## Transcription of genetic information from DNA to RNA

While bacteria have DNA molecules stored freely in the cell, eukaryotes store genetic information in the cell nucleus. Within the nucleus, DNA is thus separated from the rest of the intracellular space by a nuclear membrane, which also ensures its necessary protection. For gene expression to occur, the genetic information must be transferred from the nucleus to the cytoplasm, where protein synthesis takes place. The first step of transfer is provided by transcription, where the individual genes from DNA are transcribed into messenger RNA, or mRNA for short. The mRNA molecules are small enough to pass through the pores of the nuclear membrane and carry the information into the cytoplasm.

Like replication, transcription takes place in three basic phases, which are initiation, elongation and termination. During initiation, the RNA polymerase enzyme recognizes the promoter region that is located just before the start of the gene. From this site, RNA polymerase moves in both directions until it recognizes specific regions characterizing the presence of the coding region. Then the transcription goes into the second phase; elongation. The enzyme RNA polymerase can unravel the double strand of DNA, which, similar to replication, creates a so-called bubble. In this case, we are talking about a transcription bubble, where the separated template strand serves as a blueprint for the transcription of information into the mRNA molecule. After transcription of the entire section of DNA, transcription is terminated. The RNA polymerase is separated from the DNA molecule and the released mRNA strand is referred to as the primary transcript.

The primary transcript, or otherwise called pre-mRNA, must be edited before leaving the nucleus. A “cap“ is added to the 5'-end of the pre-mRNA, which is most often formed by a 7-methylguanosine molecule. Also, a special polymerase adds a large number of adenines to the 3'-end. Such an end is then referred to as the poly(A)-tail. Both the cap and the tail protect the mRNA from degradation.

In addition, the coding sequences of many eukaryotic genes are interrupted by sequences called introns. These are non-coding regions of DNA, which means that they do not participate in the creation of the resulting protein. Editing of the primary transcript also includes the cutting out of possible introns (splicing) by specific enzymes. Mature mRNA is thus formed only by coding sections, which are called exons (Figure 3.7). When the primary transcript of eukaryotic cells has undergone all three types of editing, including capping, tailing, and excision of introns, the mRNA can leave the nucleus and undergo translation.

It is not always the case that one pre-mRNA produces the same protein each time. In eukaryotic cells, a process called alternative splicing takes place. Its essence is the creation of different mRNAs from one primary transcript. If the gene consists of several exons, one of the exons can be cut out at the same time during the splicing of introns, resulting in different combinations of used and unused exons. Such diverse mRNAs are then translated (in some cases) into multiple, mutually different proteins (Figure 3.8). The possibility of such alternative splicing of primary transcripts saves the amount of genetic information stored in the nucleus of eukaryotic cells, since instead of genes or their parts being duplicated, one gene can encode different (albeit partially similar) proteins.

An example of alternative splicing is the sex determination of the fly Drosophila melanogaster. The main regulator of this process is the Sex-lethal (Sxl) gene, which is located on the sex chromosome X. In both sexes, one universal primary transcript is formed, which subsequently undergoes alternative splicing. It is the combination of exons of the resulting mRNA and the length of the resulting protein that determine the sex of D. melanogaster. Specifically, exon 3 contains a termination codon (a stop codon) that signals the end of translation. If exon 3 is included in the resulting mRNA, protein synthesis is terminated prematurely at this location resulting in a short product which has no regulatory function. This signals the cell to activate a set of genes that determine that the embryo will develop as a male. Conversely, if exon 3 is excised together with the introns, the resulting mRNA is translated at full length. A regulatory protein is formed that activates the genes responsible for the development of the female sex of D. melanogaster. In this way, alternative splicing can significantly influence the phenotype of the developing individual.

It is also worth mentioning the fact that eukaryotic organisms (with the exception of the simplest ones) do not differ so much in the number of genes, but they differ in their complexity, and alternative splicing is one of the mechanisms involved in increasing their complexity.