Transcription

Transcription refers to the "rewriting" of gene sequences contained in the DNA into a related but not identical language, the mRNA. The three phases of transcription are called:

• initiation
• elongation and
• termination.

Strand separation is carried out by the helicase, elongation with the aid of polymerase III (works on single strands only in the 5'-3' direction), initiation by the enzyme primase, which forms the primers. Polymerase I removes the RNA primers and replaces them with DNA nucleotides; the gaps in the new DNA are closed by DNA ligase. The mechanism is leading- or lagging-strand adapted!

Transcription is necessary because the DNA is too large and too valuable to move through the cell. During transcription, the genetic information of the DNA is transcribed into mRNA by a copying process of the RNA polymerase. The mRNA (messenger RNA) then serves as a "transcript" for translation. In this subsequent process, the mRNA is translated into amino acids on the ribosome. Genes are read at different frequencies. This allows the cell to react appropriately depending on the conditions and to produce different amounts of mRNA.

During translation, it is important to distinguish between the start codon, the normal codons and the stop codons. Translation will always begin at the start codon AUG(base triplet consisting of adenine (A), uracil (U) and guanine (G)) of the mRNA and end at one of the three stop codons(consisting of UGA, UAA or UAG).

• All other base triplets that are neither start nor stop codons and each encode a specific amino acid are referred to as "normal" codons. The start codon AUG also codes for an amino acid (methionine).
• The three stop codons are only responsible for terminating translation. They do not encode an amino acid.

The first tRNA (transfer RNA) now attaches to the mRNA at the start codon (the start codon is AUG and the first attaching tRNA accordingly takes up the amino acid methionine). The tRNA has the task of transporting the individual amino acids to the ribosome and then linking them to another amino acid. The peptide chains are formed from this sequence.

The tRNA consists of several arms. One arm binds an amino acid. The anticodon, which matches the corresponding base codon of the mRNA, is located on the other arm.

Example: The tRNA for methionine has the anticodon UAC; this only matches the base triplet AUG in the mRNA. This means that the base sequence AUG in the mRNA codes for the amino acid methionine. Each tRNA is only responsible for one amino acid at a time, analogous to its anticodon.

This means that each amino acid requires a specific tRNA in order to be transferred to the corresponding codon on the mRNA. This is followed by a second tRNA with a corresponding amino acid, which is attached next to the first tRNA. A peptide bond is formed between the two amino acids. After attachment of the amino acid, the first tRNA leaves the ribosome. The attached amino acid is now located at the end of the arm of the second tRNA together with its amino acid. Furthermore, a third tRNA together with a specific amino acid attaches to the mRNA. The process is repeated until a base triplet encoding a stop codon appears in the mRNA. There is no suitable tRNA for stop codons, so that the resulting peptide chain is now detached. The polypeptide strand is finalized.