In this blog post, we’ll explore the principles behind how parental traits are passed down to children, as well as the process of gene expression—from DNA replication and transcription to the role of RNA and protein synthesis—and examine how technologies that regulate gene expression are being utilized in life sciences and medicine today.
If you’ve spent time with your father or mother, you’ve probably heard or said things like, “My daughter really looks just like her mom,” or “My son looks exactly like his dad.” Even if you’ve never had that experience, it’s a natural biological phenomenon for parents and children to share various characteristics. This is because children inherit genetic information from their parents. In more technical terms, the phenomenon of parental traits being passed on to children is described as “the inheritance of traits.” So, what exactly is a trait? If you’ve ever played a computer game, you’ve probably spent time deciding how to customize your character’s attributes—whether to prioritize strength or intelligence. Just as games feature characteristics that distinguish characters, the observable features or characteristics exhibited by living organisms are also referred to as traits. Height, skin color, hair characteristics, and lip shape are all examples of traits. However, it is now well known that many of these human traits are not determined by a single gene but result from the complex interaction of multiple genes and environmental factors. While the genetic information inherited from parents forms an important foundation, not all traits can be viewed as exact copies of the parents’ characteristics.
So, what is inheritance? As explained earlier, inheritance is the phenomenon in which genetic information from parents is passed on to their children, influencing various traits. DNA and genes play a crucial role in this process. A gene is a specific segment of DNA that contains the information necessary to produce functional products such as RNA or proteins. However, simply possessing a particular gene does not guarantee that the trait associated with that gene will always manifest in the same way. For genetic information to translate into actual function, genes must be expressed at the appropriate time and place, and the extent of their expression is also regulated. Today, gene expression is understood not merely as a phenomenon where genes are “turned on or off,” but as a complex regulatory process that includes determining in which cells, when, and in what quantities RNA and proteins are produced. The relationship between inheritance and traits between parents and children, as explained in the original text, stems from these basic concepts of genes and gene expression.
The concept that describes the fundamental flow of genetic information within a cell is the “Central Dogma” of molecular biology. The Central Dogma essentially describes the flow of information in which genetic information stored in DNA is transferred to RNA, and the information from RNA leads to protein synthesis. However, while DNA replication is a crucial process for preserving genetic information during cell division, strictly speaking, it cannot be considered the very first step of gene expression itself. Furthermore, since processes such as reverse transcription—in which information from RNA is transferred to DNA—are now known, the Central Dogma should not be understood simply as an “uninterrupted one-way rule of DNA → RNA → protein.” The key point is that there is no general pathway by which protein sequence information is transmitted back into nucleic acid sequence information; rather, the information contained in DNA and RNA leads to functional products through various processes.
Let’s imagine DNA as a massive dictionary containing our genetic information. DNA replication can be likened to the process of creating another copy of this dictionary. Before a cell divides, it replicates its genomic DNA to ensure that each of the two daughter cells to be formed possesses a complete set of genetic information. Therefore, it is more accurate to understand DNA replication not simply as a process of creating a “backup” to prevent information loss, but rather as the process of precisely copying the entire genome in preparation for cell division. Human cells also replicate their DNA prior to division, a process involving various enzymes and proteins, including DNA polymerase.
To visualize the replication process more easily, imagine a zipper. DNA forms a double helix structure in which two strands are bound together in a complementary manner. Just like the two sides of a zipper, the two DNA strands are connected in a complementary fashion; when the strands separate during replication, new complementary strands are synthesized using each existing strand as a template. During this process, DNA polymerase synthesizes a new DNA strand based on the base sequence of the template strand. As a result, two DNA molecules are produced from a single DNA molecule, each consisting of one original strand and one newly synthesized strand. This process is called “semi-conservative replication.”
Let’s explain this using the zipper analogy again. If we label the two strands of a zipper as A and B, they are connected in a way that fits together perfectly. When replication begins, A and B separate, and using each as a template, a new strand that pairs with A and a new strand that pairs with B are created. However, it is not entirely accurate to describe the newly formed strands as simply having the “same sequence” as their original counterparts. This is because DNA uses specific complementary base pairs—such as A and T, and G and C—to form the new strands. As a result, a single strand of DNA is replicated into two DNA molecules that contain identical genetic information. The zipper analogy helps us understand the principle by which the two strands of DNA separate and new pairs are formed based on each strand.
If replication is the process for preserving the entire genome, the key processes that follow in gene expression are transcription and translation. Transcription can be likened to the process of transcribing the necessary information from the vast “dictionary” that is DNA into RNA, rather than copying the entire dictionary. Although the mechanism of transcription bears some resemblance to DNA replication, there is a key difference. In DNA replication, each of the two strands serves as a template for the synthesis of new DNA, whereas in the transcription of a specific gene, only one of the two DNA strands is used as a template. Furthermore, it is RNA polymerase—not DNA polymerase—that synthesizes RNA based on the information in the DNA.
Continuing with the zipper analogy, transcription can be thought of not as creating an entirely new zipper, but rather as producing a copy of the information—made of a different material—based on one strand of the required section. The RNA polymerase reads a specific region of the DNA and synthesizes RNA that is complementary to the template strand; once transcription of the required section is complete, the resulting RNA is separated from the DNA. In the case of genes that encode proteins, the RNA produced in this way undergoes several processing steps to become mature messenger RNA, or mRNA. In eukaryotes, including humans, mature mRNA moves from the cell nucleus to the cytoplasm, where it is used for protein synthesis. Therefore, mRNA can be understood as an information carrier that transmits the protein synthesis information stored in DNA to the ribosome.
Both DNA and RNA are composed of a chain of basic units called nucleotides. DNA uses four types of bases: A (adenine), G (guanine), C (cytosine), and T (thymine), while RNA uses four types: A (adenine), G (guanine), C (cytosine), and U (uracil). Therefore, while there are five types of bases in total when DNA and RNA are considered together, a single DNA or RNA molecule does not use all five. The key difference is that DNA uses thymine, whereas RNA generally uses uracil instead of thymine. Genetic information is encoded in the arrangement of these bases—that is, the nucleotide sequence.
When mRNA, which encodes proteins, binds to a ribosome, the process of synthesizing proteins based on genetic information begins; this is called translation. To use the dictionary analogy again, it can be described as the process of transcribing the necessary information from DNA into RNA and then converting that information into a usable product—a protein. Ribosomes read the mRNA sequence three nucleotides at a time; this group of three nucleotides is called a “codon.” Each codon specifies a particular amino acid or serves as a signal for the start or stop of protein synthesis. Therefore, rather than stating that every codon necessarily corresponds to a single amino acid, it is more accurate to understand that most codons specify a particular amino acid, while some act as translation termination signals.
Within the ribosome, amino acids are linked in sequence according to the codon information in the mRNA to form a polypeptide chain. This chain then undergoes folding and various post-translational modifications to become a functional protein. Proteins perform a wide variety of roles—such as enzymes, receptors, structural proteins, and signaling molecules—and play a crucial role in defining the characteristics of cells and tissues. Therefore, translation is the key process by which the information stored in DNA is realized as functional products known as proteins. However, the result of gene expression is not always a protein. Some genes are not translated into proteins; instead, the RNA itself performs a function. Today, gene expression is understood as a concept that encompasses not only genes that produce proteins but also processes that generate functional non-coding RNAs.
As such, DNA replication, transcription, and translation are interconnected yet serve distinct purposes. DNA replication is the process of copying the entire genome prior to cell division to pass genetic information on to the next cell; transcription is the process of transferring information from a specific gene stored in DNA to RNA; and translation is the process of synthesizing proteins by reading the information in mRNA and linking amino acids. Among these, transcription and translation are the most fundamental processes for understanding gene expression. However, in living organisms, the relationship between genes and traits is not as simple as “one gene produces one protein, and that protein directly determines one trait.” A single gene can produce different RNA and protein products; multiple genes may jointly contribute to a single trait; and environmental and living conditions can also influence gene expression and traits. Therefore, while the DNA inherited from parents serves as a crucial foundation for determining traits, it is more accurate to view actual traits as the result of complex regulatory processes surrounding gene expression.
Such complex and sophisticated processes are constantly occurring in the countless cells of our bodies. In particular, today’s technology goes beyond simply analyzing the nucleotide sequences of genes; it allows for the precise measurement of which genes are expressed in which cells and to what extent. Technologies for investigating mRNA expression at the whole-genome level, as well as those for analyzing gene expression at the single-cell level, are now widely used. This has enabled more detailed research into why cells with the same DNA perform different functions and how gene expression changes when disease occurs.
Technologies for artificially regulating gene expression have also advanced significantly. In particular, CRISPR-based technologies are expanding their scope beyond genome editing—which directly alters DNA base sequences—to include gene regulation, which suppresses or activates the expression of specific genes, and epigenome editing, which modifies epigenetic states present in DNA and histones. By binding specific regulatory factors to Cas proteins that have had their catalytic activity removed, researchers can direct the regulatory mechanism to the desired gene site, thereby lowering or increasing gene expression without cutting the DNA. Recently, active research has been underway to use these technologies to modulate specific chromatin states and control gene expression with greater precision.
These technologies have the potential for wide-ranging applications, from basic research aimed at identifying the causes of diseases to research focused on developing new therapies. In particular, epigenome editing is attracting attention because it offers the potential to regulate abnormal gene expression states without permanently altering the DNA sequence itself. However, challenges remain to be addressed, such as efficiently delivering editing tools to the desired cells and tissues, the impact on non-target sites, and the durability and safety of the regulatory effects. Nevertheless, as technologies advance beyond merely reading the information encoded in DNA to precisely observing and regulating how genes function, gene expression research continues to expand into a critical field—not only for basic science aimed at understanding biological phenomena but also for developing disease diagnosis and treatment strategies.
Ultimately, while the resemblance between parents and children begins with the DNA inherited from parents, that DNA alone does not directly determine our physical appearance. Actual traits manifest in diverse ways depending on which cells express the genetic information stored in DNA, when it is expressed, how it is translated into RNA and proteins, and how it is influenced by other genes and the environment. A person’s myriad characteristics are shaped by the interplay between the genetic information received from parents and the intricate biological processes that read and regulate it. To understand why we resemble our parents yet are not exactly the same, we must ultimately examine not only “what we have inherited” but also “how that inherited genetic information is expressed.”
