When does protein synthesis take place
The Protein synthesis or Gene expression, earlier too Protein synthesis called, is the production of a protein or polypeptide in living things. Both proteins and polypeptides, oligopeptides and dipeptides are chains of amino acids that differ in their length and sequence. They are formed on the ribosomes of living cells on the basis of the genetic information given in deoxyribonucleic acid (DNA).
Process of protein synthesis
- Main article: Transcription
In this first step in protein biosynthesis, a gene is read from the DNA and transcribed into an mRNA molecule. During this process, the nucleobases of the DNA (A, T, G, C) are transcribed into the nucleobases of the RNA (U, A, C, G). Uracil is used in place of thymine and ribose is found in RNA instead of deoxyribose.
The gene is transcribed by the enzyme RNA polymerase (and several other proteins), which requires DNA and the ribonucleotide triphosphates ATP, UTP, CTP and GTP as substrate. A continuous RNA chain (mRNA) is produced from this, complementary to a DNA strand, with cleavage of the respective two phosphate residues of the triphosphates. Codogen (DNA) → Codon (mRNA)
Different types of RNA polymerase exist in eukaryotes, since there are also genes that code not for proteins but for rRNAs and tRNAs.
In eukaryotes, transcription takes place in the nucleus, so that the mRNA has to be brought into the cytosol, since it is there that the translation is carried out. In prokaryotes, transcription takes place in the cell plasma, also known as the cytoplasm.
- Main article: Translation
Translation is understood as the translation of the base sequence of the mRNA into the amino acid sequence of the protein, which occurs on the ribosomes. In the coding area of the mRNA, three successive bases form a codon (also base triplet), which codes for an amino acid (see genetic code) becomes. The amino acids are translated sequentially according to the sequence of the codons.
Since there is no structural relationship between the codon and the associated amino acid, an intermediate piece is required which, on the one hand, binds the amino acid and, on the other hand, recognizes the associated codon on the mRNA. The tRNAs are necessary for this process as amino acid "transporters". These were specifically loaded with the appropriate amino acids by the aminoacyl-tRNA synthetases. The tRNA uses a structure called an anticodon to recognize the codon on the mRNA and specifically binds to it.
To form a peptide bond between two amino acids, they have to be brought into spatial proximity to one another. Since one or more enzymes are not able to do this on their own, the surface of a large supramolecular structure is required. This is the job of the ribosomes.
When a stop codon is reached which does not code for any amino acid, the translation is terminated.
Protein targeting and protein transport
- Main articles: Post-translational protein transport and cotranslational protein transport
Since many proteins are used as a destination. target) not the cytosol, but the extracellular space, the cell membrane, which organelles such as chloroplasts, mitochondria, peroxisomes, cell nucleus or endoplasmic reticulum have, the cell has various mechanisms to bring the proteins there. These proteins usually contain an N- or C-terminal signal sequence, which can be structured very differently depending on the target mechanism. In some cases there is no terminal signal sequence, but rather internal signals of the peptide chain, which determine the target location of the protein.
Proteins whose target is the endoplasmic reticulum (ER) is carrying a specific N-terminal sequence derived from a protein-RNA complex, the Signal Recognition Particle (SRP), is recognized. The SRP-peptide-ribosome complex is then recruited to the endoplasmic reticulum where it is recognized and bound. Translation continues through the membrane. The attached ribosomes give the impression of a "rough ER". See cotranslational protein transport.
Proteins found in the Chloroplasts must be spent have an N-terminal signal sequence that is usually early phosphorylated. The proteins Hsp70, 14-3-3 and Toc64 can also play a role in recognition and transmission through interaction with the protein precursor. After arriving on the surface of the chloroplast, the protein-precursor complex is recognized by receptor structures of the translocon apparatus of the outer chloroplast membrane (Translocon Of Outer Chloroplast Membrane, TOC). Under GTP hydrolysis, the protein is then imported into the intermembrane space or imported directly into the stroma through the translocal apparatus (TIC) of the inner chloroplast membrane. For the import into the membrane or the lumen of the thylakoids, at least 4 paths are used, which are called Sec-dependent, SRP dependent, delta-pH / Tat-dependent or spontaneous are designated.
For the Mitochondrion So far, three different import routes have been described for yeast and animal cells:
- The presequence import route whose proteins carry an N-terminal amphiphilic alpha helix. These proteins are mostly intended for the matrix, the inner membrane or the intermembrane space.
- The carrier protein import route for proteins of the inner membrane, which carry various internal signals.
- The import route of the proteins of the outer envelope membrane, which is used for the integration of proteins with a beta-barrel motif. Here, too, there are signals within the sequence.
All three import routes start at the mitochondrial translocal apparatus in the outer envelope membrane (TOM), which has different receptors. The Tom20 and Tom22 receptors recognize the N-terminal signal and pass the precursor protein on to the Tom40 pore. The Tom70 receptor recognizes the internal signals of the proteins that are intended for the outer membrane. After the import into the intermembrane space, the paths separate: the proteins with the beta barrel motif, which are intended for the outer membrane, are integrated into the membrane by the SAM complex (sorting and assembly machinery). The proteins of the other two import routes are directed to different TIM complexes: proteins with a presequence are recognized by the TIM23 complex, whereas proteins for the inner membrane are recognized by the TIM22 complex. The presequence is removed by the enzyme MPP (mitochondrial processing peptidase).
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