The Transcription

What is the transcription and what is its function?

Protein biosynthesis the first step and responsible for the rewriting of the DNA to mRNA.

Deoxyribonucleic acid (DNA) is located in the nucleus of the cell. No proteins can be made in this location. In the context of protein biosynthesis, therefore, the genetic code must be brought from the nucleus to the ribosomes (site of protein biosynthesis). This is done via the so-called mRNA (messenger RNA), which represents a complementary copy of a portion of the DNA.

Basically, transcription in the process is similar to replication. The major difference, however, is the fact that transcription results in a single-stranded strand of mRNA that affects only a portion of a gene. In contrast, replication synthethizes a whole genome and does so twice.

Transcription process

 1. Initiation: RNA polymerases bind to promoter molecules located on the sites of the genome to be copied. Before any genetic information can be read, the double helix must be unscrewed. This happens by breaking the hydrogen bond between the base pairs.

 2. Elongation: During elongation, DNA is transcribed into mRNA. The RNA polymerase migrates from 3 'to 5' and synthesized by attachment of free ribonucleotides to a DNA complementary mRNA substrand (Figure green strand), which has a 5 '-> 3' direction accordingly.

 3. Termination: In the course of transcription, the RNA polymerase encounters a terminator sequence when reading the DNA. Terminators stop the RNA polymerase and the separation of the mRNA subunit from the DNA occurs.

The further process differs for prokaryotes and eukaryotes:

For prokaryotes (organisms without cell nucleus, e.g., bacteria): the mRNA is immediately transported to the ribosomes.

Translation begins even before the transcription is completed. This is possible because mRNA and ribosomes are not separated by any cell membrane.

In the case of eukaryotes (organisms with cell nucleus, for example humans): Translation can only begin when the mRNA has come from the cell nucleus to the ribosomes. However, before this happens, the immature mRNA (as it is called immediately after completion of the termination) is still spliced ​​(splice = English). Immature mRNA consists of exons and introns. Only the exons contain important gene segments for protein biosynthesis. The introns are now removed and the remaining exons linked.

In addition, the mRNA at the 5 'end receives a cap from guanine, and at the 3' end of a poly-adenine tail of several adenine nucleotides. While the guanine cap ensures a more rapid transition from the nucleus to the cytoplasm for the now mature mRNA, the function of the poly adenine tail has not yet been conclusively explored.

Transcription is followed by translation.

Summary

In transcription, DNA is transcribed into mRNA and brought from the nucleus to the ribosomes

Transcription takes place in three phases: initiation, elongation and termination

Transcription in prokaryotes and eukaryotes differs essentially by splicing