Chapter 7DNA Fingerprinting

Forensic Science

Chapter 7 DNA FingerprintingObjectives and Vocabulary

Objectives– Explain how crime scene evidence is collected for DNA analysis– Describe how crime scene evidence is processed to obtain DNA– Describe how radioactive probes are used in DNA fingerprinting– Explain how DNA is compared for matching– Explain how DNA fingerprinting is used to determine if specimens come from

related or unrelated individuals– Explain how to use DNA fingerprinting to identify DNA from a parent, child, or

relative of another person

Vocabulary– Allele– Chromosome– DNA fingerprint– DNA probe– Electrophoresis– Gene– PCR (polymerase chain reaction)– Restriction enzyme– STR (short tandem repeat)– VNTR (variable number of tandem repeats)

Chapter 7 DNA FingerprintingIntroduction

Except for identical twins, not two people have the same DNA.Since the 1980’s DNA evidence has been used to investigate crimes, establish paternity, or identify victims of large scale disasters.DNA evidence can only be linked to only one person

Chapter 7 DNA FingerprintingHistory of Biological Evidence in

ForensicsTypes of biological evidence– Skin– Blood– Saliva– Urine– Semen– hair

Their chromosomes can be examined to determine, karyotypingEx. blood typingDNA fingerprinting is used in criminal and legal cases to determine identity and parentage.

Chapter 7 DNA FingerprintingThe Function and Structure of

DNADNA is a blueprint of life, contains the genetic material of the cellMakes proteins and copies of itselfGenetic information is stored in molecules of DNA making up structures called chromosomesMade up of two strands, know as a double helixJames Watson and Francis Crick received the 1953 Nobel Prize for describing the structure of DNA

Chapter 7 DNA FingerprintingThe Function and Structure of

DNA1. Sugar phosphate backbone2. Phosphate Group3. Nitrogenous bases joined by hydrogen

bondsAdenineThymineGuanineCytosine

Chapter 7 DNA FingerprintingThe Different DNA Bases

DNA strands are considered to be complementaryFollowing complementary base pairing rules– A pairs with T– G pairs with C

A G C G C C G A T T A G C G C T A AT AAC G C C C CG G T T G G T

Chapter 7 DNA FingerprintingThe Different DNA Bases

There are 23 pairs of chromosomes (a total of 46)One chromosome of each pair is inherited from the mother and one from the fatherTwo types of DNA– Nuclear – virtually identical

in all cells of the human body

– Mitochondrial – is passed in the cytoplasm of the egg, found in the mitochondrial, circular

Chapter 7 DNA FingerprintingGenes and Alleles

Genes are DNA sequences that have instructions that determine our inherited characteristics or traitsGenes also make up RNA– RNA single stranded– Does not include T, has

uracil (U)An allele is one of two or more alternative forms of a gene (one allele from the mother one allele from the father)

Chapter 7 DNA FingerprintingGenes and Alleles

Entire human genome is contained in the nucleusApprox. 3 billion base pairs

DNA RNA proteinsHas exons (produces RNA and proteins) and introns (“junk” DNA, may be useful in gene splicing)The nucleus of each human cell contains 23,688 genes, averaging 3,000 bp

Central Dogma

Chapter 7 DNA FingerprintingGenes and Alleles

Chapter 7 DNA FingerprintingDNA Identification

Most of the human genome is the same, but there are some variations, this allows us to identify individualsHave unique sequences in non-coded DNADNA sequences have different lengths and different sequences, these differences are called polymorphisms

Chapter 7 DNA FingerprintingDNA Identification

1984, Dr. Alec Jeffreys developed a technique for isolating and analyzing these variable areas, knows as DNA fingerprinting or DNA profilingUnique patterns look like bandsThe examination of DNA profiles can help forensic scientists decide if two or more DNA samples are from the same individual

Chapter 7 DNA FingerprintingVNTR

Variable number of tandem repeats (VNTR)Certain short sequences of DNA are repeated multiple timesThe number of repeats differ from person to personEx.CATACAGACCATACAGACCATACAGAC

Chapter 7 DNA FingerprintingSTR

Short tandem repeat (STR)High degree of polymorphism and most useful for DNA analysisMuch shorter than VNTR, usually only two to five bp in lengthDifferent number of copies of the repeat elementEx.GATAGATAGATAGATAGATAGATAGATAGATA

Preferred because of its accuracy and because small and partially degraded DNA samples may be analyzed still

Chapter 7 DNA FingerprintingDNA Profile

A DNA fingerprint can be developed with several different VNTRs and STRsFor tissue matching, two samples that have the same band pattern are from the same personFor inheritance matching, each band in a child’s DNA fingerprint must be present in at least one parent

Chapter 7 DNA FingerprintingDNA Profile

Two main purposes– Tissue matching

Exact same pattern– Inheritance matching

Follows the rules of inheritance

Chapter 7 DNA FingerprintingDNA Profile

Chapter 7 DNA FingerprintingPopulation Genetics and DNA

DatabasesPopulation genetics – the study of variation in genes among a group of individualsEx.– Asian populations, blue eyes are rare (more

common among northern Europeans)– Great Britain/US, 46% of people have O blood

but native South Americans, almost of 100% of population has O blood

Chapter 7 DNA FingerprintingCollection and Preservation of DNA

EvidenceAttention to contamination issues is necessaryCan occur if DNA from another source is mixed into the DNA from the crime scene

Chapter 7 DNA FingerprintingPreparing DNA Samples for

FingerprintingUse gel electrophoresis– When different sized

DNA fragments are separated with an agarose gel

Chapter 7 DNA FingerprintingSteps of DNA Fingerprinting

1. Extraction.extraction from the nucleus

2. Restriction fragments. cuts into smaller pieces at specific sequences

3. Amplification. PCR (polymerase chain reaction)

4. Electrophoresis

Chapter 7 DNA FingerprintingElectrophoresis

Movement based on size and charge– DNA is negatively

chargedSample is placed in a well