Gene expression

Genetic Expression is a product of multiple processes carried out from chromatin to end in functional protein

Gene expression is the process by which all organisms, both prokaryotes and eukaryotes, transform the information encoded by nucleic acids into the proteins necessary for their development, functioning and reproduction with other organisms. Gene expression is key to creating a phenotype.

In all organisms the DNA content of all its cells (except gametes) is essentially identical. This means that they contain all the information necessary for the synthesis of all proteins. But not all genes are expressed at the same time or in all cells.

Except for housekeeping genes, (genes that are expressed in all cells of the organism and code for proteins that are essential for its general function) all other genes are expressed or not expressed depending on the function of the cell in a particular tissue. For example, genes encoding proteins responsible for axonal transport are expressed in neurons but not in lymphocytes where genes responsible for the immune response are expressed. There is also temporal specificity, this means that different genes in a cell are turned on or off at different times in an organism's life. In addition, the regulation of genes varies according to their functions.

Mechanism

Chromatin Remodeling

The remodeling starts from a fully condensed state of the DNA, (which is the chromosome) and must reach a state where the DNA is exposed and physically enabled for reading and decoding; (which is the stretching of nucleosomes with "open" DNA).

Transcription

The gene itself is typically a stretch of DNA and does not play an active role. The production of copies of messenger RNA (mRNA) from DNA is called transcription, and is carried out by RNA polymerase, which adds one RNA nucleotide at a time to a growing strand of RNA. This RNA is complementary to the DNA nucleotides that are transcribed, that is, if there is a thymine (T) in the DNA, an Adenine (A) will be added to the RNA. However, if there is an A in the DNA strand in the RNA the nitrogenous base uracil (U) will be inserted instead of T. Therefore, in the mRNA complementary to the "TAC" is transcribed as "AUG".

RNA Processing

Transcription of protein-coding genes creates a primary RNA transcript at the site of the gene. This can be altered before being translated, this is particularly common in eukaryotic cells. The most common RNA processing is splicing to remove introns. Introns are segments of RNA that are not found in mature RNA, although they can function as precursors, for example, for ncRNAs, which are RNAs that carry out direct modification of nucleotides in other RNAs. Introns are common in eukaryotic genes, but rare in prokaryotes.

In the same way, the addition of the cap 5', a methyl-guanine, is carried out. And a poly-A tail, a series of adenines added to the end of the messenger to protect it from degradation in the cytoplasm.

Extensive RNA processing may be a possible evolutionary advantage of the eukaryotic nucleus. In prokaryotes transcription and translation (see below) occur at the same time, while in eukaryotes the nuclear envelope separates the two processes allowing time for RNA processing to occur.

Noncoding RNA Maturation

In most organisms non-coding genes (ncRNA) are transcribed as precursors to undergo further transformation. In the case of ribosomal RNA (rRNA), it is often transcribed as a pre-rRNA containing one or more rRNAs, the pre-rRNA is cleaved, with modifications (2'-O-methylation and the formation of pseudouridine) to specific nucleolus sites, approximately 150 different small restricted RNA species, called small nucleolar RNAs (snoRNAs), which, like snRNAs, snoRNAs are associated with proteins, forming snoPRNs. In eukaryotes, in particular, a snoPRN, called RNase MRP cleaves the pre-45S rRNA into the 28S, 5.8S, and 18S rRNA. The rRNA and RNA processing factors are in a large aggregate called the nucleolus.

In the case of transfer RNA (tRNA), for example, the 5' sequence is removed by RNase P, while the 3' end is removed by RNase P. it is removed by the enzyme Z tRNase. In the case of micro RNAs (miRNAs), miRNAs are first transcribed as primary transcripts or pri-miRNAs with a poly-A cap and tail and processed as short, 70-nucleotide, mother-loop structures known as pre-miRNAs. in the cell nucleus by the Drosha and Pasha enzymes, after being exported, it is then processed to mature miRNAs in the cytoplasm by interaction with Dicer endonuclease, which also initiates the formation of the RNA-induced silencing complex (RISC), made up of the Argonaute protein.

RNA export

In more mature eukaryotes, RNA must be exported from the nucleus to the cytoplasm. While some RNA functions in the nucleus, many RNA molecules are transported through nuclear pores and into the cytosol. In particular, this includes all types of RNA involved in protein synthesis. In some cases the RNA is further transported to a specific part of the cytoplasm, such as the synapse, which is then pulled by motor proteins via proteins that bind to specific linker sequences (called "zip codes") in the RNA.

Translation

Protein synthesis consists of two stages: messenger RNA translation, through which amino acids arrive at the ribosome on amino acid transfer RNA, where they are joined to form a polypeptide according to the nucleotide sequence of the messenger RNA. The second stage consists of post-translational modifications that the polypeptides undergo until they reach their functional state or native conformation.

Regulation

Genetic regulation comprises all those processes that affect the action of a gene at the level of gene transcription (RNA) or gene translation (protein), regulating their final products.
Regulatory processes include: chromatin alteration, histone modifications, and DNA methylation and are the molecular basis of epigenetics.

For biotechnological purposes, it is sought to interact with these regulatory processes and alter the expression of certain genes.