Protein synthesis in the cell
Each cell contains thousands of proteins. The properties of proteins are determined by their primary structure, that is, by the sequence of amino acids in their molecules.
In turn, hereditary information about the primary structure of the protein is contained in the sequence of nucleotides in the DNA molecule. This information is called genetic, and the piece of DNA that contains information about the primary structure of one protein is called a gene.
A gene is a piece of DNA that contains information about the primary structure of a single protein.
A gene is a unit of hereditary information in an organism.
Each DNA molecule contains many genes. The totality of all genes of an organism makes up its genotype.
Protein biosynthesis is one of the types of plastic metabolism, during which hereditary information encoded in DNA genes is realized in a specific sequence of amino acids in protein molecules.
The process of protein biosynthesis consists of two stages: transcription and translation.
Each stage of biosynthesis is catalyzed by a corresponding enzyme and supplied with the energy of ATP.
Biosynthesis occurs in cells at a tremendous rate. In the body of higher animals, up to 60 thousand peptide bonds are formed in one minute.
Transcription is the process of removing information from a DNA molecule by an mRNA (mRNA) molecule synthesized on it.
The carrier of genetic information is DNA located in the cell nucleus.
During transcription, a section of double-stranded DNA is “unwound”. An mRNA molecule is synthesized on one of the chains.
Informational (messenger) RNA consists of one strand and is synthesized on DNA in accordance with the rule of complementarity.
An mRNA molecule is formed, which is a copy of the second DNA strand, only in it thymine is replaced by uracil. The information encoded in DNA about the primary structure of the protein is rewritten into mRNA.
As in any other biochemical reaction, an enzyme, RNA polymerase, is involved in this process.
A DNA molecule contains a large number of genes. At the beginning of each gene there is a promoter – a special sequence of DNA nucleotides, which is determined by RNA polymerase, and from this point the assembly of the mRNA molecule begins.
The synthesis of mRNA continues until the next “punctuation mark” – the terminator. This nucleotide sequence indicates the completion of mRNA synthesis.
In prokaryotic cells, mRNA is formed in the cytoplasm, so the formed molecules can immediately participate in the synthesis of proteins.
In eukaryotes, mRNA is synthesized in the nucleus, so it first interacts with special nuclear proteins and is transferred through the nuclear membrane into the cytoplasm.
Translation is the translation of the nucleotide sequence of the mRNA molecule into the amino acid sequence of the protein molecule.
The cytoplasm of the cell must contain a complete set of amino acids necessary for the synthesis of proteins. These amino acids are formed as a result of the breakdown of proteins received by the body from food, and some can be synthesized in the body itself.
Amino acids are delivered to the ribosomes by transport RNAs (tRNAs). Any amino acid can enter the ribosome only by attaching itself to a special tRNA.
A ribosome is strung at the end of the mRNA, from which you need to start protein synthesis. It moves along the mRNA intermittently, “jumps”, lingering at each triplet for about 0.2 seconds.
During this time, the tRNA molecule, the anticodon of which is complementary to the codon in the ribosome, manages to recognize it. The amino acid that was bound to this tRNA is detached from the tRNA “petiole” and attaches to the growing protein chain to form a peptide bond. At the same moment, the next tRNA (the anticodon of which is complementary to the next triplet in the mRNA) approaches the ribosome, and the next amino acid is included in the growing chain.
The amino acids delivered to the ribosomes are oriented in relation to each other so that the carboxyl group of one molecule is next to the amino group of another molecule. As a result, a peptide bond is formed between them.
The ribosome gradually shifts along the mRNA, lingering on the next triplets. This is how the polypeptide (protein) molecule is gradually formed.
Protein synthesis continues until one of three stop codons (UAA, UAH or UGA) appears on the ribosome. After that, the protein chain is detached from the ribosome, enters the cytoplasm and forms the secondary, tertiary and quaternary structures inherent in this protein.
Since a cell needs many molecules of each protein, as soon as the ribosome, which first started protein synthesis on mRNA, moves forward, a second ribosome is strung on the same mRNA behind it. Then the following ribosomes are sequentially strung on the mRNA.
All ribosomes that synthesize the same protein encoded in a given mRNA form a polysome. It is on polysomes that the simultaneous synthesis of several identical protein molecules occurs.
When the synthesis of this protein is over, the ribosome can find another mRNA and start synthesizing another protein.
Example: the sequence of nucleotides of the template DNA strand: CHA TTA CAA.
On messenger RNA (mRNA), according to the principle of complementarity, the HCC AAU GUU chain will be synthesized, as a result of which a chain of amino acids will be built: alanine – asparagine – valine.
When replacing nucleotides in one of the triplets or rearranging them, this triplet will encode a different amino acid, and therefore the protein encoded by this gene will also change.