Which enzyme handles DNA in the transcription?

The transcription

learning goals

If you've worked through this page, you should

  • be able to describe the tasks of the transition,
  • show the difference between transcription and DNA replication,
  • be able to explain the transcription process in individual steps,
  • be able to draw and explain the structure of an operon,
  • be able to explain the functions of promoter and operator in more detail,
  • Being able to explain the roles of promoter, operator, structural genes and RNA polymerase in the transcription process.

Task of transcription

The task of transcription is to provide a working copy of a DNA segment, for example a structural gene. This working copy in the form of an mRNA is then translated into a protein by the ribosomes. But also the ribosomal RNA - the rRNA - which is an important structural element of the ribosomes, has to be provided by transcription of the corresponding DNA segments. And the tRNA molecules of a cell are not created just like that, but through the transcription of corresponding areas of DNA. Much is still unclear as far as the transcription of DNA is concerned. Are all sections of DNA transcribed? What about the so-called "junk DNA" that sits between genes and that actually makes up most of the DNA?

Difference between transcription and replication

Transcription is basically a modified replication. However, with a few differences:

First

  • It won't die entire DNA of a cell is duplicated or copied, but rather just a small partnamely a gene or an operon, a small group of genes.
    We understand a gene to be a section of DNA that is responsible for the synthesis of a protein. The term operon is explained below.

Secondly

  • When replicating will be both Strands of the double helix copied. When transcribing, only one a transcript (a copy) made of the two strands.

Third

  • Replication creates new ones DNA. The copy that is created during transcription is chemically modified DNA, so-called RNA. This RNA (ribonucleic acid) differs from the DNA (deoxyribonucleic acid) in two ways:
    a) The sugar units contain an oxygen atom more,
    b) Instead of the base Thymine the base uracil occurs in the RNA.

Fourth

  • When replicating, the DNA copy remains in the nucleus or in prokaryotes (who do not have a nucleus) near the old DNA.
    In contrast, the newly synthesized RNA migrates during transcription into the cell plasmawhere it agglomerates with ribosomes.

Transcription process

The most important enzyme in transcription is RNA polymerase, which in terms of functionality can definitely be equated with DNA polymerase. As we have just seen, however, the entire DNA is not copied during transcription, but only the part that is responsible for the synthesis of a protein (or a group of proteins), a so-called gene (or operon). To do this, the RNA polymerase must have Beginning find this gene or operon. A human cell has 46 strands of DNA in its nucleus, each a few million base pairs long. How is the RNA polymerase supposed to find the beginning of a gene?

Structure of an operon

An operon is, so to speak, the smallest functional unit in DNA. You could also say: An operon is a group of related genes with a common on / off switch.

Promoter

The promoter is at the beginning of each operon. This is a short section of DNA that is characterized by a specific sequence of bases in which the letters T and A appear particularly frequently. Above all, the base sequence / the "motif" TATATT can be found here. The promoter is the docking point for the RNA polymerase.

One reason for the frequent occurrence of bases T and A is certainly that only two hydrogen bonds are formed between T and A bases. T- and A-rich DNA regions can therefore be separated particularly easily, and that is also the first important step in the transcription of the DNA. Because of the many T and A bases, the DNA double helix can be split particularly easily into single strands at the promoter (Spektrum der Wissenschaft, March 2014, page 20).

operator

Next comes the operator region. A specific repressor protein can dock onto this DNA region according to the lock and key principle. If such a repressor protein sits on the operator, the path for the RNA polymerase is blocked and transcription is not possible. Here the cell has invented an effective method with which it can prevent the transcription of genes that are currently not needed.

Structural genes

The "correct" genes that are responsible for the proteins are attached to the operator. Such genes are also known as structural genes.

Terminator

Just as the beginning of a gene is specially marked, the end of a transcription section must also be marked. Such a specific base sequence is also referred to as a terminator.

The transcription process

The RNA polymerase diffuses around in the nuclear plasma and at some point comes into contact with a strand of DNA. The enzyme then sits on the DNA double helix like a locomotive on a track. It now slides along the DNA. As soon as the RNA polymerase encounters a promoter sequence, a more stable bond is created. The RNA polymerase recognizes a promoter by the typical TATATT motif (among others).

If the operator, which is usually located directly behind the promoter, is not blocked by a repressor protein, the RNA polymerase begins with the transcription. The bases of the structural genes are translated into mRNA. In prokaryotes, ribosomes immediately attach to the resulting mRNA and begin translation. With eukaryotes, this doesn't happen that quickly. The mRNA has to leave the cell nucleus first. Only then can ribosomes attach to the mRNA and begin translation.

When the RNA polymerase arrives at the terminator, transcription stops and the enzyme leaves the double helix. The polymerase now diffuses around aimlessly in the nuclear plasma until it happens to come into contact with the DNA again.

The transcription process - a closer look

First, the DNA double helix is ​​untwisted over a length of several base pairs. One of the two single strands is then the master copy for the mRNA; this single strand is then referred to as the codogenic strand. The RNA nucleotides are now gradually attached to this strand. More precisely, they are nucleotides with three phosphate groups, because the process of elongation (lengthening of the mRNA) is very energy-consuming, and the RNA nucleotides themselves bring this energy with them in the form of two phosphate groups, which are then split off (similar to when splitting ATP into AMP and 2 phosphate).

The RNA polymerase moves steadily during this process. A DNA-mRNA hybrid double strand with a length of exactly 9 base pairs is formed. At the tenth base, the H-bonds between the DNA nucleotide and the complementary RNA nucleotide are broken and the mRNA becomes detached from the codogenic strand.

Copy errors occur

Sometimes the RNA polymerase makes a copy error and appends the wrong RNA nucleotide to the 3 'end of the growing mRNA chain. When the RNA polymerase notices this error, it essentially goes back a little, the wrong RNA nucleotide is cut off by a special transcription factor and is then replaced by the correct RNA nucleotide.