What is DNA?
Each of an individual’s cells contains 46 DNA molecules.
23 chromosomes are inherited from the father and 23 from the mother.
DNA is packed inside chromosomes. For example, chromosome no. 3 contains 6.5% of the total DNA in each cell.
The DNA from the father’s chromosome 3 is mostly similar to the mother’s, with only a few differences called polymorphisms.
Polymorphisms help differentiate one person from another and trace ancestry.
What are STRs (Short Tandem Repeats)?
DNA has two strands, each made of four chemical bases: adenine (A), cytosine (C), guanine (G), and thymine (T).
The two strands are anti-parallel (run in opposite directions) and complementary (A pairs with T, and C pairs with G).
During DNA replication, the cell separates the strands and makes a new complementary strand.
Sometimes, mutations occur, where the wrong base is inserted, but these happen less than once per billion base-pairs per generation.
STRs are short sequences of DNA that repeat multiple times, such as “GATCGATCGATCGATC.”
STRs are often polymorphic, meaning unrelated individuals usually have different numbers of repeat units in at least some STRs.
How Do We Make Copies of DNA?
Polymerase Chain Reaction (PCR) is used to make many copies of specific stretches of DNA.
First, genetic material is extracted from a tissue sample (e.g., blood).
The sample is heated to 95°C to split the DNA strands apart.
Primers, short pieces of DNA, are introduced and bind to the DNA strands at specific locations.
DNA polymerase then synthesizes a new strand of DNA, adding complementary bases.
Taq polymerase is specifically used for this process because it can withstand the high temperature of around 72°C, which is necessary for synthesizing the new DNA strands.
The process is repeated multiple times in cycles, and each cycle doubles the number of DNA copies. With enough cycles, millions of copies are made in a short time.
What is a DNA Fingerprint?
Capillary electrophoresis uses an electric field to move DNA through a capillary tube.
Smaller fragments travel faster, which allows for the separation and analysis of DNA fragments by size.
Multiple STRs are analyzed in the same test to create a unique DNA fingerprint.
DNA fingerprints can identify individuals from various sources (e.g., blood, bones, skin cells).
They help establish parent-child relationships, identify individuals in disasters, and solve crimes.
DNA is stable and can be extracted from old remains to solve cold cases and exonerate the wrongly accused.
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