Sequence comparison, especially when supported by systematic collection, curation, and searching of biological sequence databases, has become one of the most powerful tools in modern molecular biology. With the rapid growth of DNA, RNA, and protein sequence data, comparing sequences across genes, proteins, and even species has enabled scientists to infer function, structure, and evolutionary relationships without always having to conduct exhaustive laboratory experiments. This approach rests on a fundamental principle known as the “first fact of biological sequence analysis.”
The First Fact of Biological Sequence Analysis
The first fact of biological sequence analysis states that in biomolecular sequences—whether DNA, RNA, or amino acid (protein) sequences—a high degree of sequence similarity usually indicates significant functional or structural similarity. In other words, if two sequences from different genes, proteins, or even different species are found to be similar, it is highly probable that they perform similar biological functions or adopt similar three-dimensional structures.
This principle is rooted in the evolutionary process. Evolution does not usually create entirely new molecular structures from scratch; instead, it reuses, duplicates, and modifies structures that have already proven successful. Proteins, exons, regulatory DNA sequences, morphological features, and enzymatic pathways are repeatedly reused and adapted throughout the genome of a single organism as well as across widely divergent species.
Duplication with Modification
The central paradigm explaining protein evolution is “duplication with modification.” According to this concept, new proteins and new biological functions arise mainly through repeated cycles of gene duplication followed by subsequent mutation and modification. Because of this continuous duplication process, redundancy becomes a built-in characteristic of protein sequences. This explains why a very large proportion of newly discovered sequences closely resemble sequences that are already known and catalogued in databases. Essentially, the redundancy observed in biological sequences is a natural consequence of the evolutionary mechanism by which life has diversified.
This redundancy is not limited to any particular organism. Studies conducted on relatively simple model organisms, such as yeast or fruit flies, have repeatedly demonstrated remarkable similarity with human biological systems. This is why research on simpler organisms often serves as a reliable predictor for understanding human genetics and cell biology.
Significance of Similarity and Its Limits
Although biological systems across species show strong conserved similarities, they are not identical. Differences between organisms exist, but these differences, when examined against a background of otherwise strong similarity, actually strengthen the value of biological comparison. When conserved features are found despite evolutionary divergence, it becomes easier to identify what is functionally essential versus what is variable.
Biological structures must be robust enough to tolerate variation while retaining their essential form and function. By comparing related biological objects, scientists can distinguish between:
Conserved features – regions that remain essential to structure and function across species, and
Variable features – regions that can differ without significantly altering function.
This ability to separate the “necessary” from the “incidental” is one of the most powerful applications of comparative biology.
Why Sequence-Level Comparison Is Preferred
Although biological universality exists at many levels—molecular, cellular, biochemical, and morphological—sequence-level comparison is preferred in practice for several reasons:
Availability of Data – A far greater number of protein sequences are known (through DNA sequencing) than three-dimensional protein structures, since determining 3D structure experimentally is more difficult and time-consuming.
Encoded Information – Sequences are not merely convenient substitutes; they directly encode the deeper molecular structures and mechanisms that later manifest at the cellular and biochemical levels.
Reflection of Evolution – Nowhere is the Darwinian principle of “descent with modification” more clearly visible than in the sequences of genes and their products.
Because of these reasons, searching for similarity and conservation at the sequence level provides a practical and heuristic, though not always perfect, method for identifying functional or structural universality among biological systems.
Practical Application: Database Searching
With advances in computational biology, sequence similarity searching across protein and DNA sequence databases has become the single most powerful method for inferring the biological function of an unknown gene or protein. The development of rapid heuristic algorithms and high-performance computing has made large-scale sequence comparison a routine and standard practice in molecular biology.
The general working method can be explained as follows:
When a new gene is cloned and sequenced, its DNA sequence is translated into the corresponding amino acid sequence.
This sequence is then compared against established protein and DNA sequence databases using computational algorithms.
In a substantial number of cases—historically estimated at around fifty percent—such comparison reveals sufficient similarity to suggest a probable enzymatic or structural function for the previously unknown gene.
This process has become so essential that it is now considered incomplete, and even unacceptable, to publish the sequence of a newly discovered gene without first performing such database comparisons.
Broader Scientific Impact
The widespread availability of sequence data through electronic databases has changed the very approach to biological research. Increasingly, biological investigation begins theoretically: a scientist first formulates a hypothesis based on existing sequence data and database comparisons, and only afterward conducts experiments to test that hypothesis. This represents a shift from purely experimental biology toward a more theory-driven, computationally supported model of scientific discovery.
Caveat: Limitations of the First Fact
While the first fact of biological sequence analysis is extremely powerful, it is important to understand its limitation. The relationship between sequence and structure/function is not perfectly reversible.
That is:
High sequence similarity usually implies significant structural or functional similarity (this is the first fact).
However, structural or functional similarity does not necessarily imply high sequence similarity.
This means that while similar sequences are likely to produce similar structures, it is also possible for distinctly different sequences to produce remarkably similar structures. This exception highlights that sequence comparison, though extremely useful, must be applied with an understanding of its boundaries, particularly in the deeper study of protein structure and multiple sequence comparison.
Sequence comparison, guided by the first fact of biological sequence analysis, has emerged as a central and indispensable tool in modern molecular biology. Its foundation lies in the evolutionary principle of duplication with modification, which ensures that biological sequences are inherently redundant and comparable across species. This redundancy enables scientists to infer gene function, understand protein structure, and trace evolutionary relationships through database searching, making sequence comparison one of the most transformative techniques in the study of life sciences.
