The ability of DNA to replicate autonomously and permanently exist determines its mission as a carrier of genetic information. Genetic information in the form of nucleotide sequences in its existence must be expressed through gene expression. Gene expression refers to the process by which genetic information generates proteins with specific biological functions through transcription and translation. Transcription is the process of synthesizing the corresponding RNA under the guidance of a DNA sequence; translation refers to the process of further synthesizing the corresponding protein under the guidance of RNA. The transcription process determines the sequence of newly synthesized RNA by the law of base complementation; the accuracy of the translation process is determined by the genetic code. The three consecutive nucleotides on the mRNA determine an amino acid on the protein polypeptide chain, which is called the triplet codon, which is the genetic code. (1) Transcription The product of transcription is RNA, in which mRNA is a direct template for the synthesis of proteins. Although tRNA and rRNA are not templates for translation, they are directly involved in the biosynthesis of proteins. The function of tRNA is to transport amino acids, and the function of rRNA is as a place for protein biosynthesis. Transcription is catalyzed by DNA-directed RNA polymerase, and the synthetic substrates are ATP, GTP, CTP and UTP. In the polymerization reaction, a DNA single strand is used as a template for synthesis, and polymerization is carried out according to the base complementation law (GC, AT, CG, UA), 3' OH of one nucleotide molecule and another nucleotide molecule. The alpha phosphate group of the triphosphate undergoes a nucleophilic reaction to form a phosphodiester bond, and the synthesis direction is also 5'-3'. A DNA template is only one strand in a double-stranded DNA. The template DNA strand is called an antisense strand, also called a negative strand; the DNA strand complementary to it is a sense strand, a cryptogram strand, or a positive strand. The newly synthesized RNA sequence is identical to the positive strand DNA, except that U replaces T. The transcription process begins with RNA polymerase recognition and binding to the DNA template. As the enzyme moves forward, the transcript RNA gradually prolongs until RNA polymerase reaches the termination signal, RNA polymerase separates from the DNA template, and the product RNA strand falls off and transcription is terminated. . In prokaryotes, mRNA molecules are essentially unprocessed and can be used as templates to participate in protein biosynthesis after synthesis, while tRNA and rRNA are processed on the basis of synthetic precursor molecules to become biologically functional mature molecules. . The processing of tRNA is mainly by chemical modification of certain sequences and certain bases by nucleases. The precursor molecule of rRNA is cleaved into three rRNA molecules by nuclease. The processing of eukaryotic RNA is much more complicated than that of prokaryotes, first because transcription occurs in the nucleus and translation is carried out in the cytoplasm. Transcription and translation are separated in time and space, and then eukaryotic genes are contained. The mRNA produced by its transcription must be subjected to shear processing to become mature mRNA. The processing of rRNA is carried out in the nucleolus, and the final product is the ribosomal 40S and 60S subunits; the processing of tRNA includes removal of the 5'-end leader sequence, splicing to remove introns, 3' UU substitution by CCA, base modification, and the like. The processing of mRNA mainly refers to capping at the 5' end, poly poly(A) tail at the 3' end and splicing to remove introns. In RNA viruses, a transcriptional mode opposite to conventional transcription, which is called RNA synthesis, is called reverse transcription, and it is found that the enzyme that catalyzes this process is an RNA-directed DNA polymerase, also known as reverse transcriptase. . (2) Translation Translation refers to the biosynthesis of proteins, which is the process of transforming the genetic information existing in the form of nucleotide sequences on DNA into the amino acid sequence of the protein through the genetic code. Transcription and translation can occur simultaneously in prokaryotes, but in eukaryotes transcription is carried out in the nucleus and translation is carried out in the cytoplasm. The translation process can be divided into three stages: start, extension and termination. The substances involved in protein biosynthesis mainly include three kinds of RNAs: mRNA, tRNA and rRNA, ribosome, 20 kinds of amino acids, protein factors, enzymes, free nucleotides and inorganic ions. 1. mRNA is a direct template for translation The genetic information in the mRNA molecule is transcribed from the DNA molecule, and the nucleotide sequence in the mRNA molecule is converted into the amino acid sequence in the protein molecule by translation. This information is transformed by the genetic code. Every three nucleotides on the mRNA molecule determines an amino acid on the protein polypeptide chain. These three nucleotides are called the genetic code, the triplet codon. The translation begins with the start codon AUG, and the password is read continuously in the direction of the 5'-3' of the mRNA until a stop codon is read to generate a protein with a specific sequence. There is no overlap or gap in the reading of the password, following the non-overlapping and non-interval characteristics of the genetic code, so nucleotide insertions and deletions on the DNA molecule can lead to frame shift mutation of the genetic code, resulting in structurally and functionally abnormal peptides. chain. Another property of the genetic code is degeneracy. Many amino acids have multiple codons, and the first and second nucleotides between these codons tend to be the same, mainly in the third nucleotide. It can be understood that base substitution on a DNA molecule may produce two consequences: if the mutation is on the first nucleotide of the codon, it must result in a change in the codon, thus producing a polypeptide chain with a different amino acid; if the mutation is On the third nucleotide of the codon, there is a great possibility of becoming a recessive mutation, ie changing the codon without changing the amino acid species. 2. Ribosome is the site of peptide chain synthesis The ribosome is composed of large and small subunits. The subunit contains dozens of different proteins and several rRNAs, which are embedded in a certain spatial position to become large intracellular particles visible under the microscope. Ribosomes act like a plant that moves along an mRNA template, performing protein biosynthesis. There are many kinds of proteins in ribosomes, each of which has its own function, providing all the necessary conditions for protein synthesis. 3.tRNA and aminoacyl tRNA tRNA is at the heart of protein synthesis, providing not only for the translation of each codon into an amino acid, but also for the delivery of the desired amino acid to the ribosome without errors. There are two important parts of the tRNA molecule, the anti-codon and the 3' CCA-OH end. The anti-codon has an anti-codon that can be paired with a codon on the mRNA, while the 3' CCA-OH terminus is capable of binding to a particular amino acid. Aminoacyl tRNA is an activated form of amino acid, which is produced by aminoacyl-tRNA synthetase. The enzyme has absolute specificity, allowing only specific amino acids to bind to specific tRNAs, thus ensuring the correctness of translation. Ribosome cycle Protein biosynthesis can be divided into three steps: 1 initiation: the ribosomal subunit and the starting tRNA are combined with the initiation codon and other factors to form a starting complex with the 5' start codon of the coding region on the mRNA. . 2 Extension: The relative movement of ribosomes and mRNA, with the participation of elongation factor, carries the amino acid into the ribosome by tRNA, and synthesizes the polypeptide chain encoded by the mRNA sequence. 3 termination: extension to the mRNA appears a stop code, release factor into the ribosome, release the nascent peptide chain and ribosome from the mRNA, thus completing the synthesis of a polypeptide chain. The released ribosome can be combined with the starting tRNA, the initiation factor, and the mRNA, and then another protein is synthesized, so it is called the ribosome cycle. The direction of peptide chain synthesis is from the amino terminus to the carboxy terminus, and the translational orientation on the mRNA template is 5'-3'. In the translation process, several ribosomes can be combined at the same time in each mRNA chain for peptide chain synthesis. This phenomenon is called polyribosome. 5. Processing after protein synthesis The newly synthesized peptide chain must be post-translationally processed to become a biologically active mature protein. Limited hydrolysis is the most common form of processing: the leading N-terminal methionine residue of the nascent peptide chain, which is cleaved by a specific proteolytic enzyme after the peptide chain leaves the ribosome; the translational initial product of secreted proteins and transmembrane proteins The N-terminus has 13 to 36 amino acid residues, which are called signal peptides. They are resected after transmembrane transport; some peptides in some protein precursors can be folded to form a spatial structure; polyproteins are translated. Several different proteins are produced by hydrolysis. Covalent chemical modification is another common form of processing. Certain side chains of amino acid residues can be acetylated, glycosylated, hydroxylated, methylated, nucleotideized, phosphorylated, and the like. Shanghai Chuangsai Technology has excellent performance, interleukin cytokines, fetal bovine serum, electrophoresis equipment scientific instruments, raw material drug standards, chemical reagents, cell culture consumables, Shanghai Chuangsai, mass products special promotions, welcome to inquire! Universal Steamer,Multi Layer Steamer,Large Stainless Steel Steamer,Stainless Steel Electric Steamer SUZHOU JIAYI KITCHENWARE TECHNOLOGY CO.,LTD , https://www.jiayikitchenwares.com