Numbers

Twenty-Three Pairs of Human Chromosomes

How It's Used

“The strands of DNA, spread across the 23 chromosomes in the nucleus of every human cell, comprise four chemical units, or letters, used over and over in varying chemical sequences. These replicated letters total three billion or so and form the words, or genes, that instruct cells to manufacture the proteins that carry out all the functions of human life. It is the very slight variations in the makeup of these genes that account for biological distinctions existing between individuals and that are responsible for much of the differences been sickness and health.

“The new map is expected to vastly speed identification of the tens of thousands of genes, eventually providing scientists with a clear picture of the human genome, the sum total of all human genes. In the near future, gains from this knowledge are expected to include much-earlier diagnosis of life-threatening disease, new ways to prevent illness and drug therapies tailored to treat a person's unique genetic profile, thereby ushering in a new era of medicine.”

“In 2001, a team from Harvard and two Boston hospitals identified a region on Chromosome 4—one of 23 chromosomes that make up the human genetic blueprint—that is likely to contain a gene or genes associated with extraordinary life expectancy. Subsequent studies led to the discovery that a gene called microsomal transfer protein (MTP) was responsible for the link between the chromosomal region and longevity. This gene plays an important role in cholesterol transport and one variation of the gene, common among the centenarians in the study, decreases one's risk for cardiovascular disease.”

“Coiled inside the body's cells, DNA is the chemical chain that encodes the instructions to build and operate a human in two sets of 23 chromosomes—one set passed down from each parent. The first two maps of the human genome, published by an international government-funded consortium and a private company in 2001, were based on a patchwork of DNA from several donors. Both versions were also half maps, decoding only one set of the 23 chromosomes on the assumption the two sets would hardly differ. Those maps suggested that humans were 99.9 per cent genetically identical, with only one one-thousandth of DNA information accounting for all the vibrant variety of humanity.”

“You are the current owner of one of the great wonders of the natural world: a large genome, copied almost perfectly from your parents to you, without which you would not be here. And when I say large, I mean large—3,400,000,000 large to be more accurate. This, give or take a bit, is the size of your genome measured in the number of nucleotides, the Lego-like building blocks of DNA. Buried somewhere in this DNA are genes that code for proteins, about 25,000 of them, spread out over your 23 chromosomes.”

“The Y chromosome makes its owner male because it carries the male-determining gene. Boys are born with one Y and one X chromosome in all their body’s cells, while girls have two X’s. The other 22 pairs of chromosomes in which the human genome is packaged are the same in both sexes.”

Links