DNA Fingerprinting

DNA Fingerprinting

Standard 4—The students will apply information gained about DNA, its structure and role to forensic Science • Learning Expectations • 4.1-- Apply the concepts of the molecular structure of DNA • 4.2 --Analyze the isolation and extraction of cellular DNA • 4.3--Apply concepts of restriction digestion, gel electrophoresis, and PCR • 4.4--Investigate the uses of DNA in identifying or clearing potential suspects in crimes

Introduction • No two people have the same DNA except identical twins • Since the 1980s DNA evidence has been used to investigate crimes, establish paternity, identify victims of war and large scale disasters • DNA fingerprinting, is also called DNA profiling • DNA can be extracted from small amounts of biological evidence to accurately predict whether an individual can be linked to a crime or excluded as a suspect

History of Biological Evidence in Forensics • Biological evidence such as skin, blood, saliva, urine, semen, and hair are used for identification purposes • Most lab techniques used in forensics were developed for other purposes • Blood typing techniques were first developed to make blood transfusions safe • Blood type information obtained from crime scene evidence can help to exclude suspects and help to determine if the blood comes from one person or multiple individuals

The Function and Structure of DNA • DNA is the blueprint of life and contains the genetic material of a cell • DNA holds all the information and instructions for a cell to make proteins and to make copies of itself • Genetic information of stored in molecules of DNA making up structures called chromosomes • A human chromosome is made up of two strands of DNA tightly coiled around protein molecules and itself

The Function and Structure of DNA • Because each DNA molecule is composed of two strands it is known as a double helix • James Watson and Francis Crick received the 1953 Nobel Prize for their work on describing the structure of DNA • The sides of the double helix ladder are called the backbone of each of the two strands of DNA • The backbones consist of alternating sugar and phosphate molecules • Rungs of the ladder are made up of pairs of molecules called nitrogenous bases

The Different DNA Bases 1 • DNA has 4 bases: A (adenine), C (cytosine), G (guanine), and T (thymine) • Each base is bonded to the sugar molecule on the backbone on one side and can form weak hydrogen bonds with one specific base on the other strand of the DNA • Base pair ruling: A and T; C and G • When two DNA strands form a double helix, the DNA strands are considered complementary • CGTCTA • GCAGAT

The Different DNA Bases 2 • There are 23 pairs of chromosomes in the nucleus of most human body cells • One chromosome in each pair is inherited from the mother and the other is from the father • DNA in chromosomes is called nuclear DNA—Genetic information is stored here • Another type of DNA is found in the mitochondria of the cell and unlike nuclear DNA, is inherited only from the mother • Mitochondrial DNA is passed onto the offspring in the cytoplasm of the egg cell; none of the mitochondria come from the sperm cell • Analyses of chromosomes is called karyotyping

Genes and Alleles 1 • Each chromosome contains many genes • Genes are DNA sequences that have instructions that determine our inherited characteristics or traits, such as blood type • Genes can also make another type of nucleic acid called RNA • An allele is one of two or more alternative forms of a gene, for example, one allele of a gene might code for normal hemoglobin, while another allele codes for abnormal hemoglobin • One allele comes from the mother and one comes from the father

Genes and Alleles 2 • The human genome is the total amount of DNA in a cell and is contained in chromosomes and mitochondria • DNA in chromosomes contains approximately 3 billion base pairs • Chromosomes contain DNA used to make proteins or other molecules, called encoded DNA (exons) and un-encoded DNA (introns), that do not produce protein or RNA molecules, but are important for gene splicing

Genes and Alleles 3 • The chromosomes in the nucleus of each human cell contain some 23,688 encoded genes, with each gene averaging 3,000 base pairs • This is less than 1.5% of DNA in the genome • The rest of the human genome, more than 98.5%, is made up of non-coding DNA thought to be involved in gene regulation and gene splicing, but much of this DNA has no known function and is referred to as “junk DNA” • Population studies are conducted to calculate the percentage of people who have a particular allele

DNA Identification 1 • Most of the human genome is the same in all humans, but some variations exist among individuals • Scientists are able to identify individuals based on this variation • This variation is found in the non-encoded junk DNA, which much of is in the form of repeating sequences • Individuals have unique patterns of repeated base sequences in the non-encoded DNA and certain sequences may be repeated many times • Within a human population, these differences in DNA sequences are called polymorphisms • High degrees of polymorphisms are most useful in DN A analysis

DNA Identification 2 • In 1984, Sir Alec Jeffreys at the University of Leicester observed the DNA from different individuals contains many different polymorphisms • His lab developed a technique for isolating and analyzing these variable areas that is known as DNA fingerprinting • A DNA fingerprint appears as a pattern of bands on X-ray film

DNA Identification 3 • Because the number and location of polymorphisms are unique to an individual, each individual’s DNA has a unique band pattern • The examination of DNA profiles can help forensic scientists to decide if two or more DNA samples are from the same individual, related individuals, or unrelated individuals

DNA Identification 4 • Different repeated sequences appear in different places in the genome of each individual • Forensic scientists focus on 2 types of repeating DNA sequences in the non-encoding sections of DNA known as VNTRs (variable number tandem repeats) and STRs (short tandem repeats)

VNTR • Within in the non-encoding DNA, certain short sequences of DNA are repeated multiple times • The number of copies of the same repeated base sequence in the DNA can vary among individuals • For example, if the repeated base sequence is GATACAGAC there might be 3 copies in the DNA of one individual while another individual might have seven copies • Because the number of repeats varies from one person to another these multiple tandem repeats are known as variable number tandem repeats (VNTR) • The length of a VNTR can be from 9 to 80 bases in length

STR • DNA sequences with a high degree of polymorphism are most useful for DNA analysis • A short tandem repeat (STR) is a short sequence of DNA, usually only 2-5 base pairs in length within the non-encoding DNA • The polymorphisms in STRs result from a different number of copies of the repeat element that occur in a population • Use of STRs is becoming the preferred method of analysis because of its accuracy and because small and partially degraded DNA samples can be used to identify individuals

Restriction Enzymes • Molecular scissors that cut DNA at specific base sequences

DNA Probe • Molecule labeled with radioactive isotope, dye, or enzyme, that is used to locate a particular sequence or gene on a DNA molecule

DNA Profile • VNTR and STR data from DNA fingerprints are used for 2 main purposes—tissue matching or inheritance matching • For tissue matching, two samples that have the same band pattern are from the same person • For inheritance matching, the matching bands in the DNA fingerprints must follow the rules of inheritance, each band in a child’s DNA fingerprint must be present in at least one parent

Sources of DNA 1 • DNA is found in the nucleus of cells in the human body • A perpetrator may leave biological evidence, such as saliva, blood, seminal fluid, skin, or hair at a crime scene • When evidence at a crime scene is very small it is referred to as trace evidence • Trace evidence can be consumed during forensic testing

Sources of DNA 2 • The use of polymerase chain reaction (PCR) technique helps to resolve this problem • PCR generates multiple identical copies from trace amounts of original DNA evidence

Collection and Preservation of DNA Evidence 1 • Contamination of DNA evidence can occur when DNA from another source is mixed with DNA that is relevant to the case

Collection and Preservation of DNA Evidence 2 • To avoid contamination of DNA evidence • Wear disposable gloves • Use disposable instruments for handling each sample • Avoid touching the area where you think DNA may exist • Avoid touching your face, nose, mouth when collecting DNA and packaging evidence • Avoid talking, sneezing, or coughing over evidence • Air dry evidence thoroughly before packaging • Put evidence into new paper bags or envelopes • If wet evidence can’t be dried, it may be frozen

Collection and Preservation of DNA Evidence 3 • Keep evidence dry and cool during transport • Moisture can damage DNA evidence because it encourages mold growth • Direct sunlight and warm conditions are also harmful to DNA

DNA Fingerprinting