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Chapter 10 Advanced Concepts in DNA
© 2012 Cengage Learning. All Rights Reserved
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Introduction—Objectives
Discuss the structure and function of DNA. Explain what causes variation in DNA. Differentiate between the various types of DNA analysis. Compare and contrast the methods of extracting DNA. Explain the proper methods of collecting DNA evidence at a crime scene. Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Introduction—Vocabulary
DNA extraction—process of removing DNA from a cell; procedures include Chelex extraction, organic extraction, and differential extraction homologous chromosomes—pair of chromosomes in which one chromosome was inherited from the male parent and the other was inherited from the female parent Nucleotide—subunit of DNA and other nucleic acids; made up of a 5-carbon sugar, a phosphate group, and a nitrogenous base Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Introduction—Vocabulary
polymerase chain reaction (PCR)—technique used to make billions of copies of a specific segment of DNA; allows very minute samples of DNA to be copied billions of times primer—small piece of DNA used to begin and end replication during PCR short tandem repeat (STR)—short segment of DNA in which the same sequence of two to six base pairs is repeated many times; varying numbers of repeats found among individuals Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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A 200-Year Old Mystery August 13, 1792, the French royal family is arrested and imprisoned. The king and queen were executed. The heir lived two more years. Did he die in prison or escape? DNA samples from the remains of the king, queen and suspected dauphin were compared. 5 Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Introduction Deoxyribonucleic acid (DNA)—the genetic material that makes each individual unique. Only identical twins have the same DNA DNA copies itself when cells divide DNA contains the blueprint for making proteins essential for life processes DNA is found in biological evidence DNA is individual evidence Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Structure and Function (Obj 10.2)
Nucleotides—subunits of DNA Deoxyribose Phosphate group Nitrogenous base Adenine Thymine Cytosine Guanine Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Structure and Function (Obj 10.2)
Each nitrogenous base can form hydrogen bonds with a specific base on a complementary strand of DNA Adenine always pairs with thymine Guanine always pairs with cytosine Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Structure and Function (Obj 10.2)
Nitrogenous base pairs form the rungs of a twisted ladder and carry a lot of information Sugar and phosphate groups form the edges of the ladder Genes—sections of DNA that code for a protein Proteins catalyze, regulate, and control chemical reactions in the cell. Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Structure and Function (Obj 10.2)
Alleles—an alternative form of a gene for a certain trait. An individual inherits two alleles for each gene, one from each parent. Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Structure and Function (Obj 10.2)
11 Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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What Causes Variation in DNA? (Obj 10.2)
99.5% of DNA is the same for everyone 0.5% of DNA is what makes us unique Homozygous—when two alleles are identical for a particular trait Heterozygous—when two alleles are different Marker—a sequence of DNA base pairs associated with a specific trait Matching markers from two samples determines the likelihood of an individual being the same as the reference sample Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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DNA Analysis Procedures (Obj 10.3, 10.4)
Method of extraction varies depending upon The type of biological evidence analyzed The amount of evidence available The type of cells present Chelex Extraction Used when a small amount of biological evidence is available Sample placed in boiling water with Chelex beads The cells in the sample burst open to expose and preserve DNA Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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DNA Analysis Procedures (Obj 10.3, 10.4)
Organic Extraction—removing biological evidence from materials (e.g., blood from bed sheets) Differential Extraction—separates vaginal epithelial cells from sperm cells with a mild solution of phenol and chloroform Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Measuring Quality and Quantity of DNA (Obj 10.3)
High molecular weight (HMW)—sample contains DNA in large chunks Restriction fragment length polymorphism (RFLP)—DNA analysis requiring HMW Polymerase chain reaction (PCR)—analysis used when only small amounts of DNA are available, amplification process Variable Number of Tandem Repeats (VNTR) —repeating sections of DNA Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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DNA Fingerprinting Fingerprinting has several steps
Isolate and purify DNA Restriction enzymes indicate where DNA can be cut into fragments (the RFLPs) Gel electrophoresis sorts DNA bands by size Southern blotting process transfers DNA bands from the gel to a nylon sheet Population genetics determines probability of another individual having the same profile Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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DNA Fingerprinting Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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PCR Polymerase Chain Reaction
An enzyme separates a region of DNA and then copies it billions of times Samples are heated and cooled in a three-step cycle Denaturing—(heating) separating the two DNA strands Primer annealing—(cooling) adding primers (markers) to the beginning and ends of targeted sequences Polymerization—(heating) an enzyme copies the DNA between the two primers Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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PCR 19 Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
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PCR Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
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STR Short Tendem Repeats—a short segment of DNA in which the same sequence of 2 to 6 base pairs is repeated many times Easier and quicker to replicate Repeating STR patterns are highly variable between individuals; therefore Matching multiple STRs between individuals has a very low probability Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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CODIS Combined DNA Index System An FBI nationwide database
Stores DNA fingerprints of individuals who have been convicted of certain crimes Open to forensic scientists to compare DNA fingerprints of suspects based on 13 core STR locations Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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CODIS Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
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Collecting DNA Evidence (Obj 10.5)
Biological samples Avoid contamination Retain original surface when possible Reference samples for comparative analysis Stop degradation Protect sample from environmental exposure Remove moisture by drying out and freezing the sample Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Collecting Biological Evidence
Examples Skin cells Blood Saliva Semen Hair Other possibilites? Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Collecting Biological Evidence
Guidelines Never use bare hands Avoid sneezing and coughing Use clean protective gloves for each item collected Package each sample separately Air dry evidence located on objects; store in an envelope or paper bag, not plastic bags Establish a chain of custody Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Collecting Reference Samples
Known samples taken from suspects and victims Reference samples are compared to evidence collected from the crime scene Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Testing for Blood, Semen, & Saliva
Presumptive tests used at the crime scene Luminol is the most common method used to detect blood Sprayed on objects to find bloodstains Detects blood even when diluted Chemicals and fluorescent dyes are used to visualize seminal fluid For saliva, the Phadebas test or starch iodine test detects the presence of amylase Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Mitochondrial DNA (mtDNA) Analysis
mtDNA is inherited from the mother Easier to extract than nuclear DNA Less likely to degrade Each cell has hundreds of copies of mtDNA Typically extracted from teeth, hair, bones Take care to avoid altering this evidence for other analyses Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Y-Chromosome Analysis
Passed down through paternal lineage Beneficial for these situations Sexual assault Missing person Genealogy Does not provide individual evidence Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Chapter Summary The discovery of DNA’s existence, structure, and function dates back to the 1800s. DNA, shaped like a double helix, is made up of a sugar and phosphate backbone with nucleotides on the inside. The four nitrogenous bases are adenine, guanine, cytosine, and thymine. Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Chapter Summary The genetic code is nearly universal. The code is always read three bases at a time and codes for amino acids, which combine to form proteins. DNA isolation and extraction from biological evidence such as blood, semen, and saliva must be done prior to completing a DNA profile. 32 Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Chapter Summary Polymerase chain reaction (PCR) is used to collect and analyze very minute and degraded samples. Short tandem repeats (STR) are very short segments of DNA in which the same sequence of two to six base pairs is repeated several times. The STR pattern, being highly variable from person to person, provides matches with higher degrees of certainty. Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Chapter Summary Once DNA is exposed to an environment outside the body, the DNA begins to degrade, which affects the ability to retrieve a useful result. When collecting DNA evidence, extra precautions must be taken to ensure it is kept in the best possible condition. Presumptive tests exist for the three most common types of body fluids found at a crime scene: blood, semen, and saliva. Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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Chapter Summary Mitochondrial DNA is inherited from the mother.
A Y chromosome is inherited by males from their father. Y-chromosome analysis is beneficial in cases involving sexual assault, missing persons, and genealogy. Most sexual assaults involve a male as the perpetrator. DNA tests that specifically test the genetic content on the Y chromosome can be forensically valuable. Forensic Science II: Advanced Concepts in DNA, Chapter 10 © 2012 Cengage Learning. All Rights Reserved
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