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6.9: The Human Genome

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    22481
  • Vitruvian Man

    The drawing in Figure \(\PageIndex{2}\), named Vitruvian Man, was created by Leonardo da Vinci in 1490. It was meant to show normal human body proportions. Vitruvian Man is used today to represent a different approach to the human body. It symbolizes a scientific research project that began in 1990, exactly 500 years after da Vinci created the drawing. That project, named the Human Genome Project, is the largest collaborative biological research project ever undertaken.

    Vitruvian_man.png
    Figure \(\PageIndex{1}\): Image used with permission (Public Domain; Leonardo da Vinci via Wikimedia Commons).

    What Is the Human Genome?

    The human genome refers to all the DNA of the human species. Human DNA consists of 3.3 billion base pairs and is divided into more than 20,000 genes on 23 pairs of chromosomes. The human genome also includes noncoding sequences (e.g. intergenic region) of DNA, as shown in Figure \(\PageIndex{2}\).

    2390098-1499276671-8-97-1024px-Human_genome_to_genes-it.png
    Figure \(\PageIndex{2}\): Human Genome, Chromosomes, and Genes. Each chromosome of the human genome contains many genes as well as noncoding intergenic (between genes) regions. Each pair of chromosomes is shown here in a different color. (CC BY 3.0; original author by LoStrangolatore and modified by Mandeep Grewal).

    Discovering the Human Genome

    Scientists now know the sequence of all the DNA base pairs in the entire human genome. This knowledge was attained by the Human Genome Project (HGP), a $3 billion, an international scientific research project that was formally launched in 1990. The project was completed in 2003, two years ahead of its 15-year projected deadline.

    Determining the sequence of the billions of base pairs that make up human DNA was the main goal of the HGP. Another goal was mapping the location and determining the function of all the genes in the human genome. A somewhat surprising finding of the HGP is the relatively small number of human genes. There are only about 20,500 genes in human beings. This may sound like a lot, but it's about the same number as in mice. Another surprising finding of the HGP is a large number of nearly identical, repeated DNA segments in the human genome. This number was previously suspected to be much smaller.

    Human_Genome_Project_Timeline_(26964377742).jpg
    Figure \(\PageIndex{3}\): Timeline of the human genome project. Full Version is visible at the NIH site. (CC BY 2.0; National Human Genome Research Institute (NHGRI) via Wikimedia Commons).

    A Collaborative Effort

    Funding for the HGP came from the U.S. Department of Energy and the National Institutes of Health as well as from foreign institutions. The actual research was undertaken by scientists in 20 universities in the U.S., United Kingdom, Australia, France, Germany, Japan, and China. A private U.S. company named Celera also contributed to the effort. Although Celera had hoped to patent some of the genes it discovered, this was later denied. The entire DNA sequence of the genome is stored in databases that are available to anyone on the Internet. Additional data and tools for analyzing the human genome are also available online.

    Reference Genome of the Human Genome Project

    In 2003, the HGP published the results of its sequencing of DNA as a human reference genome. Figure \(\PageIndex{4}\) illustrates the process of DNA sequencing. The sequence of the human DNA is stored in databases available to anyone on the Internet. The U.S. National Center for Biotechnology Information (NCBI), part of the NIH, as well as comparable organizations in Europe and Japan, maintain the genomic sequences in a database known as Genbank. Protein sequences are also maintained in this database. The sequences in these databases are the combined sequences of anonymous donors, and as such do not yet address the individual differences that make us unique. However, the known sequence does lay the foundation to identify the unique differences among all of us. Most of the currently identified variations among individuals will be single nucleotide polymorphisms or SNPs. An SNP (pronounced "snip") is a DNA sequence variation occurring at a single nucleotide in the genome. For example, two sequenced DNA fragments from different individuals, GGATCTA to GGATTTA, contain a difference in a single nucleotide. If this, base change occurs in a gene; the base change then results in two alleles: the C allele and the T allele. Remember an allele is an alternative form of a gene. Almost all common SNPs have only two alleles. The effect of these SNPs on protein structure and function, and any effect on the resulting phenotype, is an extensive field of study.

    2390098-1500239216-26-11-Sanger-sequencing.svg.png
    Figure \(\PageIndex{4}\): The Sanger (chain-termination) method for DNA sequencing. (1) A primer is annealed to a sequence, (2) Reagents are added to the primer and template, including: DNA polymerase, dNTPs, and a small amount of all four dideoxynucleotides (ddNTPs) labeled with fluorophores. During primer elongation, the random insertion of a ddNTP instead of a dNTP terminates synthesis of the chain because DNA polymerase cannot react with the missing hydroxyl. This produces all possible lengths of chains. (3) The products are separated on a single lane capillary gel, where the resulting bands are read by a imaging system. (4) This produces several hundred thousand nucleotides a day, data which require storage and subsequent computational analysis (CC BY-SA 3.0; Estevezj via Wikimedia Commons).

    Benefits of the Human Genome Project

    The sequencing of the human genome holds benefits for many fields, including molecular medicine and human evolution.

    • Knowing the human DNA sequence can help us understand many human diseases. For example, it is helping researchers identify mutations linked to different forms of cancer. It is also yielding insights into the genetic basis of cystic fibrosis, liver diseases, blood-clotting disorders, and Alzheimer's disease, among others.
    • The human DNA sequence can also help researchers tailor medications to individual genotypes. This is called personalized medicine, and it has led to an entirely new field called pharmacogenomics. Pharmacogenomics, also called pharmacogenetics, is the study of how our genes affect the way we respond to drugs. You can read more about pharmacogenomics in the Feature below.
    • The analysis of similarities between DNA sequences from different organisms is opening new avenues in the study of evolution. For example, analyses are expected to shed light on many questions about the similarities and differences between humans and our closest relatives the nonhuman primates.

    Ethical, Legal, and Social Issues of the Human Genome Project

    From its launch in 1990, the HGP proactively established and funded a separate committee to oversee potential ethical, legal, and social issues associated with the project. A major concern was the possible use of the knowledge generated by the project to discriminate against people. One issue was the fear that employers and health insurance companies would refuse to hire or insure people based on their genetic makeup, for instance, if they had genes that increased their risk of getting certain diseases. In response, in 1996, the U.S. passed the Health Insurance Portability and Accountability Act (HIPAA). It protects against unauthorized, nonconsensual release of individually identifiable health information to any entity not actively engaged in providing healthcare to a patient. This was followed in 2008 by the Genetic Information Nondiscrimination Act (GINA), which specifically prohibits genetic discrimination by health insurance companies and workplaces.

    Summary

    • The human genome refers to all of the DNA of the human species. It consists of more than 3.3 billion base pairs divided into 20,500 genes on 23 pairs of chromosomes.
    • The Human Genome Project (HGP) was a multi-billion dollar international research project that began in 1990. By 2003, it had sequenced all of the DNA base pairs in the human genome. It also mapped the location and determined the function of all the genes in the human genome.
    • In 2003, the HGP published the results of its sequence of DNA as a human reference genome. The entire DNA sequence is stored in databases that are available to anyone on the Internet.
    • Sequencing of the human genome is helping researchers better understand cancer and genetic diseases. It is also helping them tailor medications to individual patients, which is the focus of the new field of pharmacogenomics. In addition, it is helping researchers better understand human evolution.
    • From its launch in 1990, the HGP established and funded a separate committee to oversee potential ethical, legal, and social issues associated with the project.

    Review

    1. Describe the human genome.
    2. What is the Human Genome Project?
    3. Identify two main goals of the Human Genome Project.
    4. What is the reference genome of the Human Genome Project? What is it based on?
    5. Explain how knowing the sequence of DNA bases in the human genome is beneficial for molecular medicine.
    6. What was one surprising finding of the Human Genome Project?
    7. Why do you think scientists didn’t just sequence the DNA from a single person for the Human Genome Project? Along those lines, why do you think it is important to include samples from different ethnic groups and genders in genome sequencing efforts?
    8. True or False. The sequenced human genome does not include noncoding regions — it only includes actual genes.
    9. True or False. Knowing the sequence of the human genome can give insight into human evolution.
    10. a. What is pharmacogenomics?

      b. If a patient were to have pharmacogenomics done to optimize their medication, what do you think the first step would be?

      c. List one advantage and one disadvantage of pharmacogenomics.

    11. There are approximately 20,000 human ________ .

      A. base pairs

      B. nucleotides

      C. alleles

      D. genes

    12. Explain how the sequencing of the human genome relates to ethical concerns about genetic discrimination.

    Explore More

    For years, scientists have had the challenge of sequencing the human genome. Learn more about the human genome project here:

    Check out this video to explore the moral and ethical grey areas of genetic editing: