There are approximately three billion base pairs in a single copy of the human genome. Obtaining a complete catalog of our genes was the focus of the Human Genome Project, which announced a final reference sequence for the human genome in April 2003 (D.N.A. Box 2.1). The information from the Human Genome Project will benefit medical science as well as forensic human identity testing and help us better understand our genetic makeup.
Within human cells, DNA found in the nucleus of the cell (nuclear DNA) is divided into chromosomes, which are dense packets of DNA and protection proteins called histones. The human genome consists of 22 matched pairs of autosomal chromosomes and two sex determining chromosomes (Figure 2.3). Thus, normal human cells contain 46 different chromosomes or 23 pairs of chromosomes. Males are designated XY because they contain a single copy of the X chromosome and a single copy of the Y chromosome while females
Molecular biology's equivalent to NASA's Apollo Space Program began in 1990 when a multi-billion, 15-year project was launched to decipher the DNA sequence contained inside a human cell. The Human Genome Project began under the leadership of James Watson, the scientist who with Francis Crick first determined the double-helix structure of DNA in 1953. With joint funding from the U.S. National Institutes of Health and the Department of Energy, efforts in the United States began with examining genetic and physical maps of human DNA and other model organisms such as yeast, Drosophila (fruit fly) and the mouse.
In 1992, Francis Collins took over the helm of the Human Genome Project. Amazingly over the years the project met or exceeded its milestones and stayed under budget. In 1999, a private sector enterprise named Celera under the leadership of Craig Venter challenged the public sector to a sequencing duel. The competition in large measure drove the Human Genome Project forward leading to the announcement of a draft sequence in June 2000, its publication in February 2001, and a 'final' sequence in April 2003.
The medical community will likely be the largest beneficiaries of the Human Genome Project as we come to better understand the genetic basis for various diseases. This information raises legal and ethical issues as scientists and policy makers struggle with genetic privacy concerns and intellectual property rights. Undertaking such an enormous project has accelerated technology development and will continue to aid in the understanding of our species. Now that a reference sequence is in place, efforts have turned to understanding normal variation that occurs among different individuals in the International Haplotype Mapping ('HapMap') Project.
D.N.A. Box 2.1 The Human Genome Project contain two copies of the X chromosomes and are designated XX. Most human identity testing is performed using markers on the autosomal chromosomes, and gender determination is done with markers on the sex chromosomes. As will be discussed in Chapters 9 and 10, the Y chromosome and mitochondrial DNA, a small, multi-copy genome located in cell's mitochondria, can also be used in human identification applications.
Chromosomes in all body (somatic) cells are in a diploid state; they contain two sets of each chromosome. On the other hand, gametes (sperm or egg) are in a haploid state; they have only a single set of chromosomes. When an egg cell and a sperm cell combine during conception, the resulting zygote becomes diploid again. Thus, one chromosome in each chromosomal pair is derived from each parent at the time of conception.
Mitosis is the process of nuclear division in somatic cells that produces daughter cells, which are genetically identical to each other and to the parent cell.
Figure 2.3 The human genome contained in every cell consists of 23 pairs of chromosomes and a small circular genome known as mitochondrial DNA. Chromosomes 1—22 are numbered according to their relative size and occur in single copy pairs within a cell's nucleus with one copy being inherited from one's mother and the other copy coming from one's father. Sex-chromosomes are either X,Y for males or X,X for females. Mitochondrial DNA is inherited only from one's mother and is located in the mitochondria with hundreds of copies per cell. Together the nuclear DNA material amounts to over three billion base pairs (bp) while mitochondrial DNA is only about 16 569 bp in length.
Meiosis is the process of cell division in sex cells or gametes. In meiosis, two consecutive cell divisions result in four rather than two daughter cells, each with a haploid set of chromosomes.
The DNA material in chromosomes is composed of 'coding' and 'non-coding' regions. The coding regions are known as genes and contain the information necessary for a cell to make proteins. A gene usually ranges from a few thousand to tens of thousands of base pairs in size. One of the big surprises to come out of the Human Genome Project is that humans have less than 30 000 protein-coding genes rather than the 50 000-100 000 previously thought.
Genes consist of exons (protein-coding portions) and introns (the intervening sequences). Genes only make up ~5% of human genomic DNA. Non-protein coding regions of DNA make up the rest of our chromosomal material. Because these regions are not related directly to making proteins they are sometimes referred to as 'junk' DNA. Markers used for human identity testing are found in the non-coding regions either between genes or within genes (i.e., introns) and thus do not code for genetic variation.
Polymorphic (variable) markers that differ among individuals can be found throughout the non-coding regions of the human genome. The chromosomal
Human Genome 23 Pairs of Chromosomes + mtDNA
Located in cell nucleus
Autosomes 2 copies per cell
Nuclear DNA 3.2 billion bp
Located in mitochondria (multiple copies in cell cytoplasm)
100s of copies per cell position or location of a gene or a DNA marker in a non-coding region is commonly referred to as a locus (plural: loci). Thousands of loci have been characterized and mapped to particular regions of human chromosomes through the worldwide efforts of the Human Genome Project.
Pairs of chromosomes are described as homologous because they are the same size and contain the same genetic structure. A copy of each gene resides at the same position (locus) on each chromosome of the homologous pair. One chromosome in each pair is inherited from an individual's mother and the other from his or her father. The DNA sequence for each chromosome in the homologous pair may or may not be identical since mutations may have occurred over time.
The alternative possibilities for a gene or genetic locus are termed alleles. If the two alleles at a genetic locus on homologous chromosomes are different they are termed heterozygous and if the alleles are identical at a particular locus, they are termed homozygous. Detectable differences in alleles at corresponding loci are essential to human identity testing.
A genotype is a characterization of the alleles present at a genetic locus. If there are two alleles at a locus, A and a, then there are three possible genotypes: AA, Aa, and aa. The AA and aa genotypes are homozygous while the Aa genotype is heterozygous. A DNA profile is the combination of genotypes obtained for multiple loci. DNA typing or DNA profiling is the process of determining the genotype present at specific locations along the DNA molecule. Multiple loci are typically examined in human identity testing to reduce the possibility of a random match between unrelated individuals.
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