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After studying this chapter you should be able to: Name the parts of a nucleotide and explain how they are linked together to form DNA

Understand the concept of base pairing as it relates to the double-helix structure of DNA

Contrast DNA strands that code for the production of proteins with strands that contain repeating base sequences

Explain the technology of polymerase chain reaction (PCR) and how it applies to forensic DNA typing

Understand the concept of electrophoresis

Understand the structure of an STR

Describe the difference between nuclear and mitochondrial DNA

Understand the use of DNA computerized databases in criminal investigation

List the necessary procedures for the proper preservation of bloodstained evidence for laboratory DNA analysis

DNA: the indispensable forensic science tool

amelogenin gene amino acids buccal cells chromosome complementary base

pairing deoxyribonucleic acid

(DNA) electrophoresis epithelial cells human genome hybridization low copy number mitochondria multiplexing nucleotide picogram polymer polymerase chain

reaction (PCR) primer proteins replication restriction fragment

length polymor- phisms (RFLPs)

sequencing short tandem repeat

(STR) substrate control tandem repeat touch DNA Y-STRs

KEY TERMS

chapter 15

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Criminalistics: An Introduction to Forensic Science, Eleventh Edition, by Richard Saferstein. Published by Prentice Hall. Copyright © 2015 by Pearson Education, Inc.

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378 CHAPTER 15

The discovery of deoxyribonucleic acid (DNA), the deciphering of its structure, and the decod- ing of its genetic information were turning points in our understanding of the underlying concepts of inheritance. Now, with incredible speed, as molecular biologists unravel the basic structure of genes, we can create new products through genetic engineering and develop diagnostic tools and treatments for genetic disorders.

For a number of years, these developments were of seemingly peripheral interest to forensic scientists. All that changed when, in 1985, what started out as a more or less routine investiga- tion into the structure of a human gene led to the discovery that portions of the DNA structure of certain genes are as unique to each individual as fingerprints. Alec Jeffreys and his colleagues at Leicester University, England, who were responsible for these revelations, named the process for isolating and reading these DNA markers DNA fingerprinting. As researchers uncovered new approaches and variations to the original Jeffreys technique, the terms DNA profiling and DNA typing came to be applied to describe this relatively new technology.

This discovery caught the imagination of the forensic science community because forensic scientists have long desired to link with certainty biological evidence such as blood, semen, hair, or tissue to a single individual. Although conventional testing procedures had gone a long way toward narrowing the source of biological materials, individualization remained an elusive goal. Now DNA typing has allowed forensic scientists to accomplish this goal. The technique is still relatively new, but in the few years since its introduction, DNA typing has become routine in public crime laboratories and has been made available to interested parties through the services of a number of skilled private laboratories. In the United States, courts have overwhelmingly admitted DNA evidence and accepted the reliability of its scientific underpinnings.

What Is DNA? Inside each of 60 trillion cells in the human body are strands of genetic material called chromosomes. Arranged along the chromosomes, like beads on a thread, are nearly 25,000 genes. The gene is the fundamental unit of heredity. It instructs the body cells to make proteins that determine everything from hair color to our susceptibility to diseases. Each gene is actually composed of DNA specifically designed to carry out a single body function.

Interestingly, although DNA was first discovered in 1868, scientists were slow to understand and appreciate its fundamental role in inheritance. Painstakingly, researchers developed evidence that DNA was probably the substance by which genetic instructions are passed from one genera- tion to the next. But the major breakthrough in comprehending how DNA works did not occur until the early 1950s, when two researchers, James Watson and Francis Crick, deduced the struc- ture of DNA. It turns out that DNA is an extraordinary molecule skillfully designed to carry out the task of controlling the genetic traits of all living cells, plant and animal.

Structure of DNA Before examining the implications of Watson and Crick’s discovery, let’s see how DNA is con- structed. DNA is a polymer. As we will learn in Chapter 12, a polymer is a very large molecule made by linking a series of repeating units.

NUCLEOTIDES In the case of DNA, the repeating units are known as nucleotides. A nucleotide is composed of a sugar molecule, a phosphorus-containing group, and a nitrogen-containing molecule called a base. Figure 15–1 shows how nucleotides can be strung together to form a DNA strand. In this figure, S designates the sugar component, which is joined with a phosphate group to form the backbone of the DNA strand. Projecting from the backbone are the bases.

The key to understanding how DNA works is to appreciate the fact that only four types of bases are associated with DNA: adenine, cytosine, guanine, and thymine. To simplify our dis- cussion of DNA, we will designate each of these bases by the first letter of their names. Hence, A will stand for adenine, C will stand for cytosine, G will stand for guanine, and T will represent thymine.

Again, notice in Figure 15–1 how the bases project from the backbone of DNA. Also, al- though this figure shows a DNA strand of four bases, keep in mind that in theory there is no limit to the length of the DNA strand; in fact, a DNA strand can be composed of a long chain with millions of bases. The information just discussed was well known to Watson and Crick by

deoxyribonucleic acid (DNA) The molecules carrying the body’s genetic information; DNA is double stranded in the shape of a double helix.

chromosome A rodlike structure in the cell nucleus, along which the genes are located; it is composed of DNA surrounded by other material, mainly proteins.

polymer A substance composed of a large number of atoms; these atoms are usually arranged in repeating units, or monomers.

nucleotide The unit of DNA consisting of one of four bases—adenine, guanine, cytosine, or thymine—attached to a phosphate–sugar group.

C S

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FIGURE 15–1 How nucleotides can be linked to form a DNA strand. S designates the sugar component, which is joined with phosphate groups (P) to form the backbone of DNA. Projecting from the backbone are four bases: A, adenine; G, guanine; T, thymine; and C, cytosine.

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