Many people have contemplated getting their genomes sequenced or DNA tested, but have stopped because they are not sure what is the difference between the processes. While they are related, genome sequencing and DNA testing are different simply because DNA, genes, and genomes are all different.
This confusion is understandable as the terms DNA, genes, and genomes are often used interchangeably and in conjunction with each other. What it comes down to is that the human body is a complex organism that relies on the relationship between DNA, genes, and genomes. Stating that DNA is the “blueprint” for the human body is an oversimplification that doesn’t really explain what exactly DNA, genes, or genomes are.
If a structural architect were to say to someone that a blueprint is the set of instructions used to build a structure, it would be true. But the blueprint has many layers to it that translate the architect’s vision into a sustainable reality. So, let’s break it down and see how DNA, genes, and genomes are different, more than just a simple blueprint.
To start with, humans, like all living organisms are made of a collection of cells. These cells split and replicate themselves in the development of a complete organism. There are different types of cells in each organism that create different structures.
Different cells form to create the muscles, bones, tissues, and organs of every person. The human body is compiled of trillions of cells, all of which have to work together for homeostasis.
For a cell to replicate itself, it needs a set of instructions that will determine what the cell will become. In 1665, English scientist Robert Hooke first identified cells under a microscope. He coined the biological term “cell” because, to him, the structure resembled the small living quarters inhabited by Christian Monks in monasteries.
As the basic structural unit for life forms, all cells contain cytoplasm inside of a membrane. They are far more complex than a Monk’s quarters, with many elements that make them function.
There are two types of cells, eukaryotic and prokaryotic cells. Prokaryotic cells, like those found in bacteria, are single-celled organisms that do not have a central nucleus. Eukaryotic cells, like those found in humans, animals, and plants, all have a nucleus where the magic takes place.
Inside the nucleus of each cell are 46 chromosomes (23 from each parent) that house a delicately coiled molecular strand of DNA. The DNA within the chromosomes provides each cell with the necessary information to support life. This information includes genetic markers that determine an individual’s specific traits.
Inside the chromosome, DNA forms a 3D, ladder-like, double helix structure. The “rails” of the ladder are connected by “rungs” called base pairs. These base pairs are adenine (A), cytosine (C), guanine (G), and thymine (T). The sequence in which these bases pair up translate to an individual’s genes. Scientists believe that there are around 23,000 genes within each human DNA molecule.
In 1865 Gregor Mendel, a multidisciplinary Czech scientist, developed Mendelian inheritance, which would go on to become the basis of modern genetic science. By cross breeding pea plants with various traits, Mendel recognized how genes pass from hosts onto offspring. Mendel was the first to identify recessive and dominant genes and how they influence future generations of offspring.
It wasn’t until 1900 that Gregor’s theory of hereditary was verified, eventually being applied to human genetics. With the advancement of technology, and building off of established theories, like Mendelian inheritance, scientists were able to see how genes develop in strands of DNA. As the base pairs line up, complete with recessive and dominant influences from the 46 chromosomes housing the DNA strand, an individual’s unique genetic traits are developed.
The compilation of all of the 23,000 genetic markers carried within strands of DNA, inside the estimated 30 trillion cells in the human body makes up a genome. In 1920, while studying plant cells, German botanist Hans Winkler combined the words gene and chromosome to express the entirety of an organism’s genetic makeup. “I propose the expression Genom,” Hans wrote, “for the haploid chromosome set, which, together with the pertinent protoplasm, specifies the material foundations of the species.” A person’s genome represents all of the genetic material that makes them who they are.
Nearly all human DNA is identical. It is the 0.01% made up of the thousands of genes present in DNA that make each individual person unique. Because there are countless variations for the base pairs A, C, G, and T to connect, no two people have the exact same genome. The only time this rule is broken is in the case of identical twins, or even rarer identical triplets. In such a situation, the babies will have mirror copies of each other’s DNA.
So yes, DNA contains the blueprints needed for each cell in the body to function, but on a much grander scale. Perhaps a better analogy is to use a book, rather than a single sheet of paper used to make a blueprint.
In this analogy, the book represents the cell. The information contained within the book is organized into 46 chapters, which represent the 23 chromosomes from the mother (egg) and 23 from the father (sperm). Within each chapter is a collection of paragraphs or DNA strands.
Each paragraph (a strand of DNA) is made up of combinations of alphabetic characters. Each of these alphabetic characters represents a gene. The combination of letters (genes) and paragraphs (DNA) provides the book (cell) with the information needed to do its job.
All of the books (cells), filled with chapters (chromosomes), paragraphs (DNA), and letters (genes) are housed within a library. This library represents the entire genome. As stated in Pearson’s Ninth Edition Campbell Biology book, “The entire ‘library’ of genetic instructions that an organism inherits is called its genome.” Each organism has only one genome, or library, that represents its whole self.
The complexity of an organism determines the size of the library in this analogy. The bacterial organism Mycoplasma genitalium only has 525 genes. So, this organism’s library of books would be relatively small. More complex organisms, like humans, will have a much higher number of genes and information, resulting in a much larger library. Because Mycoplasma genitalium has such a small number of genes, it was one of the first organisms to have its full genome sequenced in 1995 by American biologist J. Craig Venter.
When a DNA test is administered, the results only reflect a few paragraphs within a few books in a library. All the test is looking for is the basic genetic markers that are inherited. These markers can include blood type, hair or eye color, and family lineage.
If a doctor were to request it, the results taken from a DNA test could be used to search for specific genetic markers. The genes being searched for could reveal information on genetic mutations that lead to certain diseases. Mutations in genes, like those that lead to diseases like breast cancer, are hereditary. This is why a DNA test can often be used in the early identification of disease risks.
To understand why genes mutate and how the process happens, and is thus passed down through dominant or recessive genes, scientists want to look at full genome sequencing, not just a snippet from a DNA test. However, fully sequencing a person’s genome, all of the genetic material carried within the trillions of cells in their body, is a massive undertaking.
That is why most people rely on DNA tests or specifically curated genome tests. It took 13 years and hundreds of scientists collaborating around the world to be able to establish the first fully sequenced human genome. It is such an undertaking that some have even compared it to the landing of Apollo 11 on the moon, a feat that took years to accomplish.
And like the knowledge gained from the Apollo 11 mission, the knowledge gained with every DNA test and full or partial genome sequenced, leads to greater insights for the future.
With the current progression of technology, being able to fully sequence someone’s genomes will soon be as simple as a DNA test.