RNA is a chain molecule formed by condensation of ribonucleotides with phosphate bonds. Ribonucleotide molecules are composed of phosphoric acid, ribose and bases. RNA has four main bases, namely, A adenine, G guanine, C cytosine and U uracil. Among them, U- uracil replaced T- thymine in DNA and became the characteristic base of RNA. If mRNA is the blueprint for synthesizing protein, then ribosome is the factory for synthesizing protein. However, there is a lack of special affinity between the 20 amino acids of protein and the bases of mRNA. Therefore, a special RNA (transferrin RNA (tRNA)) must be used to transport amino acids to ribosomes. TRNA can accurately connect the amino acids it carries to form polypeptide chains according to the genetic code of mRNA. Each amino acid can bind to 1-4 trnas, and there are more than 40 known trnas.
TRNA is the smallest RNA with an average molecular weight of about 27,000 (25,000-30,000) and consists of 70 to 90 nucleotides. Moreover, it has the characteristics of rare bases, except pseudouridine and hypoxanthine nucleoside, mainly methylated purine and pyrimidine. This rare base is generally made by special modification after transcription.
Starting from 1969, the structures of more than a dozen tRNA in yeast, Escherichia coli, wheat, mice and other organisms were studied, and it was proved that their base sequences could be folded into clover-like secondary structures (Figure 3-23), and all of them had the following * * * characteristics:
① There are G (mostly) or C at the 5' end.
② The 3' ends all end in ACC order.
③ There is a guanine-rich ring.
④ At the top of this loop, there is a anticodon loop with three exposed bases, which is called anticodon. Anticodons can be paired with complementary codons on the mRNA chain.
⑤ There are thymine rings. RNA (ribosomal RNA (rRNA) is the main component of ribosomes. Ribosomes are factories for synthesizing protein. In E.coli, the amount of rRNA accounts for 75%-85% of total RNA, while the amount of tRNA accounts for 15%, and the amount of mRNA only accounts for 3%-5%.
Generally, rRNA combines with ribosomal proteins to form ribosomes. If the rRNA on ribosome is removed, the structure of ribosome will collapse. Prokaryote ribosomes contain three kinds of rRNA: 5S, 16S and 23S.
S is the sedimentation coefficient. When the precipitation speed of particles is measured by ultracentrifugation, the speed is proportional to the size and diameter of particles. 5S contains 120 nucleotides, 16S contains 1540 nucleotides, and 23S contains 2900 nucleotides. In eukaryotes, there are four kinds of rRNA, their molecular sizes are 5S, 5.8S, 18S and 28S, and they have about 120, 160, 1900 and 4700 nucleotides respectively. RRNA is single-stranded, containing different amounts of a and u, g and c, but has a wide range of double-stranded regions. In the double-stranded region, the bases are connected by hydrogen bonds, showing a hairpin helix.
The function of rRNA in protein synthesis is not completely clear. However, there is a nucleotide sequence at the 3' end of 16S that is complementary to the leading sequence of mRNA, which may contribute to the combination of mRNA and ribosome. MicroRNAs(miRNAs) are a kind of endogenous non-coding RNA with regulatory function, which is about 20~25 nucleotides in size. Mature miRNAs are produced from long primary transcripts after a series of nuclease cleavage processes, and then assembled into RNA-induced silencing complexes, which recognize the target mRNA through base complementary pairing, and instruct the silencing complexes to degrade the target mRNA or inhibit the translation of the target mRNA according to different complementary degrees. Recent studies have shown that miRNA participates in many regulatory pathways, including development, virus defense, hematopoietic process, organ formation, cell proliferation and apoptosis, and fat metabolism.
In addition to the main RNA mentioned above, there are some other RNAs: (small RNA)
They exist in the nucleus and cytoplasm of eukaryotes, and their length is 100 to 300 bases (the longest in yeast is about 1000 bases). Many cells can contain 105 ~ 106 RNA molecules, while a few cells cannot be directly detected. They are synthesized by RNA polymerase II or RNA polymerase III, and some of them can be capped like mRNA.
There are two main types of small molecular RNA: one is snRNA (small nuclear RNA), which exists in the nucleus; The other is scRNA (small cytoplasmic RNA), which exists in cytoplasm.
Small molecular RNA usually forms a complex with protein, which plays an important role in the life activities of cells.
① Small RNA:
It is the main component of RNA splicing in eukaryotic post-transcriptional processing. Five kinds of snRNA were found in mammals, and the length was about 100-2 15 nucleotides. SnRNA always exists in the nucleus, forms RNA splices with about 40 nuclear proteins, and plays an important role in RNA post-transcription processing. Some snRNPs are closely related to splicing, which are complementary to donor and recipient splicing sites and branching sequences respectively.
Among them, snRNA located in nucleolus is called small nucleosome RNA, which is involved in the processing of rRNA precursors and the assembly of ribosome subunits.
② Small RNA:
Cytoplasmic small RNA (SCRNA) is mainly located in the cytoplasm, with various types, and participates in the synthesis and transportation of protein. SRP particle is a ribonucleoprotein particle, which consists of one 7SRNA and six protein. Its main function is to recognize signal peptides and guide ribosomes into endoplasmic reticulum. RNA (antisense RNA) is involved in the regulation of gene expression.
The above RNA molecules are all products of transcription, and the mRNA is finally translated into protein, while rRNA, tRNA and snRNA do not carry the information translated into protein, and their final products are RNA. There is also a special RNA (its classification has nothing to do with the above RNA classification)-ribozyme.
Ribozyme is a term used to describe RNA with catalytic activity, that is, chemically essential ribonucleic acid (RNA) has the catalytic function of enzyme. Ribozyme substrates can be different molecules, and some substrates are only some parts of the same RNA molecule. Ribozymes have many functions, some can cut RNA, some can cut DNA, and some have activities such as RNA ligase and phosphatase. Compared with protease, ribozyme is a primitive catalytic enzyme with low catalytic efficiency.
Most ribozymes participate in the process of RNA self-shearing and processing by catalyzing the hydrolysis of phosphate and phosphodiester bonds, and they are also specific and even have Km value.
This discovery is that the RNA part (MIRNA) left by scientists in E.coli has enzyme activity under the condition of high concentration of magnesium ion in vitro after removing this part. Non-coding RNA (ribonucleic acid), a new type of life dark matter, is called "dark matter" in life. A few days ago, Professor Ke Dashan of China discovered a new type of circular noncoding RNA, and revealed the function and mechanism of this kind of noncoding RNA. This achievement was published in the internationally renowned journal Nature, Structure and Molecular Biology. Non-coding RNA is a large class of RNA molecules that do not encode protein, but play a regulatory role in cells.
Just as there are many "dark matter" and "dark energy" that can neither be seen nor felt in the universe, there is such a mysterious "dark matter"-non-coding RNA in the "small universe" of life.
More and more evidence shows that the occurrence and development of a series of major diseases are related to the imbalance of non-coding RNA regulation.
Circular RNA molecules have only attracted the attention of researchers in recent years, while previous studies mainly focused on linear RNA molecules. A new type of circular noncoding RNA discovered in Professor Shan Ge's laboratory is named exon-intron circular RNA. In the paper, they also studied why this new type of circular noncoding RNA becomes a circular rather than a linear molecule, and found that there are often complementary repetitions at both ends of the circular sequence.