DNA

• Author: D. P. Lyle
• Pages: 179-203

DNA 179

CHAPTER 11

DNA

To say that DNA profiling has altered the landscape of crim inal investi-

gation would grossly understate its importance. Its usefulness stems from

its ubiquitous nature and unique structure. It is found in many materi-

als commonly shed at crime scenes: blood, semen, saliva, sweat, tears, ski n

cells, hair follicles, and many other human tissues. Other than fin gerprints

(Chapter 13), it is the only evidence that allows for absolute individualiza-

tion, since no two people, except identical twins, sha re the same DNA. Iden-

tical twins possess the sa me DNA but different fingerprints.

Our understanding of DNA and its uses in both medicine and forensics

is an evolving field. It is a complex subject, but in this chapter I will attempt

to simplify it and hopefully give you some degree of understanding of its

impact on the world of forensics.

The Structure of DNA

DNA (deoxyribonucleic acid) is the body’s instruction manual. Within it

lie the genes that determine height, size, eye and hair color, intelligence level,

musical ability, and virtually everyth ing else that makes each of us indi-

vidual. DNA itself is inert, in that it does nothing except store information.

180 DNA

Translating this information into action depends on other molecules, such as

various RNAs (ribonucleic acids) and protein enzymes.

The body is made up of approximately 60 trillion cells, a big number

that looks like this: 60,000,000,000,000. Each of these cells, except for the

red blood cells (RBCs) in the blood, contains a nucleus, and it is within the

nucleus that the DNA resides. Some of this DNA is packaged into units

called genes, the basic units of heredity. The genes are in turn arrang ed

along a long structure called a chr omosome.

Humans possess 46 chromosomes arranged into 23 pai rs. These pairs

are numbered 1 through 22, with the final two being the sex chromosomes,

termed X and Y.

The DNA molecule is a polymer (long string of repeating components)

of smaller molecules we c all pur ine bases. Though there are many di ffer-

ent purine b ases, only four are involved in the production of DNA. These a re

guanine, cytosine, thymine, and adenine, which are typically referred to

by their first letter: G, C, T, and A. All life is based on these four molecules.

The number of bases strung together in any given DNA strand can

be in the millions and they can hook up in any conceivable order. Like a

typewritten message, where letters are str ung together into meaningfu l

words and sentences, the order in which these bases are linked determine

the message contained within the DNA. The highly variable nature of this

pattern is what makes DNA so useful for identification.

Our DNA is double-stranded, meaning th at it consists of paired

strands of these bases that are wound together in a double helix , a spiral-

like structure that looks like a tw isted ladder (Figure 11-1). It is important

to note that when these bases pair up to form a double strand, each strand

is a “mirror image” of its mate. The reason is that the ru les of base pair-

ing dictate that C binds only with G and A only with T. It has to do with the

size and shape of these bases, but diggin g deeper into that is not necessary

for you to understand how DNA is used in the forensic world.

Using these rules, a section of double-stranded DNA might look like this:

G- C- T- T- A- C- T- A - A

C- G - A- A- T- G - A- T- T

You can see that A always lines up with T and C always with G.