FOR IMMEDIATE RELEASE
Wednesday, December 1, 1999
Contact: Cathy Yarbrough
Scientists Complete First Chapter of Book of Life with Decoding of First Human Chromosome
- A total number of at least 545 genes and 134 pseudogenes (genes
that once functioned but no longer do) were detected on the
chromosome, with 200 to 300 additional ones likely. If
representative of other chromosomes, this count suggests that the
total number of genes on all human chromosomes will not be
substantially more or less than the previously estimated number of
- The genes range in size from 1,000 to 583,000 bases of DNA with a
mean size of 190,000 bases. A total of 39 percent of the
chromosome is copied into RNA (exons and introns), while only 3
percent of the chromosome encodes protein.
- A total of 247 genes were revealed by computer analyses to be
identical to previously identified human genes or protein sequences.
Computer analysis of the chromosome 22 sequence found 150
additional genes with DNA sequence similarity to known genes. An
additional 148 predicted genes containing sequence homologous to
known genetic markers (ESTs) were identified.
- Several gene families appear to have arisen by tandem duplication.
There are families of genes that are interspersed among other
genes and distributed over large chromosomal regions.
- There is unexpected long-range complexity of the chromosome with
an elaborate array of repeat sequences near the centromere of the
chromosome. The existence of so much repetitive DNA information
could help explain how this chromosome rearranges or reshuffles its
DNA, leading to human disorders such as DiGeorge syndrome,
which includes a form of mental retardation, and how chromosome
structure changes over time.
- An unexpected finding shows several regions where recombination
is increased, and others where it is suppressed, and these will
probably play a role in health and disease.
Comparing the chromosome 22 sequence to known gene sequences of
the mouse, a lab animal frequently used to facilitate understanding of
human genetic disorders, the research team found 160 human genes that
have comparable sequences in the mouse. Examining the chromosomal
locations of the mouse genes that have counterparts on the human
chromosome 22 shows that the order of the genes along the chromosome
in the two species is genetically conserved, although the mouse homologs
of human genes on chromosome 22 are dispersed to eight different
mouse chromosomal regions.
The sequencing of the DNA of chromosome 22 was conducted as part of
the international Human Genome Project, which involves scientists in the
U.S., England, Japan, France, Germany and China.
In deciphering chromosome 22, scientists used the approach that has
been developed and widely tested by the Human Genome Project. This
approach involves sequencing overlapping cloned segments of DNA from
known locations on the chromosome.
Until now, scientists were uncertain about whether an entire human
chromosome could be sequenced in this manner. For example, they did
not know whether insurmountable problems would prevent assembling
their sequencing data. The presence of a small number of unclonable
gaps was not unexpected, but the scientists carrying out this project
adhered to the agreed upon standard that a chromosome should not be
considered "essentially complete," until the sequence of regions that are
clonable and sequenceable with current technology have been determined
to high accuracy, and the sizes of any remaining gaps have been
"That chromosome 22 was essentially sequenced by using overlapping
clones increases our confidence that the Human Genome Project will be
able to complete a 'working draft' of the DNA sequence of the human
genome in Spring 2000 and finish it by 2003," said Dr. Richard Wilson,
co-director of the Genome Sequencing Center at Washington University
School of Medicine in St. Louis and member of the research team that
deciphered chromosome 22.
The results of the Human Genome Project, which are freely accessible
through public databases such as GenBank
(www.ncbi.nlm.nih.gov/genome/seq), give scientists insight into the way
genes are arranged along a strip of DNA and paves the way for major
advances in the diagnosis and treatment of disease.
Knowing the identity and order of the chemical components of the DNA of
the 23 pairs of chromosomes that are found in almost every human cell
provides a tool to determine the basis of health and disease. "The fact that
all of this information is now freely available for scientists to use, without
the constraints of patents and fees, is of major importance, if the
knowledge of our genetic make-up is to be used for the good of mankind,"
said Dr. Michael Morgan, chief executive of the Wellcome Trust Genome
Campus, which is home to the Sanger Centre.