Transcription: The Process of RNA Synthesis

Concept: Transcription is the process of synthesizing RNA using the template strand (3′-5′ strand) of DNA. In other words, transcription is the process of transferring genetic information from the template strand of a gene to RNA according to the complementary principle.

Location: The process of transcription mainly takes place in the nucleus, on chromosomes, at a specific DNA segment (gene) in the G1 phase of interphase when the cell needs to synthesize proteins.

Stages:

• Initiation:

• RNA polymerase binds to the regulatory region, causing the gene to unwind and expose the 3′-5′ template strand.

• RNA polymerase begins synthesizing RNA at a specific location (transcription start site).

• Elongation:

• RNA polymerase moves along the template strand in the 3′-5′ direction to synthesize mRNA with a 5′-3′ direction from free ribonucleotides in the environment according to the complementary principle.

• The sequence of nucleotides on the template strand of the gene determines the sequence of ribonucleotides on RNA.

• Termination:

• When RNA polymerase reaches the end of the gene and encounters a termination signal, transcription stops and releases RNA.

• The region that has just been transcribed immediately rewinds the two DNA strands.

• After synthesis, tRNA and rRNA form higher-order structures to form complete RNA, while mRNA is directly used as a template for protein synthesis.

Differences between transcription in prokaryotes and eukaryotes:

• Eukaryotes: After transcription, pre-mRNA splices out introns, connects exons together, then adds a 5′ cap and a poly-A tail to form mature mRNA. Mature mRNA then passes through the nuclear membrane into the cytoplasm to serve as a template for protein synthesis.

• Prokaryotes: There is no processing of pre-mRNA like in eukaryotes, mRNA is directly synthesized from the gene and used as a template for protein synthesis.

Principle: Transcription in all organisms occurs according to the complementary principle and the template principle.

Significance: Transcription synthesizes various types of RNA involved in translation, thereby ensuring that protein synthesis occurs normally.

Note:

• In eukaryotes, there are many types of RNA polymerase enzymes involved in transcription.

• Mature mRNA is capped with a 5′ cap and a poly-A tail to protect mRNA from degradation and help mRNA bind to ribosomes during translation.