The tickle of curiosity. The gasp of discovery. Fingers running across the keyboard.

The tickle of curiosity. The gasp of discovery. Fingers running across the keyboard.

The World of Iniquus - Action Adventure Romance

Monday, September 30, 2013

Walking Through Forensic DNA Basics: Information for Writers


Animation of the structure of a section of DNA...
 (Photo credit: Wikipedia)
This week I had the opportunity to go down to Richmond to visit the FBI headquarters. A handful of Sisters in Crime Members were being hosted by the FBI Evidence Response Team. I want to thank the team for sharing their expertise. (Names and images are withheld for security purposes)

That night I learned a lot about processing the scene of a crime, collecting everything from foot prints BLOG LINK - Footwear Evidence to  fingerprints BLOG LINK to Fingerprint Evidence to fibers.

But the gold star belongs to DNA. DNA has risen to be THE  best available evidence because the laboratory results are statistically linked to a single person and exclude all others.

Did you know that with the improvements in science, they can now trap enough DNA in a fingerprint to create DNA profile?

If you are writing a story that involves a crime, it's likely that your investigation team will be looking for DNA to collect and use to solve the case.

Processing Blood Samples, FBI
Forensic analysis of DNA can be confusing so let's walk through this together.

STEP ONE - What Is DNA? 

First the bilology basics. I know. I know. It's the spinach on your plate. But it's important, so I'll try to make this as quick and as painless as possible.

DNA - Deoxyribonucleic Acid -
* All cells have DNA (except red blood cells)
* DNA is the genetic material making up our chromosomes
* Gene – the basic unit of heredity; a sequence of DNA nucleotides
   on a chromosome.
* Genome – the sum total of an organism’s genetic material.
* DNA encodes the instructions for when and how to make
   proteins. These proteins tell the cell how it will
   function - is it a brain cell? a liver cell?
* It looks like a twisted ladder. The uprights are made out of
   phosphates and sugars. rungs are pairs of
   smaller particles called nucleotides.

You're doing great! Hang in there!

English: DNA replication or DNA synthesis is t...
(Photo credit: Wikipedia)
* All DNA is made up of a combination of four letters
  G - Guanine
  C - Cytosine
  A - Adonine
  T - Thymine
* G and C got together A and T go together
Happy Valentines Day - Relationship Mitosis
Mitosis (Photo credit: id-iom)

Mitosis -
* DNA replication
* A cell splits by dividing like a zipper
* The new cells contain an identical set
   of cells as were present at conception
* This only changes when there is a
    random mutation
* All cells in the whole body contain the
   exact same DNA

Video Quick Study of Mitosis (1:29)
VIDEO QUICK STUDY (8:03) basic info on DNA and Mitosis - might be more information than you need, but the presentation is thorough and easily understood.

STEP TWO - There Are Two Kinds of DNA
Prokaryotes are primitive cells, without a nuc...
 (Photo credit: Wikipedia)

Nuclear DNA-
* It is found inside the nucleus packed
   into chromosomes
* It is inherited from both parents
   50% from our mom and 50% from
   our dad.
* Each parent contributes 23
   chromosomes for a total of 46.
* The dominant and recessive qualities
   gives us our traits
* A DNA profiles belongs to an
   individual and ONLY that
   individual and no one else in the world    UNLESS we have an
   identical twin. Plot twist!
* The closer we are in genetic
   relationship to someone else, the
   closer the DNA configuration.
* Each cell has one copy of nuclear DNA

Mitochondrial DNA-
* Is found in mitochondria
* 100% of mitochondrial DNA is inherited from mom.
* The mitochondrial DNA is the same in our brothers and sisters
    regardless of who fathered each child.
* All mitochondrial DNA came from your mom which came from 
   your mom's mom which came from your
   mom's mom's mom etc. How cool is that?
* Is not housed in a a set of chromosomes
* This DNA is smaller than Nuclear DNA
* This DNA is present in larger numbers than Nuclear DNA
   (nuclear DNA has just one copy per cell)
* This DNA tends to be more stable than Nuclear DNA over time.

VIDEO QUICK STUDY (1:58) Two Minute Science Lesson: How DNA Testing Works

STEP THREE - Collecting DNA Evidence

Maryland V. King Police can collect DNA without a warrant Video Quick Study (1:55)

Biological evidence is perishable
* DNA will fragment
* The best shot at getting useful DNA is in a fresh sample
* Fresh tissue has more nuclear DNA and an older sample has less

Things that cause DNA to degrade

* UV light
* Acid
* Bleach
* Hydrogen Peroxide
* High humidity
* Heat and Fire
* Anything that oxidizes biological molecules

Handle with Care:
Video Quick Study (2:16) Mentions the changes in technology and the importance of properly preserving the sample.

* If the evidence that is being collected is wet it should be air dried.
* The evidence should be stored in a PAPER BAG that can release
* Store in low temperatures (this prevents bacterial or fungal
   growth that has its own DNA)
*  Avoid as much as possible environmental contamination.

Crime Scene Techs:
* Need to understand that they can be contaminated by the crime
   scene (pick up bacteria etc.)
* Can contaminate a crime scene with their own saliva, dander, hair
* The FBI Processing unit that I visited this week uses full Tyvek
   suits to prevent contamination of the scene.
* People who were on the scene will often be asked to give a DNA
   sample called an "ELIMINATION SAMPLE"
* Most often reference samples are taken from a buccal swab. A
   square piece cotton is swabbed over the
   inner cheek. This is placed in an envelope. Why not a vial?
   Because a closed container encourages the
   growth of other life forms. 
   VIDEO QUICK STUDY (3:07) How to collect a buccal sample.
* DNA elimination samples are collected from victims and
   witnesses etc.

English: A forensic scientist at the U.S. Army...
. (Photo credit: Wikipedia)

PLEASE NOTE this elimination sample does NOT get run through the system. It is merely there for the scientists to compare with the sample they are analyzing. Your character will NOT be located and charged with a previous crime. There are all kinds of privacy laws that prevent this from happening, according to the FBI agents we were interviewing.

Death Investigators: What if you have a body to identify?
If the investigators have an idea who this might have been they can get a swab from a blood relative and try to find markers that they share. If no relative is available, then they could go to the person's home and try to collect DNA there from a toothbrush or razor, etc.

What if the house burned down and DNA from that location is not available? Well, investigators could try to find stored DNA samples, for example a woman's pap smear might be stored or perhaps your character had given blood to the Red Cross.

Here's a VIDEO QUICK STUDY (6:09) that gives a simplified description - good overview. 
STEP FOUR - Extracting and Processing DNA

Extracting DNA isn't hard you can do it right now in your own kitchen. Don't believe me?
Video Quick Experiment for You to Try (2:46)
Video Quick Study - Separating DNA from Blood (6:37)

In extracting DNA from Crime Scene or reference samples, the goal is to find the 1% that differs from everyone else. This gives an individuals DNA Fingerprint or DNA Profile.

99.9% of our DNA is the same in all humans. Only .1% differentiates us

* DNA is stiff and brittle. In order to avoid damaging it, a

   technique is employed that breaks down the cell around it.
* In a crime scene laboratory being meticulous is paramount.
VIDEO QUICK STUDY - (6:19 - but go right to 3:00 mark) this
   process is better explained through this video than writing the
   steps because of the equipment


RFLP Analysis - Restriction Fragment Length Polymorphism not widely used since the mid-90s
* A large sample is required
* Results are not compatible with major DNA databases
* The proces uses molecular scissors to cut where a TA next to a
   CG combos. Since everyone has different DNA profile's the
   material created will be of different lengths. (15-35 repeating
   base pairs)
* This is then processed with Gel Electrophoresis
   Video Quick Study (5:18) Frankly this is the quickest, easiest
   way of understanding the process

To correct the problem of sample size, scientists developed PCR - Polymerase Chain Reaction
* This was developed in 1983 as a method for replicating DNA
VIDEO QUICK STUDY (3:32) sorry this has no audio, but it is
   the best quick study I could find
* PCR takes advantage  of the DNAs double strand and its simple
* Uses a thermocycler to double the amount of DNA with every
* Only one strand of DNA can make a billion copies in four hours


STR - Short Tandem Repeat
* It still uses PCR to make copies
* STR focuses on the smaller repeating units in DNA
* STR uses only about 3-7 repeating base pairs (where RFLP
   needed 15-35)
* Can be used on much smaller samples than RFLP
* Can be used on more degraded samples than RFLP
* Only 18 cells are needed to get a DNA profile - this is why they
   can pick up DNA from a fingerprint, according to our FBI
* Uses Capillary Electropheresis -  done in a column (not in a flat
   gel as in electrophoresis)

THERE ARE 13 DIFFERENT CORE LOCATIONS WITHIN DNA that captures most of humans variability in STR

CODIS FBI Logo (Photo credit: Wikipedia)

* Each core location has its own
   established probability of being
   found in the larger population.
* When all 13 different probabilities
   are added together the statistical
   probability are in the quadrillions.
* These can be put into the CODIS
 (Combined DNA Index System)
  developed by the FBI in the late
  90s. Pay attention to this date if
  your story happens before this time

Recently they've developed a way to tell if the DNA comes from a man or a woman by looking at the sex chromosomes. (XX or XY) They do this by looking at the amelogenin gene on the sex chromosome. It is longer in a man than woman but this is enough to tell the difference. This is important because sometimes there is a mixed sample of DNA, for example if they do a vaginal swab for sperm.

STEP FIVE - Is It a Match?

Reading the computer output is outside of the scope of this article - though I will address this in the near future.

CODIS 13 point profile
CODIS 13 point profile (Photo credit: Wikipedia)
The information gathered is analyzed and a statistic is assigned.
Population Genetics gives us
    statistics about the likelihood of this
    profile occurring in a population. It is
    a PROBABILITY that the same
    profile is replicated in the population
    and not coincidence.
* The more stretches of DNA that are
   compared between two people the
   statistics possibility of error narrows.
   It can become so narrow that it
   EXCLUDES everyone
   else from the pool of possibilities
   EXCEPT an identical twin.

Okay how was that? Not bad right? We made it to the end of the DNA pathway. Now as you sit on the bench to catch your breath, let's review some points at which a plot twist could mess everything up:

* Has the prosecution kept tight control of its chain of who has
   custody from collection to end result?
* Was the biological material from which the DNA was extracted
   identified? For example, in a rape case was it semen that is being
* Is the testing laboratory accredited?
* Has the testing laboratory  been audited by an outside agency?
* Has the testing laboratory  undergone a proficiency test - and was
   it a blind test?
* Did the laboratory run the required control samples?
* Was there examiner bias? Did the tech talk directly to the
   investigator or was it blind?
* How large was the DNA sample? Was it from a pool of blood?
   Or was it from a fingerprint?
* What if your character -- either victim or accused -- has a relative
    involved in the research (or an ex-wife eek!)?
* Was more than one DNA profile mixed in the sample?

So many ways to play with DNA evidence! 

Intermountain Forensics:
This is very good! If we could add to your collection: CODIS Core Loci is now 20 markers and keep an eye out on next generation sequencing and genetic genealogy for the future of forensic DNA. Nice work!

Thank you so much for stopping by. And thank you for your support. When you buy my books, you make it possible for me to continue to bring you helpful articles and keep ThrillWriting free and accessible to all.


  1. Fiona, thanks for another great post.

    Does water, particularly salt water, degrade DNA? For example, if a person was dumped in the ocean, would you be able to tell the identity of the killer from any evidence on the body?


  2. Hi Eden,
    In terms of DNA degradation, time and temperature are important. Being in water might complicate the DNA identification in that there are other organisms with their own DNA contacting the sample. If they can not use the nuclear DNA they might turn to mitochondrial DNA which can narrow the family but is less statistically applicable to identifying an individual. Here is an article that puts a body in the water for almost a week. According to Cookie at WPA 2013, that's about when a sunken body will float - depending on conditions.

    For my readers who don't know Eden, she is a Canadian based writer with a mystery due out this year.

    Hope this helps! Thanks for coming by my blog.
    Cheers ~ Fiona

  3. How close is the DNA of identical twins?

  4. Hi Randall, Identicals (twins, triplets) have the same DNA.

    1. That's what I was thinking. That seems like it could lead to some interesting possibilities.

  5. Following up on a DNA question from a writer -
    So we know that there is no DNA in a red blood cell because it has no nucleus. But I thought for sure it would contain mitochondrial DNA and at least that would be a path that the authorities could follow - removing suspects from the possibility list. And I was WRONG!

  6. Absolutely fascinating, as is this entire blog! Thank you so much. I'm interested that twins share the same DNA, but is it true that they have different finger-prints? I'm sure it is. And what about the iris? I'm coming back on here for many a re-visit!

    1. Irises and therefore the biometrics of the iris are individual. Here's an article to help you understand.

      Thank you for your kind support of my blog and my writing.

  7. Nominated! Good luck Fiona, sounds like a wonderful opportunity.


  8. Excellent resource for DNA! I wish my chapter of Sisters in Crime would have a field trip like this. Maybe soon... I hope. Bookmarking for future use. Thanks for sharing!

    1. Hi Sue,

      We've had some really interesting opportunities through our SinC chapter. I love being part of that organization :)