QUESTION 11 (3 marks) DNA fingerprinting (profiling) is commonly used as a foren

Question

QUESTION 11

(3 marks)

DNA fingerprinting (profiling) is commonly used as a forensic technique in criminal investigations, for example comparing one or more individuals' profiles to DNA found at a crime scene in order to assess the likelihood of their involvement in the crime.

The following DNA profiles were generated from blood found at the crime scene, and from 3 individuals who were people of interest (this is a real DNA profile!).

(a) After comparing the DNA fingerprints, which individual would you conclude is more likely to have been present at the crime scene, and perhaps warrants further investigation?

Suspect #1 Suspect #2 Suspect #3

(b) After comparing the DNA fingerprints, which individuals are less likely to have any involvement in this crime as their DNA was NOT found at the crime scene?

Suspect #1 Suspect #2 Suspect #3   and Suspect #1Suspect #2Suspect #3

Explanation / Answer

The genetic fingerprint (also called DNA test or DNA analysis) is a technique used to distinguish between individuals of the same species using samples of their DNA. The technique is based on the fact that two human beings have a large part of their DNA sequence in common and to distinguish two individuals we can exploit the repetition of highly variable sequences called minisatelites or VNTR. Two unrelated humans will be unlikely to have the same number of minisatellites at a given locus. In the SSR / STR profiles (which is different from genetic imprinting) the polymerase chain reaction (PCR) is used to obtain enough DNA to detect the number of repetitions at several loci. It is possible to establish a selection that is very unlikely to have arisen by chance, except in the case of identical twins, who will have identical genetic profiles.

The genetic fingerprint is used in forensic medicine to identify suspects with blood, hair, saliva or semen samples. The microsatellites show greater variation than the rest of the genome since they contain sequences in different repetitions and with a different degree of recombination due to the instability of the locus.

There are several techniques for identifying the "genetic fingerprint". For this purpose we have:

RFLP analysis

The analysis consists of making a Southern (or Southern blot, a method used to verify if a certain DNA sequence is present or not in a sample of analyzed DNA) and to use specific probes to detect the VNTR (variable number of repetitions in tandem).

First, the DNA to be analyzed is separated from other materials. It must then be cut into fragments of different sizes using restriction enzymes, which are proteins that cut the DNA without damaging the bases. The fragments are ordered by size using gel electrophoresis. DNA, which has a negative charge, advances in the electric field. Smaller molecules move more rapidly through the gel, so they will be located farther from the origin than larger fragments. Then by heat or alkaline solution, gel is applied in order to denature the DNA and separated into individual fragments. Once this is done, the DNA is now ready to be analyzed using a radioactive probe hybridization reaction.

To make the radioactive probe, the DNA polymerase is needed. The DNA to be subjected to radioactivity is placed in a test tube. Next, the polymerase is added to the tube. It dissolves and is waiting for it to start working. As the DNA polymerase patches break the DNA, the current ones are replaced by the new nucleotides in the tube. Each time the sample has a guanine base, the cytosine will be put into radioactivity. In the repetition of DNA, the polymerase also becomes radioactive. The radioactive parts are ready for use. Now the radioactive probe can be used to create a hybridization reaction. Hybridization occurs when two genetic sequences are linked because of the hydrogen found in the base pairs. There are two of these between adenine (A) or thymine (T) and three of cytosine (C) or guanine (G). In order to carry out the hybridization, the DNA must be denatured.

The radioactive denatured DNA and the probe should be placed in a plastic bag with saline and sealed tightly. The probe will adhere to the DNA denaturation wherever it is found. The probe and the DNA do not have to fit exactly. This process ends up making a DNA pattern of the fingerprints. Everyone has a VNTR that they have inherited from one of their parents and the VNTRs are unique to each person.

PCR analysis

It consists in applying the PCR technique to amplify specific regions with the appropriate primers. With the invention of the polymerase chain reaction (PCR), genetic technology had an important advance in the capacity to recover information from very small samples. PCR involves the amplification of specific regions of DNA using temperature and a thermostable polymerase enzyme together with fluorescence labeled in a specific DNA sequence. There are commercial kits that use single nucleotide polymorphisms (SNP) of available discrimination, these PCR use kits used to amplify specific regions with known variations and hybridization with anchoring probes on cards, resulting in a color spot corresponding to a order in particular. The main criticisms of the RFLP method affect its slowness and the large amounts of DNA it requires to obtain useful results. This led to PCR-based methods that require smaller amounts of DNA that can also be more degraded than those used in RFLP analysis. PCR assays are extremely valuable in identifying new members of a gene family.

AmpFLP

It is an amplification technique of polymorphic regions, preferably the D1S80 locus. These analyzes can be automated, and allow the easy creation of phylogenetic trees based on the comparison of individual DNA samples. Based on the variable number of tandem repeats (VTNR) to distinguish various allele polymorphisms, they are separated into a polyacrylamide gel used in an allelic ladder (as opposed to a molecular weight). Bands could be visualized by silver gel staining. A popular place for fingerprinting is the D1S80 locus. Like PCR-based methods, degraded DNA or very small amounts of DNA can cause allelic abandonment.

STR Analysis (Short Tandem Repeat)

It consists of the amplification of sequences with small tandem repetitions that are not conserved within the species. Once amplified, the fragments are separated to check the number of repetitions (by capillary or gel electrophoresis), so that repeating patterns can be distinguished that can be comparable and associable.

This method uses highly polymorphic regions that have short repeated DNA sequences. Because different people have different numbers of repeating units and these regions of DNA can be used to differentiate between people. These STR loci are attacked with sequence specific and amplified by PCR. The DNA fragments that are then detected result and separated by electrophoresis. There are two methods of separation and detection, capillary electrophoresis (CE) and gel electrophoresis.

The STR polymorphisms shown in each region are in themselves very common, in general, each polymorphism is shared by around 5 - 20% of people. When we observe multiple loci, it is the only combination of these polymorphisms of an individual that makes this method of discrimination an instrument of identification. The STR regions that are tested on a person become a discrimination of the test.

From one country to another there are different STR systems in the DNA analysis that are in use. In North America the systems that amplify the CODIS 13 core loci are almost universal, whereas, in the United Kingdom, the SGM + system, which is compatible with the national DNA database is in use. Whichever system is used, many of the STR regions used in the test are the same. These DNA analysis systems are based around multiplex reactions, which many STR regions will be tested at the same time.

DNA structure.

Capillary electrophoresis (movement through the application of an electric field) allows the injection of DNA fragments into a thin (capillary) glass tube filled with polymers. The DNA is extracted through the tube by the application of an electric field, the separation of the fragments occurs in such a way that the smallest fragments travel faster through the capillaries. The fragments are then detected using fluorescent dyes which is attached to the first ones used in the PCR. This allows multiple amplified fragments to be executed simultaneously, something known as multiplexing. Then sizes are assigned using the DNA size as labeling standards that are added to each sample, and the number of repetitions is determined by comparing the size of the allelic ladder, which contains a sample of all common sizes of possible repeats. Although this method is expensive, with greater capacity of the machines and a higher performance than those that are being used to reduce the cost / sample it is possible to reduce the delays that exist in many government crime facilities.

Gel electrophoresis is one of the acts that is used using similar principles known as "CE", but instead of using a capillary, the large polyacrylamide gel is used to separate the DNA fragments. An electric field is applied, as in the EC, but instead of running all the samples through a detector, the smaller fragments run near the bottom of the gel and all the gel is scanned and stored in a computer. This produces an image that shows all of the bands corresponding to different sizes and repeats the allelic ladder. This method does not require the use of size standards, since the allelic ladder runs along with the samples and serves for this purpose. The visualization can be through the use of marked fluorescent dyes first in the silver stain or by the gel before scanning. Although it is profitable and can be quite high performance, to apply the ITS silver stain is suspended. In addition, many laboratories are phasing out gels in favor of the EC, making the cost of the machines more manageable.

The real power of STR analysis lies in the statistical power of discrimination. In the US, there are 13 central loci (DNA sites) that are currently used for discrimination in CODIS. Because these places use a variety independently (with a certain number of repetitions at one locus does not change the probability of having any number of repetitions elsewhere), the product of the probability rule can be applied. This means that, if someone has the ABC type DNA, in which the three loci are independent, we can say that the probability of that type of DNA is the probability of having type Sometimes the probability of having the type B times is the probability of having the type of C. The most prevalent method of DNA fingerprinting used today is based on PCR and uses short tandem repeats (STR). This method uses highly polymorphic regions that have repeated short DNA sequences (the most common is 4 repeated bases, but there are other lengths in use, including bases 3 and 5). Because different people have different numbers of repeating units, these regions of DNA can be used to discriminate between people. These STR loci are attacked with first sequence-specific and amplified by PCR. The DNA fragments that are then detected result and separated by electrophoresis. There are two methods of separation and detection, capillary electrophoresis (CE) and gel electrophoresis.

The STR polymorphisms shown in each region are very common, in general, each polymorphism will be shared by around 5 - 20% of people. When we observe multiple loci, the unique combination of these polymorphisms of an individual makes this method of discrimination an instrument of identification. The other STR regions that are tested on a person become a discrimination of the test.

Analysis of the Y chromosome

Specific primers have been created for regions of the Y chromosome that amplify sequences inherited only by the father. Therefore, this type of analysis only serves to compare family members by the father and men.

Mitochondrial analysis

It is used in highly degraded samples, since mitochondrial DNA has more copies than nuclear. The HV1 and HV2 regions are amplified and are always compared with relatives on the maternal side, since the mitochondria are the exclusive inheritance of the mother. Forensic scientists extend the HV1 and HV2 region of mtDNA and then each region is sequenced in a single nucleotide and the differences are compared to the reference.

This accounts for the theoretical scientific explanation, however it is impossible to answer the question, because the file (xid-6089869_1); it is not visualized in the question as such.

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