98.5 percent pure chimp

Phi Kappa Phi Forum, Summer 2003 by Tiffany-Castiglioni, Evenlyn

Science & Technology

Evelyn Tiffany-Castiglioni

On April 13, 2003, the human genome project succeeded in providing an index for the atlas called the human genome, which comprises all the genetic material in each human cell. The project is the largest biology study in history. Having indexed this massive atlas and also those of the laboratory mouse, rat, and a few simpler species, scientists are intrepidly making plans for the long list of other similar atlases that lack an index. The chimpanzee is high on the list of priorities. Protein and DNA similarities identified during the past thirty years have shown that the genomes of humans (Homo sapiens) and the common chimpanzee (Pan troglodytes) are about 98.5 percent the same. With the technologies developed to carry out the human genome project, we can now focus on that 1.5 percent of difference. Why do we have such a deep interest in how genetically similar we are to chimpanzees, or rather how different we are from them? In scientific debate, the philosophical and evolutionary aspects of this question are curiously more urgent than the implications for human health and disease.

Before examining scientific motivations for mapping the chimp genome, let us consider what we have learned from mapping the human genome. The genome, which is the complete set of DNA in a species, has been studied at several levels of detail, from the equivalent of continents to neighborhood street maps. The continents are the chromosomes of a cell, large packages of DNA and protein that can be seen with a standard light microscope. Twenty-four different chromosomes exist in the human genome, and each cell of the body, except for the mature red blood cell (which has no nucleus) contains a genome. The chromosomes occur in pairs, including twenty-two duplicate pairs of autosomes and one pair of sex chromosomes - the X and Y. Females have two X chromosomes, and males have one X and one Y. Thus, each nucleated cell has two sets of the human genome, except sperm and ova, which each have a single set. Chromosomes are readily identifiable by species based on their numbers, shapes, and staining patterns. These chromosomes, or "continents," have areas densely populated with genes, like cities, as well as sparsely populated areas, akin to deserts.

The neighborhood street maps are the DNA sequence itself. The address of each house represents a coding unit, or codon, consisting of three chemical bases of DNA called nucleotides. Codons, as well as non-coding chains of nucleotides, are linearly arrayed to form strands of DNA, one strand running through each chromosome. The human genome has about 3.2 billion pairs of nucleotides. Yet there are only four different nucleotides, and these form an elegant, efficient code with some built-in redundancy. The redundancy comes from there being only twenty different essential amino acids to code for, but sixty-four different codons (4 x 4 x 4). Each codon specifies an amino acid or a stop signal that tells transcriptional machinery of the cell how to assemble a protein out of amino acids. Scientists have looked at short segments of DNA and determined the sequence in which the nucleotides are strung together for 99.9 percent of the human genome. A few gaps still need to be filled because they present unusual structures that the current technologies cannot readily unravel. Also missing from the human genome map is intermediate detail. For example, we do not yet have long strings of accurately sequenced DNA, so we do not know all gene orders and orientations. In effect, we are missing details about the borders of counties and countries.

The human genome project has yielded some surprising results. The biggest surprise is that there are only about 30,000 genes, rather than 80,000 to 100,000 as previously estimated. Genes are the basic units of heredity that code for proteins, such as enzymes, peptide hormones, receptors for signals, and structural proteins. A second finding is that genes make up only 2 percent of the human genome. The rest is apparently non-coding and may provide chromosomal structural integrity, as well as hold clues for evolution and gene-regulatory functions. The proteins for which more than half of these genes code are currently unknown. A third finding is that out of 3.2 billion pairs of nucleotides, each of us has 2-3 million that differ from those of any other human, except for an identical twin. Most of these differences have no biological significance because they do not occur in genes and therefore do not change protein structure. In short, the human genome is 99.9 percent the same in all people. This last finding is particularly interesting in comparison with the chimpanzee, because even though extant populations are small and geographically limited, partial sequence data indicate that the chimp has much greater genetic variation within its species than does the human.

Now let us look at the estimate that the DNA sequence of our genome differs only 1.5 percent from that of the chimp. That makes the chimp our closest living biological relative, closer than the Rhesus monkey, which differs from humans by 7 percent. For reference, the laboratory mouse (Mus musculus) differs from humans by 40 percent. Some scientists look at 1.5 percent and say that cannot be right. Indeed, the estimate may need some refinement. One team of investigators has made a reasonable, though not airtight case, that chimps and humans differ by as much as 5 percent, based on recently described insertions and deletions within selected long DNA sequences that were not detected by sequencing short segments of DNA.