13.1: Deoxyribonucleic Acid and Forensic Biology
- Page ID
- 53179
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)When Edmond Locard postulated his principle of evidence exchange, the concept of using deoxyribonucleic nucleic acid, or more commonly referred to as DNA, as a means of identifying individuals specific to a crime was seventy years in the future; yet he could not have been more correct to understand the critical importance of the detection of trace and transfer evidence to DNA analysis today.
DNA is the foundational component for the entire genetic composition of every human and animal. Every person’s DNA is individual to that person, except in the case of identical twins. This makes DNA the king of forensic identification. A crime laboratory technician can develop profiles of individuals by analyzing selected DNA sequences called loci. These samples can be extracted from evidence found in a crime scene. Since DNA is found in most cells in the human body, even an infinitesimal sample of bodily fluids or tissue can yield a complete DNA profile. As previously noted, DNA can be extracted from blood, saliva, semen, sweat and other secretions, skin tissue and cells, bone, teeth, finger and toenails, and, in some cases, urine and fecal matter.
Of course, the complete topic of DNA would better merit a complete chapter or entire book in itself and would more appropriately be authored by a microbiologist as opposed to a humble forensic specialist, but for the purpose of this textbook we will keep it as primary as possible.
According to Andy Brunning in his 2015 article, What Makes up the Structure of DNA, the double-helix model of DNA consists of two intertwined strands. These strands are made up of nucleotides, which themselves consist of three component parts: a sugar group, a phosphate group, and a base. The sugar and phosphate groups combined form the repeating ‘backbone’ of the DNA strands. There are four different bases that can potentially be attached to the sugar group: adenine, thymine, guanine and cytosine, given the designations A, T, G and C. The bases are what allow the two strands of DNA to hold together. Strong intermolecular forces called hydrogen bonds between the bases on adjacent strands are responsible for this; because of the structures of the different bases, adenine (A) always forms hydrogen bonds with thymine (T), whilst guanine (G) always forms hydrogen bonds with cytosine (C).
Cells in the human body are constantly in a cycle of generation, division, intracellular death, and regeneration. DNA replication allows this cycle to occur. During this process, the two strands of DNA split, using two single strands as a guide to construct a new version of the complimentary strand, with A pairing with T, and G pairing with C. Enzymes, called DNA polymerases, replicate the bases of one strand by using the information from the opposite strand.
Forensic laboratories use this polymerase chain reaction process, or PCR, to ‘copy’ regions of DNA in a test tube. Today, even the smallest sample of DNA can be replicated into the billions in a matter of a few hours.
We will discuss the collection of DNA in the next chapter.