Short
answer
Your DNA is like an instruction manual for how
to build you.
Most DNA is basically the same in
everyone - two eyes, two ears, one nose, etc - but there is enough
variation in certain parts of the DNA that everyone turns out
slightly different from everyone else.
Because of this, DNA can be used to
identify individuals. On TV crime shows, detectives regularly make
use of DNA evidence to match a criminal to the skin or sperm cells
they left behind at a crime scene.
At DNA Labs, we are more interested
in another feature of DNA - the fact that you inherit it from your
parents. Knowing this, we can test related people and establish a
family connection.
In the case of a paternity test, for
example, if we test a child and its mother, we can compare where
the two match. This tells us which parts of the DNA have come from
the mother. The remaining parts must therefore have come from the
father. If we look at a man's DNA and it matches those remaining
points, the man is the father. If the parts don't match, the man
could not have supplied the child's DNA and is therefore not the
father.
DNA can also establish relationships
between more distant relatives as they all will have common DNA
inherited from an ancestor.
Long
answer
DNA is an acronym for deoxyribonucleic
acid.
This complicated-sounding name is
actually a good description of the molecule. DNA is a long string
of nucleotides, hence the nucleic part of the name. Each nucleotide
is made up of a sugar molecule, a phosphate group and a nitrogenous
base. The sugar molecule is deoxyribose.
While the sugars and phosphates stay
the same all along the molecule, the nitrogenous bases vary. There
are four types of nitrogenous bases in DNA - adenine (A), thymine
(T), guanine (G) and cytosine (C). These four different bases
combine in a variety of ways to form a code. This code contains all
the information about your genetic makeup. If you were to read it,
it would look something like this:
...CGTAGCTTACTTTAGGCTAGCAAACGCATC...
DNA can be further broken down into
genes - short sections of DNA (though much longer than this
example) that can be decoded by the cell to mean something. A
single gene might be responsible for any aspect of your body's
function or appearance, like making a certain protein, or the
colour of your eyes.
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Long strings of DNA, twisted around each other to
form a double helix and containing hundreds of genes each, coil up
in the form of chromosomes. In normal human cells there are 46
chromosomes There is one pair of sex chromosomes (two X chromosomes
for females and one X and one Y chromosome for males) and 22 pairs
of autosomes (non-sex chromosomes) numbered 1-22.
Packaging the DNA into chromosomes ensures that
the genetic material is copied and distributed evenly when a cell
divides. Chromosomes can only be seen under the microscope when a
cell is dividing.
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Using DNA for
identification
Most DNA is pretty much the same between
people - two eyes, one nose, two legs, etc - but there is enough
variation to make every person unique. The uniqueness of DNA makes
it an almost foolproof method of identification. On TV crime shows,
detectives regularly make use of DNA evidence to match a criminal
to the skin or sperm cells they left behind at a crime
scene.
It is also possible to make use of
the knowledge that half of a person's DNA comes from their mother
and half from their father to test for possible relationships
between people. In the case of paternity testing, DNA taken from a
child is compared to that of its mother. Common points on the two
samples show those parts of the DNA the child has inherited from
its mother. Uncommon points must therefore have come from the
father. When a man's DNA is tested we can see that if the points
match, he is the father. If they don't, he is not.