Ribosomal RNA (Structure Diagram of Ribosomal RNA)
In 1968, Robert Hawley, Marshall Nillen-Berg and Hagobin Horana won the Nobel Prize in Physiology or Medicine prize.
Shortly after the publication of the DNA double helix structure in 1953, the famous Russian-American physicist George Gamow gave a manuscript to Francis Kerry, one of the discoverers of the DNA double helix structure gram. In the manuscript, he proposed for the first time the correspondence between the four bases A, T, C, and G of DNA and the 20 amino acids of proteins, which greatly inspired Crick.
A Physicist Joins
George Gamov was born in Russia in 1904 and graduated from Leningrad University. At the age of 28, he was elected as the youngest academician of the USSR Academy of Sciences. Gamow emigrated to the United States in 1934, where he became a professor of physics at George Washington University and other schools. Gamow’s research is extremely broad, including atomic decay, the origin of the universe, and the genetic code. In these fields, he proposed a series of pioneering theories, such as advocating and developing the Big Bang theory, predicting the cosmic microwave background radiation, etc. Later, two young American engineers, Arnold Penzias and Robert Wilson, observed this radiation at the origin of the universe and won the 1978 Nobel Prize in Physics. Unfortunately, at this time Gamow has been dead for 10 years.
However, Gamow is best known for his work in popularizing science. Gamow published 25 books in his lifetime, 18 of which were popular science books, including “Adventures in the Physical World” and “From One to Infinity”. Today, these popular and humorous popular science masterpieces are still best-selling all over the world, influencing generations of teenagers.
Although Schrödinger mentioned the concept of “genetic code book” in his 1944 book “What is Life”, few scientists at that time, including Schrödinger himself, paid attention to the correspondence between DNA and proteins. After reading Watson and Crick’s article in the journal Nature, Gamow turned his attention from his own field of quantum mechanics and cosmology to the genetic code of life, and began to think about the relationship between DNA and protein. relation.
Based on the intuition of a theoretical physicist, Gamow boldly infers that there are only 20 amino acids that make up proteins, and speculates that the three nucleotides (triplets) connected by the DNA chain encode one amino acid, because If a nucleotide corresponds to an amino acid, it can only encode four amino acids, and if two connected nucleotides correspond to an amino acid, it can only encode 16 amino acids, which are less than the 20 amino acids that make up a protein. If four linked nucleotide combinations correspond to one amino acid, there will be 44 quadruple nucleotide combinations, encoding 256 amino acids, far exceeding the number of known amino acids. Only when three connected nucleotides correspond to one amino acid, there are only 64 triplet combinations, closer to 20 amino acids. In October 1954, Gamow published his views in the journal Nature, in which heIt is proposed that every 4 nucleotides form an empty hole, and amino acids are embedded in it, like a “key and lock”. At the same time, he gave Crick a copy of the article, which aroused Crick’s interest in studying the genetic code.
At this time, Gamow and James Watson, one of the discoverers of the double helix structure of DNA, initiated the establishment of the RNA tie club, consisting of 20 famous scientists, each representing an amino acid, wearing exclusive tie. Gamow stood for alanine, Watson for proline, Clark for tyrosine, and Irving Chargaff, who discovered the base composition of DNA, for lysine. In this club, scientists often discuss their unpublished ideas or research. Inspired by Gamow, Crick began to study the genetic code in earnest, submitting a manuscript to the club in 1955. Crick affirmed Gamow’s theoretical contribution on triplet codons and multiple codons encoding the same amino acid, and pointed out that DNA codons do not directly correspond to protein amino acids, but require the help of intermediate RNA.
The correspondence between the genetic code and amino acids, every combination of three nucleotides corresponds to one amino acid.
Genetic Central Dogma
At this time, Watson had recognized the important role of RNA in protein synthesis and shifted the research focus to his postdoc Research object – on viral RNA. Watson proposed in 1953 that DNA must first transmit information to RNA, and then RNA can guide the synthesis of white matter, because DNA is in the nucleus, and the place where protein is synthesized is the ribosome in the cytoplasm. Both Watson and Crick realized that this RNA was like a “messenger” designed to deliver the genetic information on the DNA. What is this messenger RNA?
At first, Crick wasn’t sure what messenger RNA was. He once believed that ribosomal RNA (rRNA) was messenger RNA, and that each ribosome synthesized a protein using rRNA as a template. But this contradicted some scientific findings at the time, because no matter what kind of cells, rRNA sequences are basically the same, but they can synthesize proteins with completely different amino acid sequences.
In 1959, Arthur Pardee, Francois Jacob and Jacques Monod of the Pasteur Institute in France speculated that there might be an unstable messenger molecule that is easy to degrade through experiments on the lactose operon. Generally, Escherichia coli needs to grow in a medium containing lactose, and Escherichia coli needs to synthesize galactosidase in the body to decompose lactose for energy. Paddy et al. found a strain of E. coli with a mutation in the beta-galactosidase gene that was unable to break down lactose due to the lack of beta-galactosidase. This mutant cannot grow in media containing lactose, but if the normal β-galactosidase gene is added to the medium, these mutant E. coli can synthesize large amounts of β-galactosidase in a few minutes enzymes and start growing on lactose medium. Paddy et al speculated that,Adding the normal beta-galactosidase gene doesn’t make new ribosomal RNA, but there must be a messenger molecule that breaks down quickly so that beta-galactosidase can be synthesized, but they still can’t say what this messenger molecule is .
At a small seminar held at King’s College, Cambridge University in the United Kingdom, Crick was so excited when he saw Paddy’s research report that he immediately realized that messenger RNA was not ribosomal RNA. Crick colleague and partner, South African biologist Sydney Brener, also attended the meeting and decided to isolate this messenger RNA. In 1961, Brenner collaborated with Jacob and Matthew Messelson at Caltech. After repeated experiments, he finally captured this fleeting messenger molecule, which turned out to be an RNA molecule complementary to DNA, called messenger RNA.
At this time, Crick realized that the genetic information of the DNA in the nucleus has two directions, one is DNA self-replication, that is, in the process of dividing the cell into two, the double-stranded DNA will be opened to form Single-stranded DNA, then each DNA single-stranded DNA serves as a template to replicate another complementary DNA single-stranded DNA, forming two new double-stranded DNAs, each double-stranded DNA randomly enters a progeny cell, and each double-stranded DNA contains an original single-stranded DNA. Another direction is the process by which DNA guides protein synthesis, in which messenger RNA burns away the DNA’s genetic information and transports it from the nucleus to ribosomes in the cytoplasm. The ribosome then uses the messenger RNA as a template to sequentially assemble various amino acids into long chains, eventually forming a unique empty structure, the protein.
This is a far-reaching law of genetic information transmission proposed by Crick in 1958 – the main content of the central law, that is, genetic information is transmitted from DNA to RNA (transcription), and then from RNA to Proteins (translation), which also includes the self-replication of DNA, and states that “once information has been transmitted to a protein, it cannot be transmitted again”. However, other scientists later discovered that not only DNA can replicate itself, but some RNA can also replicate itself, and RNA can be reverse-transcribed into DNA under the action of reverse transcriptase. In 1970, based on these new findings, Crick modified the central dogma, which is still in use today.
However, how do the various amino acids wandering in the cytoplasm be assembled on the ribosome in an orderly manner?
As early as 1955, Crick proposed a linker hypothesis in a paper submitted to the RNA Tie Club, that is, there is a linker connecting amino acids and RNA templates in the cytoplasm. Its main function is to find and grab amino acids in the cytoplasm and transport them to ribosomes for assembly. Soon, other scientists discovered that there is indeed a kind of RNA that functions as an adapter, that is, transfer RNA, which travels in the cytoplasm in the form of intersecting “hairpin” structures. One end can recognize the genetic code of the messenger RNA, and the other end is used to bind the corresponding amino acid. The discovery reflects the Crick’s superhuman intelligence and insight.
Crick concluded in a paper published in the “Nature” magazine in December 1961: The genetic code consists of triplet bases, and each triplet is arranged in sequence without overlapping. The genetic code is read from a fixed starting point, and multiple triplets can correspond to the same amino acid. At this time, Crick has undoubtedly become the most active scientist in the field of genetic code research. His research enthusiasm has inspired many young scientists to join in the work of deciphering the genetic code, among which Marshall Nirenberg is the best.
Cracking the Genetic Code
Nirenberg was born in April 1927 in New York to a Jewish family. He has been interested in biology since childhood. In 1952, he received a master’s degree in zoology from the University of Florida, then transferred to biochemistry, and received a doctorate from the University of Michigan in 1957. In the same year, Nirenberg entered the National Institutes of Health for postdoctoral research. He officially entered the National Institutes of Health in 1959 until his death in January 2010.
Before 1959, Nirenberg mainly studied how sugar molecules were transported and metabolized in the body, as well as the purification of enzymes, but he had little experience in gene regulation and protein synthesis. However, when he saw the work of Paddy, Jacob and Mono on the lactose operon, he suddenly became very interested in molecular genetics. After some deliberation, he decided to join the research team that is cracking the genetic code, which he considers “one of the most exciting areas of research in biochemistry”. When he told his colleagues about this decision, some colleagues warned him that a newly hired researcher needs to produce research results as soon as possible in order to keep his job and rush into unfamiliar fields. Once you fail to achieve any results, you may be dismissed by the institute soon, and it will be even more difficult to find a new research position. Nirenberg himself knew that this was a very risky decision. Faced with fierce competition from outstanding scientists from all over the world, he was afraid of failure, but his desire to explore the unknown and make extraordinary breakthroughs prevailed.
Soon, Nirenberg devoted himself to research work. At first, he tried to repeat the method of Paddy, Jacob and Mono, but the result was not satisfactory. At this time, Nirenberg accidentally noticed that two scientists at Harvard University had invented a cell-free protein synthesis system, which made him overjoyed. The cell-free protein synthesis system uses enzymes to break down the bacteria’s cell wall, leaving the bacteria’s other components intact. By adding carbon-14 isotope-labeled amino acids, it is possible to observe how proteins are synthesized in vitro. Nirenberg was acutely aware of the importance of this method in the study of the genetic code. He imagined that if endogenous DNA or RNA was destroyed and some exogenous DNA or RNA fragments of known sequence were added, it would be possible to observe the role of these DNA and RNA in protein synthesis.
Nirenberg plans to build this system in two years, because the experimental process is very tedious and requires a lot of experiments to explore the conditions, and he is the only one who needs to learn everything from scratchpeople. After about a year and a half of hard work, Nirenberg finally built the initial system when German postdoc Heinrich Matthaei joined Nirenberg’s lab. Originally, Matthaei was mainly engaged in botany research, had experience in isotope research, and was very interested in in vitro protein synthesis, so he found Nirenberg. Alone, of course, Nirenberg was more than happy, and soon formed a directing duo.
In early 1961, Nirenberg asked Matthaei to add a section of polyuric acid (…UUUUUU…) to 20 test tubes of the cell-free protein synthesis system. At the same time, all 20 amino acids were added to each test tube, one of which was labeled with a carbon-14 isotope. They wanted to see what kind of polypeptide chains could be synthesized. The reason for choosing uridine acid is because uridine acid is a unique nucleotide of RNA, which can distinguish whether DNA or RNA directly guides protein synthesis.
One day, Matthaei excitedly called *** to Nirenberg who was on a business trip. He found that polypeptide chains were synthesized in only one test tube, to which carbon-14 isotope-labeled phenylalanine was added. Nirenberg was overjoyed when he heard Maathai’s ***, and came back early to witness this important moment. The two teachers found that polyuridylic acid RNA and ribosomal RNA are indispensable for the synthesis of this phenylalanine polypeptide chain, and DNase cannot affect the synthesis of the polypeptide chain in this system. When RNase was added, protein synthesis was terminated, further evidence of the presence of messenger RNA. However, Nirenberg and Maathai don’t seem to realize the implications of their findings. In May 1961, they published their main results early in a little-known journal, Biochemical and Biophysical Research Letters, which had just started publishing but didn’t attract much attention. In addition, they did not mention in the paper the most critical finding of this study-polyuridine RNA strands may carry the genetic code for phenylalanine.
Three months later, Nirenberg participated in the Fifth International Biochemistry Congress held in Moscow, and reported the research results of their master and apprentice to about 35 participating scientists in a branch venue. He barely knew the scientists involved, and most of the others didn’t know him either. Luckily, the host for this venue is Francis Crick. After listening to Nirenberg’s report, Crick couldn’t hide his excitement. He caused a stir by inviting Nirenberg to repeat his report at a larger seminar the next day. Nirenberg remembered the scene fondly. He later recalled, “It was unbelievable! Everyone gave me a standing ovation, and five years later, everyone regarded me as a science rock star.” That same month, his work was republished in the more influential journal Proceedings of the National Academy of Sciences. Indeed, with his courage and hard work, Nirenberg has turned from a layman in the field of protein synthesis into a great scientist who has attracted much attention.has made a great contribution.
Combining Nirenberg and his own research, Crick deduced that the triplet “UUUU” is the codon encoding phenylalanine, which was recognized by the participants. In this way, one of the codons was officially born. Inspired by Crick, Nirenberg hopes to decipher the codons for the other 63 amino acids and draw in large numbers of young people, up to 20. Soon, Nirenberg followed the previous method to decipher the triplets “AAA”, “CCC” and “GGG” that code for lysine, proline and glycine, respectively. The decoding of other amino acids was not so easy, but through continuous technical improvement, by 1966, the codons of other amino acids were also cracked, including three stop codons that did not encode any amino acids.
It is worth mentioning that, inspired by Nirenberg’s research, the American biologist Hargobind Horana (Hargobind Horana) who was born in Punjab, British India (now Pakistan) invented A chemical synthesis method that provides another effective way to decipher the genetic code. Later, this method was also used to synthesize an artificial gene, which was expressed in E. coli for the first time, thus opening a new era of genetic engineering technology.
In 1965, American biochemist Robert Holley (Robert Holley) analyzed the primary structure and secondary structure of transfer RNA, proving that Crick’s conjoined “linker” is involved in protein synthesis It played an important role. Three years later, Nirenberg, Horana, and Holley shared the Nobel Prize in Physiology or Medicine “for the unraveling of the genetic code and its role in the synthesis of urogenium *** in proteins.”
So far, the genetic code of messenger RNA has been completely cracked, but the cracking of the code of life has just begun. How does life start from DNA with a single structure and linear arrangement, and with the help of messenger RNA and other RNAs, synthesize proteins with different structures and functions, and evolve into a colorful and complex world of life? Can we change these codes of life, correct genetic diseases in humans, or even create new life? The answers to these questions have begun to emerge, but there are still many unsolved mysteries of life, attracting groups of scientists.
Southern Weekend Special Writer Tang Bo