Genes Direct the Synthesis of Proteins

Section 1: Genes Direct the Synthesis of Proteins

How do genes direct the synthesis of proteins? We know that genes are segments of DNA with genetic effects. DNA mainly resides in the cell nucleus, while proteins are synthesized in the cytoplasm. So, how is the genetic information carried by DNA transmitted to the cytoplasm? And once the genetic information reaches the cytoplasm, how does the cell interpret it?

Transcription of Genetic Information

How do genes in the nucleus direct protein synthesis in the cytoplasm? Scientists hypothesized that an intermediary substance acts as a messenger between DNA and proteins. Later, it was discovered that such a substance exists in cells—RNA.

What is RNA, and why is it suitable to serve as a messenger for DNA? RNA is another type of nucleic acid, and its molecular composition is quite similar to that of DNA: it is also composed of basic units called nucleotides, and these nucleotides contain four types of bases, which makes RNA capable of accurately transmitting genetic information. Unlike DNA, however, the five-carbon sugar in RNA is ribose instead of deoxyribose (Figure 4-1), and RNA lacks the base thymine (T), which is replaced by uracil (U) (Figure 4-2). RNA is generally single-stranded and shorter than DNA, allowing it to pass through nuclear pores and move from the nucleus to the cytoplasm.

This RNA that serves as the messenger for DNA is called messenger RNA (mRNA). Additionally, there are transfer RNA (tRNA) and ribosomal RNA (rRNA) (Figure 4-3).

How is genetic information from DNA transmitted to mRNA?

Through research, scientists discovered that RNA is synthesized in the nucleus by RNA polymerase using one strand of DNA as a template—a process called transcription. The basic process of transcription can be explained using mRNA as an example. When a cell begins to synthesize a particular protein, RNA polymerase binds to a segment of DNA that encodes the protein, causing the DNA double helix to unwind and expose the bases. Free ribonucleotides in the cell pair with the exposed bases on the DNA template strand according to complementary base pairing rules, and under the action of RNA polymerase, they are sequentially linked together, forming an mRNA molecule (Figure 4-4).

Translation of Genetic Information

After mRNA is synthesized, it passes through nuclear pores into the cytoplasm. The various amino acids free in the cytoplasm are then assembled into proteins with a specific sequence of amino acids, using mRNA as the template. This process is called translation.

As you already know, the sequence of nucleotides in nucleic acids contains genetic information. The essence of translation is to convert the nucleotide sequence of mRNA into the amino acid sequence of a protein. Think about how you use an English-Chinese dictionary: by relying on the correspondence between English words and Chinese characters, you can translate an English text into Chinese. To understand how mRNA is translated into proteins, we first need to establish the correspondence between the bases in mRNA and the amino acids.

What is the relationship between bases and amino acids?

DNA and RNA contain only four types of bases, while proteins in living organisms are composed of 21 different amino acids. How can these four bases determine the 21 amino acids? If one base were to specify one amino acid, then four bases could only determine four amino acids, which is clearly insufficient. If two bases specified one amino acid, four bases could determine 16 (i.e., 4²) amino acids, which is still not enough. If three bases specified one amino acid, four bases could determine 64 (i.e., 4³) amino acids, which would be sufficient to account for the 21 amino acids needed to build proteins.

This speculation was a step toward decoding the genetic code. Later, through a series of hypotheses and experiments, scientists eventually cracked the genetic code, discovering that each set of three adjacent bases on mRNA determines one amino acid. These sets of three bases are called codons (Figure 4-5), and scientists compiled a codon table with all 64 codons (Table 4-1).

Once mRNA enters the cytoplasm, it binds with the "assembly machinery" of proteins—the ribosome—to form a "production line" for synthesizing proteins. With the "production line" in place, "workers" are needed to produce the product.

How are the amino acids free in the cytoplasm transported to the protein "production line"?

The "workers" that transport amino acids to the "production line" are another type of RNA—tRNA. There are many types of tRNA, but each type can only recognize and transport one specific amino acid. tRNA is much smaller than mRNA and has a unique molecular structure: the RNA strand folds into a shape resembling a cloverleaf, with one end carrying an amino acid and the other end containing three adjacent bases (Figure 4-6). These three bases of each tRNA can pair complementarily with a codon on the mRNA and are called anticodons.

Figure 4-7 shows the "production line" of protein synthesis. Notice that the ribosome moves along the mRNA. The binding site between the ribosome and mRNA creates two binding sites for tRNA.

Once the polypeptide chain is synthesized, it detaches from the ribosome-mRNA complex and typically undergoes a series of steps to fold into a protein molecule with a specific spatial structure and function. The protein then begins to perform various functions essential to the life of the cell.

In the cytoplasm, translation is a rapid and efficient process. Typically, multiple ribosomes can bind sequentially to a single mRNA molecule, synthesizing multiple polypeptide chains simultaneously (as shown in the figure on the right). This means that even a small number of mRNA molecules can quickly synthesize large quantities of protein.

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