Introduction to Forensics: The Science of "When"

Welcome to one of the most fascinating parts of your Biology course! In this chapter, we explore how scientists use biological clues to solve mysteries. Whether it’s a crime scene or identifying remains, forensic biology helps us answer two big questions: Who was it? and When did they die?

We will look at how bodies change after death and how we can use a person's unique genetic code (DNA) to identify them. Don't worry if this seems like a lot—we will break it down step-by-step!

1. Decomposition and Carbon Recycling

When an organism dies, it doesn't just disappear. It undergoes decomposition. This is a vital process for life on Earth because it is part of carbon recycling.

Microorganisms (like bacteria and fungi) break down the organic molecules (proteins, carbohydrates, and lipids) in the body. As they respire, they release \(CO_{2}\) back into the atmosphere. This carbon can then be taken up by plants during photosynthesis, keeping the carbon cycle moving. Without decomposition, the planet would be covered in dead matter and new life wouldn't have the nutrients it needs to grow!

2. Determining Time of Death

Forensic scientists use several "clocks" to estimate how long a body has been dead. This is known as the Post-Mortem Interval (PMI).

A. Body Temperature (Algor Mortis)

From the moment of death, metabolic reactions stop. Since these reactions produce heat, the body begins to cool down until it matches the temperature of its surroundings.

Key Points:

  • The cooling usually follows a specific curve (often S-shaped or sigmoid).
  • Factors affecting cooling: Body size (larger bodies cool slower), clothing (insulates the body), and the surrounding temperature (a body in a freezer cools faster than one in a desert).

B. Muscle Contraction (Rigor Mortis)

After death, muscles go through a period of stiffening called rigor mortis.

How it works: In a living person, muscles need ATP to relax. After death, ATP production stops. The muscle fibers become "locked" in a contracted state. This usually starts 2–4 hours after death, peaks at about 12 hours, and then disappears as the muscle tissues start to break down (decompose).

C. Forensic Entomology (The Study of Insects)

Insects are often the first "witnesses" to a death. Different bugs arrive at different times.

  • Fly Life Cycles: Blowflies often arrive within minutes of death to lay eggs. By studying the stage of the fly's life cycle (egg, larva/maggot, pupa, or adult) and the temperature of the environment, scientists can work backwards to find the time of death.
  • Quick Tip: If you find a pupa that takes 10 days to develop at \(20^{\circ}C\), and you know the temperature has been \(20^{\circ}C\), the person likely died at least 10 days ago!

D. Succession on a Corpse

Just as a forest grows back after a fire in stages, different species of bacteria and insects colonize a body in a predictable order. This is called succession.

As the body decomposes, the conditions change (e.g., pH levels, moisture, and available food). The first group of insects (like blowflies) makes the body attractive to the next group (like beetles), which then makes it attractive to others. By looking at which species are present, scientists can estimate the time since death.

Key Takeaway: Time of death is rarely determined by one factor alone. Scientists combine evidence from temperature, stiffness, insects, and decomposition stages to get the most accurate estimate.

3. DNA Profiling: The Genetic Barcode

Every person (except identical twins) has a unique DNA sequence. DNA profiling is a way to look at specific parts of DNA to identify an individual.

Step 1: Obtaining the DNA

DNA can be collected from tiny samples like blood, hair follicles, or cheek cells. Because these samples are often very small, we need to make many copies of the DNA first.

Step 2: PCR (Polymerase Chain Reaction)

Think of PCR as a molecular "photocopying machine." It amplifies (copies) specific sections of DNA so we have enough to test.

The Process:

1. Denaturing: The DNA is heated to about \(95^{\circ}C\) to break the hydrogen bonds and separate the two strands.
2. Annealing: The temperature is lowered (to around \(55^{\circ}C\)) so that primers (short DNA sequences) can bind to the start of the section we want to copy.
3. Extension: The temperature is raised to about \(72^{\circ}C\). An enzyme called DNA polymerase adds free nucleotides to build the new DNA strands.

Step 3: Gel Electrophoresis

Once we have enough DNA, we need to separate the fragments to see the "profile." We use gel electrophoresis.

How it works:

  • DNA fragments are placed into a gel.
  • An electric current is applied. Because DNA is negatively charged, it moves toward the positive electrode.
  • Size matters: Small DNA fragments move through the gel easily and quickly, while large fragments move slowly and stay near the start.
  • The result is a pattern of bands.

Step 4: Creating the Profile

The fragments are "visualized" (made visible) using fluorescent dyes or radioactive probes. This creates a pattern of bands that looks like a barcode. By comparing the bands of a crime scene sample to a suspect’s sample, we can see if they match.

Common Mistake to Avoid: DNA profiling is NOT the same as DNA sequencing. Profiling looks at the size of specific fragments to create a pattern; it doesn't "read" every single base pair of the entire genome!

Summary Checklist

Before moving on, make sure you can:

  • Explain the role of microorganisms in decomposition and carbon recycling.
  • Describe how body temperature and rigor mortis change after death.
  • Explain how insect life cycles and succession help determine time of death.
  • Describe the steps of PCR (heating, primers, DNA polymerase).
  • Explain how gel electrophoresis separates DNA fragments based on size and charge.

Did you know? Forensic entomology can even tell if a body was moved! If you find a type of fly on a body that only lives in the woods, but the body was found in a city apartment, you know the body was moved after death.