DNA Structure, Replication, and Forensic Profiling

DNA (Deoxyribonucleic Acid): The hereditary molecule found in the nucleus of almost every cell which contains instructions to build an organism.
Forensic Application: DNA is unique to individuals, unlike unreliable "junk science" such as bite mark analysis or hair microscopy used in the Roy Brown case.
Chromosomes: Structures made of a single DNA molecule wrapped around proteins.
Human Chromosomes: Typical cells contain $23$ pairs ( $46$ total); one chromosome in each pair is inherited from each parent.

Nucleotides: The building blocks of DNA, consisting of a sugar, a phosphate, and a base (Adenine [A], Thymine [T], Guanine [G], and Cytosine [C]).
Double Helix: Two strands of nucleotides held together in the middle by hydrogen bonds between complementary bases.
Backbone: Formed by strong covalent bonds between the sugar and phosphate groups of nucleotides.
Complementary Base Pairing: $A$ always pairs with $T$ , and $G$ always pairs with $C$ .
Uniqueness: Variation in the sequence of nucleotides along the DNA strand makes every individual unique.

Natural Replication: A semiconservative process where the original DNA strands separate, and each serves as a template for a new strand, resulting in two double helices each containing one old and one new strand.
Enzymes: * Helicase: Unwinds the DNA double helix. * DNA Polymerase: Reads the template and adds complementary nucleotides to the new strand.
PCR: Developed by Kary Mullis, this laboratory technique amplifies small DNA samples using cycles of heating (to separate strands) and cooling (to bond primers and allow polymerase to work).
PCR Ingredients: DNA sample, primers, nucleotides, and DNA polymerase.

Genome: The complete set of genetic instructions, containing about $3 imes 10^9$ nucleotide pairs.
STRs (Short Tandem Repeats): Noncoding regions of DNA where specific sequences repeat multiple times; the number of repeats varies between individuals.
Gel Electrophoresis: A technique that separates PCR-amplified STRs by length; shorter fragments move farther through the gel than longer ones.
Discrimination Power: While $5\%$ to $20\%$ of people may share the same STR length at one site, analyzing $15$ sites simultaneously provides a high degree of certainty for identification.

Question: If you apply radioactive labels to phosphorus in a DNA molecule and allow it to replicate once, how much labeling is in daughter molecules?
Response: Each daughter molecule would contain half of the original radioactive labeling because replication is semiconservative.
Question: Is PCR identical to natural DNA replication?
Response: No; PCR occurs in a test tube, uses different temperature controls (heating/cooling), and amplifies only specific regions of DNA.
Question: Can DNA profiling differentiate identical twins?
Response: No, because their DNA sequences are identical (unless rare mutations occur during cell division).