DNA Genetic Analysis
DNA testing is a genetic analysis technique that makes it possible to identify a person from a small amount of biological samples.
This analysis therefore makes it possible to define a genetic identification fingerprint which is based on the following fact:
- Although humans have a large majority of DNA in common
- Each individual has a unique part in their DNA
To understand how the lab sheds light on our genetic profile, you have to start from the ground up and understand what exactly DNA is.
DNA is a long molecule of information that is rolled up and condensed into a ball present in our cell nuclei. A person has in each of these cells 46 balls of DNA that we call Chromosome: 23 inherited from our biological mother and 23 from our father.
It is important to note that DNA manages almost all of our biological functions ; it is the carrier of our genetic information and also provides manufacturing instructions.
DNA is therefore a molecule which is made up of nucleotides at its base. Not to go into details, nucleotides are organic components made up of chemical elements . In total there are 4 kinds of nucleotides symbolized by the letters A, C, G, and T: ( Adenine, Cytosine, Guanine and Thymine )
How reliable is a DNA test?
The reliability of the results during a DNA test will depend on several factors:
1. Laboratory accreditation
Checking the laboratory's accreditation allows you to be sure of the analytical methods that scientists use when looking for a parentage link. An accreditation is an international standard that a laboratory can acquire after verification of the entire process by a committee external.
Accreditation gives the laboratory the possibility of genetic analyzes which may be legally legal.
2. The declaration of your situation
Be sure to communicate well before your order on the family situation, your doubts and the possible relations between the participants. The result of the DNA test will depend on your declaration because it is linked to a deduction of possibilities.
3. The type of test
All DNA tests are not equal in terms of the odds ratio they offer, and depending on the basic situation several tests are possible and some more reliable than others. As a general rule, it is always advisable to do DNA testing directly with the affected person.
4. The type of sample
The reliability of the results does not depend on the type of sample, but not all samples reliably provide enough genetic information to make a DNA test.
In our DNA, nucleotides come together in a more complex structure to form what are called amino acids . In addition, this combination of nucleotides into amino acids regroup together again to ultimately form the proteins in our body .
Proteins perform a multitude of functions within the body . They manage all our biological activity through the creation of molecules , the management of vital functions or the transmission of information . Depending on their roles, they can have different names (enzymes, myosin, histones, etc.)
A gene is a strand in the DNA molecule . It is a segment of DNA which, depending on its expression, defines the role of its cell. (heart cell, liver cell or brain cell ...)
The role of the gene according to its cell thus determines existence of two main functions on the DNA molecule:
- Coding zones, which are used to produce and create new proteins
- The non-coding zones, which rather have a role of protein regulation
It is considered that the non-coding areas cover about 98% of our DNA.
Still in the same logic, the genes are just as rigorously positioned in a specific way on the DNA. This facilitates the localization of a particular gene because its position remains unchanged in all human beings.
When we locate a specific gene we speak of Locus .
- For a coding DNA area , the locus is identified with the name of the protein used.
- For a non-coding DNA zone , each locus is nomenclatured according to a very precise code:
The D18S52 locus: is located on chromosome 18, it supports a nucleotide sequence that cannot be found elsewhere (S) and it bears the number 52 or D19S52 .
DNA is therefore a complex structure that brings together different chemical components in a double helix organization. Two long parallel and complementary strands of nucleotides linked together by molecular bonds.
Know that in DNA all the architecture and the relationship between nucleotides is rigorously hierarchized by chemical elements:
- In front of a G, there is always a C and vice versa
- In front of an A, there is always a T and vice versa
This rigorous organization allows the molecule to be duplicated very easily.
Of course, humans are all part of the same species with a very large number of similarities in the structure of nucleotides. However, our extraordinary diversity is rooted in minute variations .
These variations between individuals are called polymorphisms, and analyzing a DNA sample allows us to observe these variations and compare them. Taking two individuals at random, we find about 1 variation every 1200 nucleotides.
There are two types of variations depending on the coding or non-coding area of DNA.
- When the polymorphic variation is in a coding zone, the variation is visible on the protein of the locus (structure of the nucleotides in amino acid and the amino acids in protein)
- When the polymorphic variation is in a non-coding zone, the variation is visible by the number of repetitions of the nucleotides. We then speak of length polymorphism.
The length polymorphism are repetitive sequences of nucleotides which can be repeated several times in a row such as for example the group: AAGTA which can vary from one person to another and thus be repeated 11 times in one person, 14 times in a second person or 15 times in a third.
The term used to refer to the variations between individuals is " Allele ".
During an analysis, an individual will each have two alleles for each genetic trait. One allele representing the variant present on the paternal chromosome and one allele representing the variant present on the maternal chromosome.
A repeating polymorphism sequence consisting of a minimum of 10 nucleotides is commonly referred to as VNTR (Variable Number Tandem Repeats) or minisatellite .
A polymorphism sequence which repeats with a small number of nucleotides, one then speaks of STR (Short Tandem Repeat) or microsatellite .
Short STR-like sequences have come to dominate genetic analysis because of their advantages:
- they are numerous (approximately 50,000 sequences of this type in human DNA)
-they can be analyzed simultaneously (multiplex analysis)
On the other hand, they sometimes suffer from a limited polymorphism.
Repetitive Sequences : VNTR and STR
The first step in a genetic analysis is the extraction and purification of DNA . To do so, it is necessary in a way remove the DNA molecule from its support and dissolve any substances that could interfere with the progress of the analysis. The scientists then immerse the sample in an aqueous medium which will eliminate all external substances to ultimately only retain the DNA molecule.
Depending on the type of analysis, STR sequences are carefully chosen and cut with a natural protein: Restriction Enzyme .
Restriction enzymes are special proteins because they come from bacteria and have the ability to cut DNA molecules at the level of specific sequences depending on the enzyme chosen. They are therefore tools widely used in genetic engineering and in biology laboratories.
The second step is PCR amplification . It is a method allowing from a scarce sample to quickly copy precise sequences of DNA in very many copies . This copying technique, which allows the quantity of DNA to be doubled in a very short time, is possible thanks to the discovery of the enzyme called " DNA polymerase (Protein) which allows the perfect reconstruction of a DNA helix previously separated.
PCR amplification is carried out with the mixture of the following active ingredients:
- DNA sample : segment previously cut
- Additional nucleotides
- DNA primers : single strands complementary to the sample to be copied
- DNA polymerase : enzyme that recognizes primers and assembles nucleotides to copy target DNA
The mixture is subjected to rapid temperature variations in a programmed cycle.
Each cycle consists of 3 stages:
- 90 ° denaturation to separate the DNA helices in 2
- The ybridation h at 45 ° to fix the primers to the DNA fragments
- 72 ° elongation which allows reconstruction of missing DNA by DNA polymerase enzyme using added primers and nucleotides
At each cycle, the number of copies is doubled . In 30 or 40 cycles, millions of copies of the target sequence are obtained.
PCR (polymerase chain reaction)
The third step is the separation of DNA by electrophoresis . This method makes it possible, under the effect of an electric field, to separate proteins from DNA according to their sizes and their molecular weight. The experiment can be done in a tube or in a gel, and allows the observation of the migration of the DNA sequence depending on its composition.
The fragments move according to their size towards the positive pole, because the DNA is negatively charged. The smaller the fragment, the faster (and therefore far) it migrates. All the fragments of the same size form a detectable line and make it possible to characterize their DNA content.
Thus, depending on the result of the migration after electrophoresis, the laboratory is able to determine the composition of the fragment: the number of nucleotides and its number of repeats.
Analysis of the fragments obtained by electrophoresis forms the genetic fingerprint of a person which can then be compared to determine a parentage link. Since the length of a particular fragment can vary from one individual to another, with the exception of identical twins, the probability that two people have the same genetic fingerprint is almost zero (1 in 3 billion).
Read the results of a DNA test
The result of a DNA test schematizes genetic analysis, making it possible to identify a person by their genetic fingerprint, or genetic profile. Each laboratory must provide on the result according to the test, the information identifications which make it possible to establish the link of biological filiation between individuals.
Mitochondrial DNA Analysis
Mitochondrial DNA (mtDNA) testing is a genetic analysis that does not use the genetic information found in our DNA. It is a non-standard test, which is not carried out on the nuclear DNA present in our cells like previously, but which focuses on the analysis of the DNA of mitochondria.
Mitochondria are ancient bacteria that have gotten inside a cell to form a symbiotic relationship, several million years ago. This relationship to transform the bacteria into a real organelle for the cell.
The organelles are small compartments specialized in certain functions that regulate the life and activity of the cell. The mitochondria today specialize in the production of energy for the cell.
Mitochondrial DNA therefore corresponds to the DNA which is found inside the mitochondria, hence its name, and not to the DNA present in the nucleus of the cell which is the support of the genetic heritage of 'an individual.
Mitochondrial DNA is circular DNA that has several hundred mitochondria per cell and each contains about ten copies of its DNA. Thus it will therefore be present in several thousand copies while nuclear DNA is only present in two copies. For this reason, mitochondrial DNA can be isolated from old samples where very degraded where nuclear DNA is not detected.
Unlike nuclear DNA, mitochondrial DNA does not contain repetitive sequences and inter-individual variations are sometimes visible on a single nucleotide . The polymorphism of mitochondrial DNA is therefore a structural polymorphism (and not of repetition like that of nuclear DNA).
The analysis is carried out on these polymorphisms present in a non-coding region called the control region.
The sequences are amplified by PCR in order to then be detailed, which makes it possible to obtain the complete sequence of nucleotides. : it is the technique of sequencing.
This technique is enough tedious to be put in place since it is necessary to order the whole area of the DNA to determine a variation which sometimes occurs on a single nucleotide (on average there are approximately 8 nucleotides difference out of the 600 analyzed).
The analysis of mitochondrial DNA does not identify 100% an individual, c p ar everal people can have the same mtDNA. But it can become very useful for checking a family link or searching for origins.
Indeed the particularity of this DNA is that it is transmitted strictly through the maternal route . Indeed, during fertilization, when the maternal egg and the paternal sperm merge, only the egg has mitochondria and allows transmission.
This means that all the members of a sibling will therefore have the same mitochondrial DNA transmitted by their mother, who herself inherited it from her mother and that across a person's entire maternal line.
We could then think that all the human species has the same mitochondrial DNA since it is transmitted unchanged to subsequent generations throughout the maternal line. Although at the origin of our species, everyone had, the same mtDNA sequence, we know that naturally mutations appear from time to time. When this occurs in reproductive cells, the mutation is likely to be passed on to offspring.
It is thanks to mutations that accumulate during the development of the human species in the DNA of the descendants of each family, that strangers (unless you are parents by maternal route), will always have mitochondrial DNA. different.