What is DNA? When and how was it discovered? How do we use it to determine relationships between people? These are some questions you might be asking yourself. Let’s take a look at the basics of DNA.
What is DNA?
DNA stands for deoxyribonucleic acid. It is found in almost all living organisms, from humans to insects, algae to trees. DNA is found in every cell in the human body (with only a few exceptions), and although it is made up of only four different building blocks called nucleotides, it carries all of our genetic information. It is made up of molecules called nucleotides that essentially form a code for building and maintaining our bodies. Strands of DNA are tightly bundled into structures called chromosomes. Each human cell contains 46 chromosomes or 23 pairs of chromosomes. One chromosome from each pair was passed to you by your mother, and the other chromosome in the pair was passed to you by your father. There are special cells in the body that contain only half of your chromosomes (23 in total, no pairs) – sperm cells and egg cells. It is during conception that these cells come together to give a child all 46 chromosomes; 50% from its mother’s egg cell and 50% from its father’s sperm cell. Interesting fact for you: There are over 37 trillion cells in the human body, and each contains 46 chromosomes. This means that if you could unravel all of your chromosomes and line the DNA up end to end, it would reach from the Earth to the Sun and back again over 600 times. That’s a lot of DNA!
Although we know that we have an abundance of DNA in our bodies, over 99% of our DNA is the same from one individual to the next simply because of the everyday biological processes our bodies undergo and our physical similarities (we all have the same organs in similar locations within our bodies, we all have limbs and teeth that grow similarly, etc.). Because of this, the majority of our DNA is not useful for identification purposes. There are, however, small areas of DNA that are highly variable between individuals. They are so different, in fact, that we can use these specific areas of DNA to identify one individual from the next. This is what is meant by a person’s unique DNA make-up – the combination of these small, highly variable areas on the DNA molecule that is different in every single individual (with the exception of identical twins). In a DNA laboratory, lab staff can get a precise view of these areas, allowing them to very accurately distinguish one individual from the next.
When and how was DNA discovered?
In the mid-1800s, Charles Darwin proposed his theory that living organisms were much more likely to survive and reproduce if they were well suited to living in their specific environments. Organisms less suited to their environments were less likely to pass their characteristics onto future generations of their species. He called this process natural selection and theorized that this natural selection causes species to change gradually, or evolve, through generations.
The basic principles of the way in which characteristics are passed from generation to generation were discovered by Gregor Mendel in the mid-1800s. He worked with pea plants, cross-breeding them to discover which physical traits were more likely to be seen in the next generation, and which traits were usually hidden. To explain his findings, he coined the terms ‘dominant’ and ‘recessive’ – dominant traits being those likely to be seen and recessive traits being those less likely to be seen. What he didn’t know was the specifics of how these traits are passed from one generation to the next, only suggesting that there were “invisible” factors at play.
Toward the end of the 1800s, a Swiss physiological chemist named Friedrich Miescher was studying the protein components of white blood cells when he accidentally discovered a unique substance in the nuclei of those cells. He noticed that this substance had a very high phosphorus content and did not share the chemical properties of the proteins he was studying. He named this discovery “nuclein” and, although, at the time his work wasn’t appreciated by the scientific community, it’s now known that what he discovered was DNA.
In the very early 1900s, Mendel’s work and theories were rediscovered by different researchers across Europe. After a few decades of further research and more discoveries, it became fairly well known that traits were passed from parents to offspring by what were called ‘genes’. In 1944, an immunochemist named Oswald Avery was studying bacteria when he discovered a substance that was neither a protein nor a carbohydrate but was determined to be a nucleic acid called deoxyribonucleic acid, (DNA). Avery was able to determine that this DNA plays a huge role in translating specific characteristics during reproduction, but he did not yet know the specifics of how this occurred.
A scientist named Erwin Chargaff had read Avery’s paper in the 1940s and was so interested in the chemistry of nucleic acids that he decided to devote all his attention to these structures. He was eventually able to analyze DNA from a number of different species and discovered two major characteristics regarding the chemistry of nucleic acids. He discovered that the composition of DNA is different from species to species and that in double-stranded DNA the number of guanine units equals that of cytosine units and the number of adenine units equals that of thymine units. These discoveries proved to be quite important and are now called “Chargaff’s Rules”.
In the mid-1900s there was a woman named Rosalind Franklin researching X-Ray diffraction. During her studies, she was eventually able to produce two sets of photographs containing images of DNA strands from which she could calculate the dimensions of the structure as well as note that it was likely helical in shape. She was also able to determine that the outside of the strands contained the phosphates. Although Franklin was very close to discovering the true nature of the structure of DNA, she was ultimately beaten to the mark by James Watson and Francis Crick. Watson and Crick used Franklin’s findings and, with a little more research, were awarded a Nobel Prize in 1962 for discovering that DNA has a double helix structure. A double helix looks something like a twisted ladder. It is on the rungs of this ladder where the DNA code can be found. One of the most important properties of DNA is that it can make copies of itself. The molecule can split down the centre of the ladder rungs, leaving two separate halves. Each half of the ladder serves as a pattern for duplication of the genetic code, a quality that is essential in cell division and the creation of new cells. It is DNA’s unique structure that allows for DNA fingerprinting.
Following Watson and Crick’s discovery, there was a rush in the scientific community, as many scientists were eager to be the first to decipher the genetic code. There were many discoveries and breakthroughs related to DNA research throughout the second half of the 20th century, including the development of rapid DNA sequencing techniques, finding a genetic marker linked to Huntington’s disease, identifying a gene linked to increased susceptibility to breast and ovarian cancer, the beginning of the Human Genome Project, the cloning of a sheep named Dolly, and many more. But one of the most important discoveries, at least as it relates to relationship testing, was in 1985 by a man named Alec Jeffreys. Jeffreys determined that specific regions of DNA (called loci) contain sequences of base pairs that are repeated over and over again in succession. He also showed that the number of times these sequences are repeated can differ from one individual to the next. He then developed a technique that is used to determine the length of these repeated sections, essentially determining how many repeat units a person had at specific loci. This discovery turned out to be the way in which human identification is done today.
Another major development in DNA research was invented in 1985 by Dr. Kary Mullis. This was the invention of the polymerase chain reaction (PCR). PCR is a process by which specific regions of DNA can be labelled with a fluorescent tag and copied millions of times. PCR is done on an instrument called a thermal cycler, which can repeat cycles of heating and cooling to specific temperatures for specific periods of time. Chemicals are added to a DNA sample, the sample is then put onto the thermal cycler, and the thermal cycler runs through a program, heating and cooling the sample as specified. For every cycle, specific regions of DNA that vary from one individual to the next are copied and labelled. PCR creates so many copies of the specific regions of DNA that they can be isolated and detected by very sensitive instruments. This process is used by all DNA laboratories to this day.
How is DNA used to determine relationships between people?
Using DNA to determine relationships between people is highly accurate and therefore the gold standard of relationship testing. The most common relationship tests are those who determine a child’s parents. In most cases, it is the father’s relationship to the child that is called into question, but there are a number of situations in which the mother’s biological relationship to the child needs to be proven, such as some immigration cases or surrogate cases. A paternity test determines whether or not a man is the biological father of a child, while a maternity test determines whether or not a woman is the biological mother of a child. In either of these parentage tests, the DNA of the child is compared to the DNA of the alleged parent. The child must have 50% of the alleged parent’s DNA to be considered a biological child of that parent, as we receive half of our DNA from our mother and the other half from our father.
Other relationships can also be determined by DNA testing, like grandparentage, avuncular relationships and sibling relationships, however, the accuracy of these relationships can be lower due to potentially less DNA being shared between the individuals.
The 23rd pair of chromosomes is responsible for determining the sex of an individual. In females, this pair consists of two X chromosomes, while males have one X chromosome and one Y chromosome. Because of this, a mother only has X chromosomes to pass onto her child, while a father can pass either an X or a Y chromosome. The sex of the child will depend on which chromosome it has inherited from its father. Although there are some minor differences in the Y chromosome between generations, they are minimal, and for our purposes here, we’ll consider the Y chromosome unchanged from one generation to the next. With that in mind, in a biological family consisting of a mother, father and three sons, all of the sons will have the same Y chromosome, and this will be a match to their father’s Y chromosome. If this father has any brothers, they too will have the same Y chromosome that he has, and the same Y chromosome as their father (the three sons’ grandfather on their father’s side). Regarding identification, the Y chromosome can be an excellent tool in determining whether or not a male is part of another male’s family. It cannot be used, however, to determine which brother fathered a particular son, as the two brothers will have the same Y chromosome.
There is another type of DNA in human cells that is only inherited from our biological mother. This is called mitochondrial DNA, or mtDNA, and it is stored in organelles in our cells called mitochondria. Because cells of both men and women contain mitochondria, you might think that you can inherit this DNA from either parent. After fertilization, however, the mitochondria in sperm cells are usually destroyed by the egg cell. Because of this phenomenon, only the mother’s mtDNA can be passed onto the next generation, and therefore we can trace the maternal lineage of individuals. Using the previous example of a biological family consisting of a mother, father and three sons, all of the sons will have the same mtDNA, and this will match their mother’s mtDNA but not the mtDNA of their father. If this mother has any siblings (male or female), they too will have the same mtDNA that she has, which will also match their mother’s (the three sons’ grandmother on their mother’s side). So mtDNA has a different inheritance pattern than the Y chromosome because mtDNA is in both men and women. However, it is only passed through the mother’s lineage.
Although DNA technology has its limits, it is such an amazing tool for identifying individuals and for confirming or excluding relationships between them. DNA technology is an ongoing field of study and research, and as the years go on it is very likely that it will become more robust and more discriminatory than it already is. The instruments and chemical components necessary to determine an individual’s DNA fingerprint are currently quite expensive, and the process can be very time consuming, but laboratories and researchers are working on how to make the entire process from collecting an individual’s DNA sample to getting the DNA results much faster and more efficient. There is much to come in the future of DNA technology, so stay tuned.