Exact Same DNA
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DNA, or Deoxyribonucleic Acid, is the genetic material in our cells. • No two people (except identical twins) have the exact same DNA. DNA patterns from four sets of twins – which are identical? DNA fingerprinting is also known as DNA profiling. It is a technique used by scientists to distinguish between individuals of the same species using only samples of their DNA. i. The process of DNA fingerprinting was invented by English geneticist Alec Jeffreys at the University of Leicester in 1985. DNA fingerprinting has been used to help solve crimes by linking suspects to crimes and by helping identify bodies of victims. In addition to crime investigation, DNA fingerprinting can also be used to: 1.establish paternity and parentage DNA paternity testing can indicate that a man is highly likely to be the father with about 99.9% accuracy or that he is excluded as being the father with 100% accuracy. In addition to crime investigation, DNA fingerprinting can also be used to: 1.establish paternity and parentage 2.identify victims of war and large scale disasters 3.study biodiversity of species 4.track genetically modified crops 5.settle immigration disputes Most lab techniques used for DNA fingerprinting were not intended for these purposes, but instead were developed for use in the medical field for diagnosis and treatment of diseases. Small amounts of biological evidence left at crime scenes, called trace evidence, are the source of DNA needed for DNA fingerprinting. • Biological evidence includes saliva, blood, semen, skin, hair roots, body tissue cells, and even urine. DNA is a nucleic acid, found in chromosomes, in the nucleus of your cells. • Most human cells have 23 pairs of chromosomes, or 46 total • Exception is sperm and egg cells which have only 23 chromosomes because they are not in pairs Each chromosome pair can be broken into smaller segments called genes. • Genes control the traits of the organism, and therefore can vary between individuals. i. DNA has four different nitrogenous bases: 1.Adenine (A), 2.Thymine (T), 3.Guanine (G), 4.Cytosine (C). i. These bases form pairs according to the base pairing rule: 1.Adenine binds only with Thymine 2.Cytosine binds only with Guanine 3.These pairs are considered to be ‘complementary’. i. If one strand of DNA has the genetic code: A T C T G C its complementary strand would read: DNA is made when these two strands twist together in a shape called the double helix. The sides of the helix, referred to as the backbone of DNA, are made up of alternating sugar and phosphate molecules. a. The sugar in DNA is called deoxyribose, from which DNA got its name. The rungs of the helix are made up of the paired nitrogenous bases (A-T, C-G), which help to code the DNA with instructions for the cell. DNA in chromosomes is called nuclear DNA • Nuclear DNA is inherited from both the mother and father, and is virtually identical in all cells of an individual’s body Mitochondrial DNA is in the form of a circular loop, and unlike nuclear DNA, is inherited only from the mother i. The human genome is the total amount of DNA in a cell. It is contained in both the nucleus and mitochondria. ii.The human genome consists of approximately 3 billion base pairs. iii.The purpose of the genome is to code the blueprint for the human body and how it works. It is responsible for telling the body to make proteins and other needed molecules. However, not all of the 3 billion bases are useful code. a. Exons are encoded DNA, with directions to build molecules i. Exons made up only 1.5% of the entire genome. ii. Together, they code for about 24,000 different genes. iii.The rest of the genome is made of introns. b.Introns are un-encoded DNA, that do not code for the production of molecules. i. These segments make up 98.5% of DNA and are often referred to as ‘junk DNA’. ii. Despite its nickname, introns may function in gene splicing and therefore may have an important role in biology after all. iii.Introns are also very useful in forensic science! Most of the human genome is the same in all humans, but some variation exists among individuals. • Most of the variation in DNA is found in the non-coding segment, or introns. • Much of the non-coding DNA is in the form of repeated base sequences, and some of these sequences can be repeated many times. Polymorphisms are the non-coded DNA segments that contain unique patterns of repeated base sequences that that are unique to individuals. Within the non-coding sections of DNA are short sequences of bases repeated multiple times. • The number of times the sequence repeats varies between individuals. Two types of repeating DNA sequences: • Variable Numbers of Tandem Repeats (VNTR) are 9 to 80 bases in length • Short Tandem Repeats (STR) are 2 to 5 bases in length VNTR and STR data from DNA fingerprints can be analyzed for two main purposes: i. Tissue matching 1. Comparing DNA evidence from a crime scene with DNA taken from a suspect 2. How it looks: Two samples that have the same band pattern are from the same person i. Inheritance matching 1. Comparing family members’ DNA for proof of familial relationships 2. How it looks: Each band in a child’s DNA fingerprint must be present in at least one parent (50% from mom, 50% from dad) a. Often the amount of evidence left at a crime scene is very small and therefore considered to be trace evidence. b. A problem with analyzing trace evidence is that many forensic tests will destroy the evidence sample, as is the case with DNA. c. Therefore, prior to DNA fingerprinting, PCR is used. PCR stands for Polymerase Chain Reaction. • PCR is a technique that makes thousands of copies of segments of DNA that investigators want to analyze. Crime scene (template) DNA is mixed with nucleotides, an enzyme known as DNA polymerase, and primers. • Primers are short segments of complimentary DNA that base-pair with the template DNA upstream of the region of interest and serve as recruitment sites for the polymerase Cycles of denaturation, annealing and extension are repeated to achieve exponential amplification of the target sequence, allowing for billions of DNA copies to be produced in just a few hours. Because DNA is found in all cellular material and cells are very small, contamination is an important issue to address when collecting and preserving DNA evidence. To avoid DNA evidence contamination, crime scene investigators should: i. Wear disposable gloves and change them often ii. Use disposable instruments for handling each sample iii.Avoid talking, coughing, or sneezing over evidence iv.Do not touch your face or body when collecting or packaging evidence v. Air-dry evidence before packaging. If evidence cannot be dried, it may be frozen. vi.Avoid using plastic bags to store evidence that contains DNA; use paper bags or envelopes vii.Keep evidence cool and dry during transportation and storage. Avoid direct sunlight which can damage DNA. Step One: Extraction Collect DNA of the (from scene or DNA known) Step Two: Cutting the DNA into restriction fragments • RFLPs, Restriction Fragment Length Polymorphisms Step Three: Amplification (making many copies of those fragments)- use PCR Gel Electrophoresis • a process that separates RFLPs Step Four: Electrophoresis according to their length, creating a DNA Fingerprint. Video: Gel Electrophoresis Video: Loading the Gel Video: Reading the Results What the above DNA looks like after gel electrophoresis i. Note: one well should contain a control, a solution containing DNA fragments of known lengths, called Marker or Standard DNA. ii. Other wells should contain DNA from crime scene, victim, and suspects. DNA fingerprints look like a striped column. In order for DNA fingerprints to match, the columns must have bands in the exact same places with the exact same widths. • Each state keeps DNA profiles of individuals who have been convicted of certain kinds of crime such as rape, murder, and child abuse. • The military maintains DNA profiles of all service men and women. • The United States’ Electronic database of DNA profiles is called CODIS, Combined DNA Index System. As of February 2011, CODIS contained 9.4 million profiles from DNA from convicted felons of violent crimes. Of 360,000 profiles from DNA at crime scenes, CODIS made 138,700 hits. .