Proteins such as enzymes are essential for cellular function. But how do eukaryotic cells take the information contained in DNA and turn it into a protein? Describe the sequence of events that need to take place to make a fully functioning protein such as an enzyme?
Information about one protein occupies a certain part of the DNA chain; such regions are called genes. Recall that DNA and proteins are completely different compounds. DNA monomers are 4 nucleotides, and the links of protein chains are amino acids. In this case, DNA never leaves the nucleus, and the ribosomes are the site of protein synthesis.
Thus, to implement genetic information, DNA in the nucleus works in conjunction with ribosomes in the cytoplasm. Already in 1953, F. Crick established that the flow of information from DNA to ribosomes occurs through RNA, this is the central dogma of molecular biology. Ribonucleic acid (RNA), which is found both in the nucleus and in the cytoplasm, takes on the role of a transmitter of genetic information into proteins.
There are two processes:
Transcription - rewriting DNA genes into a copy of the mRNA gene (messenger RNA)
Translation - protein synthesis based on mRNA gene copy information
Transcription is the first step in protein biosynthesis from gene to protein. If a cell requires a protein, first the corresponding section of DNA (gene) in the cell nucleus is read and a copy is made from it. This process is called transcription. The mechanism of the process is similar to DNA self-duplication. However, only a small part of the DNA sequence is read. Therefore, RNA molecules are much shorter than the DNA double helix. Transcription is catalyzed by the enzyme RNA polymerase. It recognizes the start and end signs of a gene on a DNA strand based on certain base sequences.
Under the action of special enzymes, the section of the DNA molecule with the necessary gene is untwisted, the bonds between the chains are broken. Further, on one of the chains, the process of assembly of the RNA molecule begins according to the principle of complementarity. Recall that a feature of the RNA molecule is the replacement of the Thymine nucleotide with Uracil nucleotide in the nucleotide sequence. This mechanism ensures error-free rewriting of a section of the DNA molecule. This copy, called m-RNA (template or messenger), which then migrates from the nucleus into the cytoplasm to the ribosomes.
The resulting mRNA thus encodes, in the form of its specific nucleotide sequence, the instructions for synthesizing the amino acid sequence of the resulting protein.
Broadcast. The nucleotide sequence of an mRNA molecule determines the amino acid sequence of the polypeptide. Translation of the "language" of a nucleic acid into a protein "language" is guaranteed by translation by ribosomes.
To convert this information into protein synthesis, in addition to ribosomes, amino acids, and m-RNA, another type of RNA is needed - transport RNA (t-RNA). The t-RNA molecule is relatively small, only about 80 nucleotides long, and carries amino acids to the ribosomes. A t-RNA molecule always binds only one specific amino acid. Therefore, there are at least 20 different types of t-RNA in a cell. The t-RNA molecule at one of its vertices has a region through which recognition occurs according to the principle of complementarity of the m-RNA codon. This site was named anticodon.
Ribosomes are a complex complex of proteins and r-RNA. Ribosomes in the cytoplasm are strung like beads on the mRNA strand. As soon as the ribosome reaches the start codon, protein synthesis begins. According to the principle of complementarity, t-RNA recognizes the corresponding m-RNA codon, then the next t-RNA molecule is next to it. The amino acids they brought in are bound by a special enzyme, and the ribosome jumps to the next codon. This process continues many times until one of the stop codons is reached. The ribosome splits into two subunits and is ready for the next synthesis. Even before the completion of peptide synthesis, the chains begin to fold into a secondary and tertiary structure. This leads to the formation of protein molecules with a specific biological function.
This process can be repeated many times, and the cell can reproduce the required protein for itself at the right time. Thanks to these mechanisms, the regulation of biochemical processes in the cell, its renewal and self-reproduction is ensured.