Human DNA is astoundingly complicated. The four bases of DNA
make up the six feet of genetic material in every cell. This
material is spread out over 23 chromosomes. The three billion base
pairs in a single human cell make up 60,000-70,000 genes, plus the introns
(about 95%) whose function still remains a mystery. It can be
plainly seen that constructing a genetic map is a daunting task.
Getting Started
Many people want to know exactly whose DNA geneticists are mapping.
The Human Genome Project is not trying to map
a specific person's DNA, but DNA in general. 99.9% of the DNA of
every person on earth is identical. Scattered throughout this
identical DNA is the 0.1% (3,000,000) of base pairs that are vary from
person to person: the variations and mutations that make one human
different from another. These variations provide the key to
unlocking the human genome. By comparing the DNA of many different
people, especially those with genetic disorders, scientists hope that they
can map the human genome by 2005.
Genetic Linkage Maps
The first step toward mapping the genome is to establish where one gene is
in relation to another. This is done by studying people with genetic
disorders. During meiosis, chromosomes pair up
and exchange parts of their DNA, mixing the parents' chromosomes into a
single chromosome that will be passed on to the offspring in the
form of a sperm or an egg cell. This process is called
"crossing over". Two traits that are often inherited with
each other are almost definitely very physically close to each other on
the chromosome, because the closer two genes are to each other, the more
likely they are to move together when the chromosomes mix. Genes
that often move together when chromosomes cross over are said to be
linked. Although a genetic linkage map does not show a gene's
location, it can show the odds that one specific gene will be inherited
with another specific gene; in fact, the distance between genes on genetic
linkage maps is not measured in terms of physical distance, but in terms
of what the probability is that two genes will cross over together.
Contig Maps
Once scientists determine which genes are linked together, they can begin
to form a contig(uous) map of the chromosome. A contig map is
designed to show the general physical location of specific genes on a
chromosome; the distance between genes on a contig map is measured in
terms of physical distance, as opposed to the genetic linkage map, which
measures the probability of two genes being inherited together. The
margin of error when measuring the distance between two genes
on a contig map is 500,000 base pairs. A contig map can also show
which blocks of chromosomes are mostly coding genes and which blocks are
mostly introns.
Genetic Sequencing
The ultimate goal of the Human Genome Project is to produce a genetic
sequence map of the human genome. At the project's conception in
1990, computer technology was not good enough to map the entire genome in
anywhere close to 15 years. As computing technology improves and
scientists' technique is revised, the dream of a genetic sequence is
drawing closer and closer to being realized. Of course, with genetic
sequencing, the problem again arises, "Whose DNA is going to be
mapped?" When the scientists reach the areas where individuals'
DNA differs, all the participants' individual DNA will be sequenced and
catalogued.
What Next?
Even after the Human Genome Project is finished, there will be the task of
interpreting all the data. Scientists still need to learn about the
function of introns, about the different possible combinations of DNA that
yield a specific trait, and about the effects of mutations. The
Human Genome Project in no way marks the end of discovery in the human
genome. |