DNA Fingerprinting

What is it?
1. Is a process which extracts DNA from the cells of a person of interest and turns it into a pattern of bands (lines)
  1. DNA BANDS ---------->
  2. This pattern can be compared with the DNA fingerprints of the cells left at a crime scene - this leads to identifying or eliminating suspects.
    1. It can also be used in law courts to prosecute a criminal case.
      1. The prosecuting lawyer will try to show that the person being tried, was at the crime scene and left his or her cells on the weapon or victim.
      2. The defending lawyer will try to discredit the DNA fingerprint by questioning how the cells were collected and how the DNA fingerprint was produced in the laboratory.
    2. DNA fingerprints can also be used to establish the paternity of children; to identify people who have been killed in natural disasters and to work out how closely related plants and animals are to each other (the more bands which are the same, the more closely related the plants or animals are).
Steps involved in the production of a DNA fingerprint
1. 1. Cells (tissues) are collected e.g. from crime scene.
  1. Typical cells include: salvia (contains cheek cells), blood (contains white blood cells with nuclei), semen (contains sperm with half the amount of DNA but also contains skin cells of a person) and lastly hair (contains skin cells).
  2. 2. DNA is extracted using a laboratory technique which mashes the cells to release DNA and then separates out the DNA from other chemicals in the cells.
    1. 3. Restriction enzymes are added to the DNA.
      1. the restriction enzymes run along the length of the strands of DNA, cutting the DNA when a particular code is found. When they have finished the DNA strands are in fragments of different lengths.
      2. 4. The DNA fragments are dropped into the channels of an apparatus containing agarose gel (jelly).
        5. A Gel electrophoresis is used to separate the DNA fragments according to length.
        The smaller the DNA fragment, the further it will move through the gel.
Steps for producing Gel Electrophoresis
1. 1. Put a small amount of agarose into a flask.
  1. 2. Add some liquid buffer to the flask.
    1. 3. Place the flask containing the buffer and agarose mixture inside the microwave, heat the mixture until the agarose melts into the buffer.
      1. 4. Pour the melted agarose mixture into the mould - make sure the mould has tape on each end as this holds in the melted agarose.
        5. Place the comb into the gel on one end, and let the gel cool and solidify - when the gel's solid remove comb.
        6. Pour buffer into the electrophoresis box - place the gel still in its mould, in the electrophoresis box.
        7. With a clean pipet tip, use the microwave to suck up some loading buffer, then add it to the DNA sample.
        8. Suck up some of the DNA sample into the pipet tip, eject the DNA sample into the first well of the gel, using a clean pipet tip use the micropipettor to suck up some DNA size standard, transfer the DNA size standard into the next empty well.
        9. Plug the black cord from the electrophoresis box into the matching outlet on the power supply.
        10. Stain the DNA in your gel using DNA staining solution.
        11. Drag the gel out of the mould and put it into the DNA staining solution.
        12. Remove the gel from the staining solution and place it on the UV light box, then record the final results.
The collection of cells at a crime scene
1. Tissue (cells) is collected at the crime scene by police forensic team. Here are some of the rules for the procedure they follow:
  1. 1. Personal Protective Equipment (PPE) must be worn at all times as a single hair or drop of sweat from an unprotected person could leave an unknown DNA sample at the scene (The scene could be contaminated).
    1. 2. When collecting DNA samples, the forensic investigator should change gloves and forceps after each item is collected in order to avoid cross-contamination between items of evidence.
      1. 3. Biological evidence should be labeled and stored under laboratory conditions.
        ie. in a cool, dry area, free of moisture (to prevent the growth of mould which can affect the DNA testing as it contains its own DNA).
        4. Liquids should be placed in glass collection tubes and refrigerated.
        5. DNA samples must be collected from (a) the victim (b) any suspects (c) any other persons whose DNA may be found on the item of evidence.
Obtaining a DNA sample from a person
1. A person may provide samples of their DNA using a buccal swab.
  1. The inside of a person's cheek is swabbed with a cotton bud for 30 seconds.
    1. This is then placed in a sterile container and sent to the forensic laboratory.
Analysing a DNA fingerprint
1. 99.9% of the DNA fingerprint will be exactly the same for all humans.
  1. Therefore, forensic investigators look at certain sections of the fingerprint which are known to be different for different humans. Most use 10-13 different sites on the fingerprint.
    1. Identical twins will usually have the same DNA fingerprints at these sites.
      1. DNA pattern at these sites are compared with the DNA fingerprints of the cells found at the crime scene.
        The probability of any person's DNA matching the one at the crime site used to be considered as 1 in 5 million.
        However, new techniques used today (STR) which can sometimes identify the race of the person from the fingerprint can reduce this probability e.g. to 1 in 10 000.
Compairing DNA fingerprints
1. DNA samples gathered at a crime scene can be compared with the DNA of a suspect to show whether or not he or she was present.
  1. Researchers are able to read the fingerprint and match it to others.
    1. They do this by placing the xray on a light background, and comparing the RFLP lengths in the DNA from the crime scene, to the DNA of the suspect.
    2. Diagram of DNA fingerprint comparing -------->
Genetic Code Table
1. The genetic code is the set of rules by which information encoded within genetic material is translated into proteins by living cells.
  1. The genetic code is highly similar among all organisms and can be expressed in a simple table with 64 entries.
    1. <--------- Genetic Code Table
DNA Tour - Genome Spots
1. The two genes that produce red and green light-sensitive proteins are located on the X chromosome. Mutations in these genes can cause colour blindness. Colour blindness is a common inherited sex-linked disorder that affects a person’s ability to see or recognise certain colours. Eight to ten percent of all males and one half of a percent of all females are colour-blind.
  1. INHERITANCE: colour blindness is a sex-linked recessive disorder. The genes for colour vision are carried on the X chromosome. Females have two X chromosomes and will not be coloured blind if they have only one mutated copy of the gene. Males, with only one X chromosome, will be colour blind if they inherit the mutated gene.
    1. INCIDENCE: roughly 10% of men have some form of colour blindness. Women with their two X chromosomes are much less likely to have the disorder - less than half of one percent of women are affected.
      1. TREATMENT: currently there is no treatment for colour blindness.
        SYMPTOMS: people who are colour blind are not able to see the full range of colours. Colour vision deficiencies may range from a mild loss to extreme. Red - green colour blindness is the most common, followed by blue - yellow. Those most severely affected cannot see any colour, but this condition - achromatopsia - is rare and often associated with other vision problems.
        TESTING AND SCREENING: simple eye charts with colours embedded in patterned shapes can detect colour blindness. These tests are a routine part of most vision testing.
        CAUSE: colour blindness is caused by mutations in a gene or genes on the X chromosome. The most common of colour blindness is an inability to distinguish red from green. The genes for pigments that are sensitive to red and green light lie directly next to one another, so mix ups in their DNA sequences may rapidly occur during egg or sperm formation.

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DNA Fingerprinting

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