DNA basics

What is DNA?

DNA (deoxyribonucleic acid) is a chemical found inside living things, such as plant, animals and bacteria. Living things are made of cells where cells make up tissues and organs. The DNA chemical occurs as molecules inside each cell. Inside the cell, DNA is packaged into chromosomes. Each chromosome is basically a very long DNA molecule that is tightly compacted.  Each species has a different number of chromosomes, with humans having 23 pairs of chromosomes. The totality of DNA packaged into chromosomes is the genome. 

DNA itself is a very long molecule, made up of two-strands wound around each other, called a double-helix. Each strand is made up of a long string of units joined together. There are four different units in DNA, labelled with the letters A, T, G, and C, which stand for the identifying bases for each unit, adenine, thymine, guanine and cytosine. 

Previously, it was commonly thought by geneticists that all the information required to build an organism is stored in DNA, which is then passed down to the next generation. The reality is now understood to be far more complicated. DNA can no longer be considered the blueprint for life, but instead one of a list of ingredients needed to build an organism. Information is also stored in molecules that regulate a cell’s DNA, which in turn can be influenced by environmental factors and life experience. 

What is my genome?

Your genome is the totality of all your DNA that is packaged inside your cells. Most of your DNA is in the nucleus of your cells. The nucleus is a spherical compartment, which contains DNA in the form of chromosomes. A small amount of DNA is contained in other parts of the cell called mitochondria, which play a role in supplying chemical energy to the cell. Each species has a different genome and each individual’s genome is also slightly different. 

Your genome is passed down from one generation to the next during reproduction, and accounts for some of the resemblance between parents and their children, however it is not the sole carrier of information passed down the generations that is required to build an organism (see ‘What is DNA’ blog for further details). 

The genome contains within it certain segments of DNA that constitute genes (see ‘What is a gene?’ for details), as well as other regions that do not encode for genes, but instead play other roles such as regulating the behaviour, or activity of genes. Some DNA elements are used in criminal DNA sample analyses to identify a person based on small differences in sequences that can be used to distinguish people. For example, repetitive sequences called Short Tandem Repeats (STRs), exist in all people, but the number of repeats vary between individuals. Similarly, differences in individual DNA units (nucleotides) can also distinguish individuals. These individual nucleotide differences are called Single Nucleotide Polymorphisms (SNPs). SNPs are also investigated to make estimations of someone’s ancestry, as well as disease risk. However, as detailed in our ‘What can you tell from my DNA’ blog, such information is limited in its ability to make such inferences and predictions. 

What is a gene? 

A gene refers to a particular segment of DNA that is associated with a particular trait, for example a certain gene can be associated with conferring eye colour. Genes are commonly described as conferring such traits, by providing the information for the production of a particular protein that performs a specific function. Proteins are chemicals which perform different functions within organisms. In the case of eye colour, the protein produced would be a specific pigment. 

However, the original theory of genes has now been recognised as outdated and oversimplified. Genes are not the only determinants of traits, and not all the information to build an organism is kept within the gene sequence that is passed down from generation to generation. It is now understood that additional information, outside of a gene, is also required. For example, information that tells the body to produce the eye pigment only at a particular stage of a baby’s developmental stage, and in the correct eye cells, is not stored in the gene but in other chemicals that are also inherited. Certain information, determined by someone’s particular experience, such as exposure to pollutants, stress, or lack of food, can also have impacts on future generations, showing that inherited traits are not conferred solely by genes.

Further, the definition of a gene is becoming increasingly blurred by recent discoveries that they function in complex networks with other chemicals and genes. Moreover, it is now understood that one gene may have many functions; that it may combine with other genes to take on additional functions; and that these functions may vary depending on the environment, such as the gene’s role in a skin cell versus a brain cell. 

What can you tell from your DNA?

Some information can be inferred from your DNA. Each person’s DNA can be used to identify an individual, with this information used by law enforcement for example, in order to identify criminal suspects. 

DNA testing services for ancestry testing are also on the market, offering estimations on genetic origins that pertain to modern nation states or so-called ‘ethnic’ groups. However, such claims suffer various scientific pitfalls that make such claims controversial and scientifically unsubstantiated. Your DNA is compared to those that have been collected to be part of a “reference panel group” that consists of DNA taken from individuals across different geographical regions. Depending on the reference panel group used, your ethnicity estimates can vary, as seen when reference panel groups are updated, or when people use different testing companies with their own reference panels. This shows the limited accuracy of such testing methods on the market.

The limited accuracy of such ethnicity testing is due to various factors. For starters, there is no biological definition of ethnicity to associate particular genetic patterns to, and people are often mixtures of various ancestral populations. Taking the UK as an example, Germanic, French, Roman (including North African peoples such as Hadrian) and Scandinavian people settled in the UK such that a person from modern day England or Scotland, might have DNA from all those regions. Further, studies have shown that genetic diversity is greater within rather than between so-called ethnic groups, making claims of a genetic basis for ethnicity problematic. Borders are also human inventions which have continuously changed over time, and can hardly be associated with any potentially genetically defined group.  

In reality, while tests claim to make ‘ethnicity estimates’, this is a deceptive term. What they actually test, is if your DNA shares the same genetic patterns as other individuals from a particular region. 

Genetic testing for health predictions suffers similar biological pitfalls. To date, genetics has been a poor predictor of most diseases in most people. Although there are some important exceptions, most diseases are not predictable from people’s genes. This is because in most cases, multiple genes play only a small and complicated role in most diseases, or in their reaction to drugs. 

Can my DNA identify family members?

Half your DNA is inherited from your mother and half from your father. This means that DNA tests can be used to find out who you are related to, by looking at how much of your genome is shared with others.

There are various genetic tests that can be performed to identify family members. For example, small differences in DNA patterns that distinguish individuals from each other, called Single Nucleotide Polymorphisms (SNPs) (see “What is my genome?”), are usually similar between related individuals. As such, these patterns will indicate whether people are closely related or not. 

Criminal investigations sometimes compare a forensic DNA profile, taken from a sample obtained at a crime scene with family members who are already on the DNA database in order to track down a suspect. These forensic DNA profiles are based on repetitive sequences called short tandem repeats (STRs), rather than on SNPs. The number of repeats at a particular location on the genome varies between individuals so, if enough locations are used, forensic DNA profiles can distinguish between individuals. Because relatives share some of their DNA, family members can also be identified.

Testing of the Y chromosome, which is the male sex chromosome that is passed down exclusively from father to son, can be used to gain information on the male ancestral lineage. Repetitive sequences that are present only on the Y chromosome, called Short Tandem Repeats (STRs), exist in all people, but the number of repeats vary between individuals and remain similar between family lines, and can therefore be used to trace male lineages. 

Other tests can be performed to gain information on the female lineage by looking at mitochondrial DNA (see “What is my genome?”), which is a form of genetic material inherited from the mother in both men and women, allowing for identifying female lineages. 

Is my DNA unique?

Human DNA is almost identical between all individuals (estimated at 99.9 %). However, small differences in each individual mean that your DNA can be distinguished from other individuals and can be used to identify people as well as close relatives who will share similar DNA patterns. However, most differences are small, involving only a few DNA units being different between people. It is unlikely that your genome will have long stretches of DNA that another person lacks, for example. 

The small differences between individuals are often used for criminal investigations to match for example, a forensic DNA profile from a sample taken from a crime scene, to that of a suspect, or to a DNA profile on an existing database. 

Why is consent important?

Consent is important to protect against human rights abuses by the state or private companies. Without fully informed consent, there is the possibility that personal genetic information may be shared with third parties such as law enforcement, border control enforcement, or private companies who wish to profit from the data, even across countries and continents, without people being aware that this can happen.

Regulations are still under development with regard to genetic testing and some people are not aware of what can happen to their genetic information. For example, DNA information from ancestry testing companies can be given to law enforcement in certain circumstances. DNA given with consent for research purposes has also been used for the development of commercial products. In the case of health tests, such as nutrigenic tests, commercial companies may attempt to make unregulated or unproven health/disease claims based on your DNA, with the aim of selling products back to the customer, such as nutrient supplements. In many countries, the police and security services can take DNA from some people without consent, but the safeguards to prevent misuse vary widely in different countries. 

Such examples highlight the importance of consent being more than merely saying “yes”, but instead being a process that seeks to fully clarify and inform people on the purposes of the data collection, and with whom any data may be shared with at the time your DNA is given, or at some point in the future. 

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