What Is a Ribonucleic Acid?
Ribonucleic acid (abbreviated as RNA, or Ribonucleic Acid), is a genetic information carrier that exists in biological cells and some viruses and virus-like viruses. RNA condenses from ribonucleotides via phosphodiester bonds to grow chain-like molecules. A ribonucleotide molecule consists of phosphate, ribose, and bases. There are four main types of RNA bases, namely A adenine, G guanine, C cytosine, and U uracil. Among them, U (uracil) replaces T in DNA.
- Unlike DNA, RNA is generally a single-stranded long molecule that does not form a double helix structure, but many RNAs also need to form a certain secondary structure or even a base pairing principle.
- In 1990, some scientists
- In cells, RNA is mainly divided into three types according to the structure and function, namely tRNA, rRNA, and mRNA. mRNA is a template for protein synthesis transcribed from DNA sequences; tRNA is the recognizer of the genetic code on mRNA and the transporter of amino acids; rRNA is the part that makes up the ribosome, and ribosome is the machinery for protein synthesis.
- There are many different types and functions of small RNAs in cells, such as components
- Transcription refers to the process of unraveling the double strands of DNA so that RNA polymerase can synthesize the corresponding messenger RNA (mRNA) according to the base sequence on the DNA. This process is required when the human body needs enzymes or proteins. The messenger mRNA is used to take the codon out of the nuclear model. The ribosome can further use the messenger RNA (mRNA) to translate and synthesize the required proteins. The bases of DNA are A (adenine) and G (guanine). ), C (cytosine), T (thymine), and the base of RNA does not have T (thymine), replaced by U (uracil), that is, A (adenine), G (guanine), C (cytosine) and U (uracil). In DNA, A and T are connected by two hydrogen bonds, and G and C are connected by three hydrogen bonds. However, RNA has only U and no T. If it is A, the mRNA will be U, which is the position to replace the original T. As shown in the figure below, if the base sequence of the right DNA is 'AAACCG', and the left DNA will become 'TTTGGC' due to pairing However, because RNA has no T base and only U, the sequence corresponding to the synthesized mRNA is 'UUUGGC' because DNA is too Large, unable to enter and exit the nuclear membrane (the membrane of the cell nucleus), so the presence of mRNA is needed to allow the mRNA to pass through the nuclear pores (holes in the nuclear membrane) to the cytoplasm for translation (the process by which the ribosome synthesizes proteins). It is an indispensable and important response to both humans and animals and even bacteria.
- Various amino acids that are free in the cytoplasm use mRNA as a template to synthesize proteins with a certain amino acid sequence. This process is called translation. 
- First amino acids are combined with tRNA to generate aminoacyl-tRNA
- Then the start of the polypeptide chain:
- The mRNA goes from the nucleus to the cytoplasm. Under the action of the initiation factor and Mg, the small subunits bind to the starting site of the mRNA. The methionyl (methionine) -tRNA anti-codon recognizes the start code AuG on the mRNA. (mRNA) binds complementary, and then the large subunit also binds. The ribosome can accommodate two codons at a time. (Formyl methionyl in prokaryotes)
- Then there is the extension of the polypeptide chain:
- The aminoacyl-tRNA corresponding to the second code enters the A position of the ribosome, also known as the acceptor position, and the hydrogen bond of the code and the anti-code complement each other. Under the action of the polypeptide chain transferase (transpeptidase) on the large subunit, the amino acid carried by the tRNA donor (P position) is transferred to the amino acid at position A and forms a peptide bond (CO-NH), The tRNA leaves the P position and leaves the P position and re-enters the cytoplasm. At the same time, the ribosome moves forward along the mRNA, and the new code is at the A position of the ribosome. The corresponding new aminoacyl-tRNA enters the A position again. The peptide bond hangs the dipeptide on this amino acid to form a tripeptide, and the ribosome moves forward, thereby gradually progressing, and repeating this cycle, the nucleotide sequence on the mRNA is changed to the amino acid sequence.
- Finally, the termination and release of the polypeptide chain:
- The extension of the peptide chain is not endless. When the stop code appears on the mRNA (UGA, U amino acid, and UGA), no corresponding amino acid is transported into the ribosome, the synthesis of the peptide chain stops, and it is recognized by the termination factor, enters the A position, inhibits the transpeptidase effect, and makes the polypeptide The strand and tRNA are hydrolyzed and released, all released along the central tube of the large subunit, leaving the ribosome. At the same time, the large and small subunits are separated from the mRNA, and can then be combined with the mRNA initiation code, can be free in the cytoplasm or can be degraded, and the mRNA can also be degraded.