What is the transcription and what function does it perform?
Transcription (lat. Transcribere) is the first step in protein biosynthesis and is responsible for the transcription of DNA into 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 part 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 mRNA strand that affects only a subset of a gene. In contrast, replication synthesizes an entire genome, and does so twice.
1. Initiation: RNA polymerases bind to promoter molecules that are 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 dissolving the hydrogen bond between the base pairs.
2nd elongation: During the elongation, DNA is transcribed into mRNA. The RNA polymerase migrates from 3 'to 5' and synthesized by addition of free ribonucleotides to a DNA complementary mRNA substrand (Figure green strand), which has a 5 '-> 3' direction accordingly.
3. TerminationIn 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:
at prokaryotes (Nucleated organisms, 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.
at eukaryotes (Nucleated organisms, such as humans): Translation can not begin until the mRNA has passed from the nucleus to the ribosomes. However, before this happens, the immature mRNA (as it is called immediately after completion of the termination) still splits (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 are connected.
In addition, the mRNA at the 5 'end receives a cap of guanine, and at the 3' end a poly-adenine tail of several adenine nucleotides. While the guanine cap for the now mature mRNA ensures a faster transition from the cell nucleus to the cytoplasm, the function of the poly adenine tail has not yet been conclusively explored.
Transcription is followed by translation.