eukaryotes Only)
Before being translated into a protein, the primary RNA transcript (pre-mRNA) undergoes modifications:

5’ Capping: Addition of a 7-methylguanosine cap for stability and ribosome recognition.

Splicing: Removal of introns (non-coding regions) and joining of exons (coding regions).

Polyadenylation: Addition of a poly-A tail at the 3’ end to protect the mRNA from degradation.

Key Differences in Transcription Between Prokaryotes and Eukaryotes
Feature Prokaryotes Eukaryotes
RNA Polymerase Single RNA polymerase Three types (RNA Pol I, II, III)
Promoter Elements -10 (Pribnow box) and -35 sequences TATA box, CAAT box
Location Cytoplasm Nucleus
RNA Processing None Capping, Splicing, Polyadenylation
Termination Rho-dependent or Rho-independent Polyadenylation signal
Conclusion
Transcription is an essential process that converts genetic information into RNA, setting the stage for protein synthesis. While the core mechanism is similar in all organisms, eukaryotic transcription is more complex due to RNA processing and regulation.