Deoxyribonucleic Acid

Ever wonder what DNA stands for? DeoxyriboNucleaic Acid. Words of the day.

Explaining DNA matching:  The DNA strand.

Step 1: What is DNA? The basics.

  • nuclear DNA is found in a cell’s nucleus
  • mitochondrial DNA is found in mitochondria
  • comprised of 4 chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T)
  • 3 billion bases per human

Step 2: What is DNA sequencing? Things you should know.

  • more than 99% of DNA bases are the SAME in all humans
  • the sequence of the chemical bases in each human are in a different order though
  • A always goes with T (base pair) 
  • C always goes with G (base pair)

Step 3: Make a nucleotide. Ingredients include:

  • base pairs
  • sugar molecule
  • phosphate molecule

Step 4: The nucleotides form a double helix, which looks like a ladder with long strands.


Step 5: DNA can replicate itself via the sequence. In other words, a new DNA cell replaces an old DNA cell with an exact replica.

Explaining DNA matching: Why DNA? STRs.

  • Since DNA is in every cell of the human body, we don’t need a huge sample to get information from it. Forensic scientists use Short Tandem Repeats (STRs), which are small chemical base sequences, to make an identification.

Explaining DNA matching: How? Polymerase Chain Reaction (PCR).

  • Mix chloroform and phenol and apply it to the DNA sample so that the DNA strand can be isolated from the other material in the nucleus.
  • Add a human enzyme, polymerase, to the extracted DNA to form a catalyst. The reason for this is that the polymerase enzyme will replicate the extracted DNA. The PCR enhances the DNA enough so that the forensic scientist can analyze it.
  • This is better than just having a whole bunch of DNA, like a larger sample, because the DNA strand is now isolated from the other material in the nucleus. I imagine it like a digital photo being zoomed in on and cropping out all the stuff you don’t want.  

Explaining DNA matching: Recipe for analyzing.

  • use a restriction enzyme to cut up the DNA
  • use electrophoresis to sort DNA by size
  • pour it into a tube of gel
  • add a positive charge to the bottom of the tube and negative charge to the top of the tube
  • DNA moves towards the positive charge (bottom) creating band like interruptions in the gel
  • use the bands to compare patterns with other samples

So, aside from the facts above, Salado Puerto did a really, really, really, really excellent job presenting the difference between a DNA match and the statistical probability. She said that a match is not always a match using genetic methods, especially in a large-scale population, performing a massive comparison. She also said that a match differs from a statistical probability. FAs frequently use Bayesian Analysis for probability and typicality statistical analysis, and she mentions this too. There are false negatives, false positives, and likelihood ratios that are being sought out. These probabilities are what are used in court to convict someone of a crime. Are these adventitious matches (something by chance  or extrinsic)? What is the best way to make a match? A match is a positive identification. When you get results back from a DNA test, the results are supposedly 99.9% correct. Most people accept that as a positive match, but Salado Puerto pointed out that it’s not 100% and there is a chance that the match is wrong. Do you want to convict someone of a crime? A life sentence? based on a high probability that they were the one who actually committed the crime? Stay tuned..


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