||Twenty Questions About DNA Sequencing
(And The Answers)
1. Q: What is DNA?
A: The hereditary substance found in all living
cells is deoxyribonucleic acid, or DNA. It is composed of four different
chemical subunits, called bases, that are represented by the letters A,
T, C, and G. The order of the bases provides coded instructions for everything
a cell does. Determining the order of the bases along the DNA strand is
2. Q: Whatís a genome?
A: Genome refers to the complete set of genetic instructions
carried within a single cell of an organism. In most cases, the DNA in
each cell is the same. Every living thing has a genome, though they can
vary from quite small (a few thousand bases for some viruses) to quite
3. Q: How many bases, or letters, are in the human genome?
A: About three billion. If all the letters were printed
out, it would fill a stack of books as high as the Washington Monument.
The mouse genome also has 3 billion bases. A worm has 100 million, a fruit
fly has 160 million, and the yeast used in making bread has 12.5 million.
4. Q: Why do scientists want to sequence the human genome?
A: DNA sequence contains the instructions for everything
a cell does, from the moment of conception until death. Changes in DNA
spellings can increase your chances of developing an illness, protect you
from getting sick, or predict the way your body will handle medicines.
Once scientists can read the DNA instruction book, they will be able to
better understand and treat diseases.
5. Q: What is the Human Genome Projectís plan for human DNA sequencing?
A: The Human Genome Project began sequencing
the DNA of laboratory organisms to help refine the technology needed to
begin the larger, more complex human genome. In addition, we hope to understand
human genes by comparing human DNA with DNA from mice, flies, roundworms,
and yeast. These comparisons make the human sequence much easier to understand.
Two years ago, the first funds were awarded to begin sequencing human DNA.
Sequencing goals of the Human Genome Project 1998-2003 plan include increasing
capacity to deliver a complete human genome sequence by the end of the
year 2003, two years earlier than originally planned.
6. Q: How much human sequence has been produced so far?
A: A working draft of the complete Genome was finished in June, 2000. Scientists expect to have a completely finished sequence of the human genome
by the end of 2001. All sequence produced by Human Genome
Project scientists is in a freely accessible database. Celera Genomics, a privately held company, is responsible for a portion of the genomic mapping. Information from this company is not in the public domain.
7. Q: How much has it cost?
A: In 1990, when the Human Genome Project began,
DNA sequencing cost $10 per base. The process has been streamlined and
automated and better machines read the sequence fragments quickly and more
cheaply. These advances, together with sophisticated computer programs
that help interpret the data, have dramatically reduced the cost to roughly
50 cents per base for high-quality "finished" sequence. This figure covers
the cost of shotgun sequencing, as well as the cost of closing the gaps
and finishing the sequence. Altogether, the NIH genome project has spent
about $84 million on human DNA sequencing as of June 1998.
8. Q: What does "shotgun" sequencing mean?
A: Shotgun sequencing refers to a method that
uses enzymes to cut DNA into hundreds (or thousands) of random bits. That's
necessary because today's automated sequencing machines, though faster
and cheaper than older "by-hand" methods, still decipher only relatively
short DNA fragments about 500 bases long. Scientists then have to piece
the sequenced fragments back together, bit by bit, so it looks like the
original genome. Computers usually perform that step. The Human Genome
Project shotgun approach is applied to cloned DNA fragments that already
have been mapped so that it is known exactly where they are located on
the genome, making assembly easier and much less prone to error.
9. Q: How much has the federal government spent on the Human Genome
Project so far?
A: Almost $1.5 billion since it began in 1990.
That includes funding for both the Department of Energy and the National
Institutes of Health programs, which have used those funds to develop detailed
genome maps, identify and map half of the 80,000 or so human genes, sequence
the genomes of several laboratory organisms, develop new technologies,
and fund social impact studies.
10. Q: What else does the Human Genome Project do?
A: The project has developed maps of the human
genome, that is, a set of closely spaced molecular landmarks to guide gene
hunters. Project scientists also are mapping the genome of the laboratory
mouse. Genome researchers have identified almost half of the 80,000 or
so human genes, and completed the DNA sequence of two important microbial
genomes, the bacteria E. coli and baker's yeast. By the end of the
year, the first genome sequence of a multi-celled animal (the roundworm)
will be completed.
As new human sequence is deposited, scientists are using
this information to understand how genes function, how genetic material
varies between people, and how certain genetic variations can predispose
to disease. Just as important, the public Human Genome Project spends 5
percent of its research budget on studies of the ethical, legal, and social
implications of new genetic technologies. Funding also is provided to improve
current sequencing technology.
11. Q: How will scientists benefit from the sequence provided
by the Human Genome Project?
A: First, the Human Genome Project will provide
a truly complete and highly accurate reference genome sequence. That level
of completeness is necessary to provide researchers with the best possible
tool for understanding the function of genes and their role in human health
and disease. Second, the public effort will sequence mapped DNA fragments
derived from known locations in the genome, so scientists can be certain
which region they are studying. Finally, the public project will post its
sequence in a freely accessible database within one day of obtaining the
data, allowing all scientists immediate access to the data.
12. Q: Whose genome is being sequenced?
A: The Human Genome Project is sequencing DNA
collected from a number of volunteers whose identity will remain secret
to protect their privacy.
13. Q: Who has access to the human DNA sequence?
A: Scientists around the world participating
in the public sequencing effort have agreed to submit their data to public
databases within one day of verification. That means any scientist, whether
at a university, corporation, or government lab, can log on by computer
and have full access to the sequence data.
A: Some parts of the human genome are easier to sequence
than others. One strategy is to sequence the easy parts and leave the rest.
Though that would provide some useful data, it will leave gaps in areas
that are likely to contain important biological information. The Human
Genome Project has promised to fill in the gaps, or "finish" the complete
human DNA sequence.
15. Q: If only part of the genome contains genes, why sequence the
A: The more we study DNA, the more we understand how
it carries out its amazing work. Besides the part that codes for protein,
genes are made up of several parts that control their activity. Sometimes
all the parts are clustered in the same DNA neighborhood, but other times,
they may be scattered far from each other. Some parts are very easy to
spot and some aren't. So, having a complete genome sequence that includes
all of a gene's parts is the best way to study how they work. Already we
have learned that critical clues to cancer, diabetes, and osteoporosis
lie in the areas of the genome that do not code for proteins.
16. Q: What areas of the sequence will be finished first?
A: Efforts to produce a high-quality, accurate, finished
sequence will focus initially on those areas in the genome that contain
the most genes. The sooner the gene-rich areas are sequenced, the faster
researchers around the world can use that important information to study
17. Q: What is "quality" sequence?
A: High-quality sequence is complete, with all the difficult
areas "finished." Further, the international community of Human Genome
Project researchers has adopted a "4A" standard for quality sequence:
14. Q: What does "finishing" the sequence mean?
The sequence must be accurate, that is, the DNA
spellings must be correct. The publicly funded genome project will ensure
accuracy of 99.99 percent or better.
Other measures of quality reflect how useful the sequence
is to researchers performing experiments.
18. Q: Why did they decide to speed up the public sequencing project?
A: The sooner the public effort to sequence the human
genome is complete and deposited in a publicly accessible database, the
sooner it will benefit all scientists working to understand and treat disease
at a molecular level. Sequencing technology has improved dramatically in
recent years and pilot sequencing projects have been a tremendous success.
The time is ripe to accelerate full-scale sequencing of the human genome.
19. Q: What happens after the genome is sequenced?
A: Completing the DNA sequence of the human genome will
provide scientists with a tool for genetics akin to the Periodic Table
of the Elements. Instead of 100 chemical elements, though, the genome contains
about 80,000 elements, the human genes. Once you have the table of the
elements, you can begin to figure out how the genes function, how genes
interact, and how they contribute to common disorders.
20. Q: Can information about my own DNA be used to harm me in any
A: Studies have shown that people have lost jobs and
health insurance and suffered other types of discrimination based on their
genetic makeup. A very important goal of the Human Genome Project is to
ensure that new abilities to read and interpret genetic information are
used to benefit people not to harm them. That's why the project has set
aside 5 percent of its research budget to study and address the ethical,
legal, and social implications (ELSI) of genome research. While those studies
have provided policy makers with valuable knowledge for developing laws
to prohibit genetic discrimination in health insurance and employment,
and to protect the privacy of personal genetic information, much still
needs to be done in both the legislative and research arenas to ensure
that genetic information is used safely and appropriately.
The sequence must be assembled. Large-scale DNA
sequencing relies on the accurate assembly of smaller lengths of sequenced
DNA into much longer regions so it reflects the original piece of genomic
Because human DNA sequence must also be affordable,
a portion of the genome project research budget funds technology development
to reduce costs as much as possible.
Finally, high-quality, finished DNA sequence must be
accessible. Human Genome Project sequence data is deposited in public
databases on a daily basis.
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Editor: Susan K. Boyer, RN
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