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10.5.1. Mechanism of transcription

Mechanism of DNA transcription

The transcription of DNA is the first process of gene expression, whereby the information is transferred in the sequence of the DNA to the protein sequence using various RNA as intermediates. During gene transcription, DNA sequences are transcribed into RNA by an enzyme called RNA polymerase that synthesizes a messenger RNA that maintains DNA sequence information. In this way, DNA transcription could also be called messenger RNA synthesis.

The transcription is the passage of a DNA sequence to a sequence of RNA, either mRNA , rRNA or tRNA. In this process, the following intervene:

  • A strand of DNA that acts as a template. Only one of the two nucleotide strands of DNA is transcribed (template strand) while the other (informational or coding strand) is not transcribed.
  • Ribonucleotides triphosphate of A, C, G and U ( ATPGTP, CTP and UTP). They are joined by an ester bond between the phosphoric acid of carbon 5' of a ribonucleotide triphosphate, and the -OH group, located in position 3' of the last ribonucleotide that has joined the RNA chain that is being formed.
  • Enzymes. The process is catalyzed by RNA polymerases .

The RNA - dependent DNA polymerase is the enzyme responsible for synthesizing RNA. It is characterized by:

  • It joins ribonucleotide triphosphate with phosphodiester (nucleotide) bonds, always in the 5'→ 3' direction. The energy necessary for the formation of phosphodiester bonds is obtained from the hydrolysis of ribonucleotides triphosphate.
  • The RNA polymerase uses a chain DNA as template to synthesize RNA of the nucleobases complementary. The RNA polymerase binds each ribonucleotide with corresponding deoxyribonucleotide. As the newly synthesized new RNA strand is complementary to that of the DNA that is used as a template, it is almost the same as the informational strand, but taking into account that the complementary base of A is U in RNA. That is, the new RNA is like the informational chain of DNA but instead of T it has U.
  • The RNA polymerase begins to act on zones of the specific DNA called regions or genes promoters.

The transcription in prokaryotes has some differences with performing the eukaryotic cells , but both is performed in four successive phases: