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are simply long stretches of DNA. The genome refers to all the DNA in an
organism; the set of 46 chromosomes in humans, for example. While DNA
and chromosomes are physical entities, genes are conceptual.
The human genome is composed of chromosomes; we get 23 chromosomes
from our mother and 23 chromosomes from our father, making
for 23 pairs. While 22 of the 23 chromosome pairs are similar in both
men and women, the 23rd pair is quite different and
determines the sex
of an individual. For the 23rd pair of chromosomes, women
have two
X-chromosomes while men have one X- and one Y-chromosome. Because
X-chromosomes contain more DNA than Y-chromosomes, they are physically
larger than Y-chromosomes. Having too many or too
few chromosomes can affect gene regulation and cause diseases. Down's
Syndrome, for example, occurs in individuals with three copies of
chromosome 21.
The roles of X- and Y-chromosomes are important in understanding
sex-linked diseases. Women do not have Y-chromosomes, so diseases that
are caused by defective genes on the Y-chromosome can only occur in men.
Additionally, men only have one X-chromosome, so mutations in genes on
the X-chromosome are more likely affect males, because the second
X-chromosome in women can sometimes compensate for mutations on the
first. Color blindness, caused by a mutation on the X-chromosome, is
more common in men than women for this reason.
The structure of DNA is two intertwined strands that form a double
helix. The two strands of DNA are said to be complementary because the
sequence of one strand indicates what the sequence of the opposite
strand is. Each strand of DNA figuratively resembles a long piece of
tickertape. Instead of being printed on, DNA is physically composed of
four different chemical units that encode information. These four
chemical units, adenine,
cytosine, guanine, and thymine, are often abbreviated as A, C, T, and G,
respectively. Just as the English language can be expressed in
twenty-six letters, the genetic code is expressed in these four chemical
units. A DNA sequence refers to the specific order of A's, C's, T's
and G's in a stretch of DNA.
A gene is a conceptual segment of DNA that codes for a genetic
(inheritable)
characteristic. There are two essential elements of genes: coding and
regulatory elements. The coding elements of genes are first transcribed
as mRNA and then translated into protein. Regulatory elements
affect the rate at which genes are transcribed and translated.
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