Human genome

The human genome refers to the complete set of genetic "blueprints" encoded in DNA that the human body uses to build and maintain its cells. These instructions are made up of more than three billion pairs of DNA strands found in each nucleus of the body's more than thirty trillion cells. Human cells decode the DNA sequence to create genes that oversee numerous tasks from creating specific proteins to determining a person's hair and eye color. The genome of each human is unique; no two people on Earth share the exact genetic sequence. In 1990, scientists from around the world began a project to map the DNA sequence of the human genome and identify the genes contained within the body's cells. The project was completed in 2003, opening up new avenues of scientific study and leading to possible breakthroughs in medicine and the treatment of disease.

Background

The field of genetics began in the mid-nineteenth century with the work of an Austrian monk named Gregor Mendel. In experiments performed on pea plants, Mendel noticed that certain physical traits were passed along from parent to offspring. For example, when green pea plants were crossed with yellow plants, the resulting offspring were always green. However, when these green offspring were interbred, some of their offspring were yellow. Mendel theorized that some aspect of heredity was being transmitted through several generations of pea plants. His work was published but was largely ignored by the scientific community until the early twentieth century. In 1906, botanist William Bateson named the science genetics, from the Greek word genesis, or "origin."

In the 1860s, Swiss chemist Johann Friedrich Miescher discovered a substance in white blood cells that he called nuclein because of his belief that it originated from the cellular nucleus. About a decade later, German chemist Albrecht Kossel classified nuclein as a nucleic acid, a complex organic compound linked together in a chain-like structure. Kossel isolated the components of the substance and gave it a name—deoxyribonucleic acid (DNA). At first, scientists did not equate these discoveries with the concept of genetic inheritance, but that changed when Mendel's work was rediscovered.

In 1902, two scientists, American Walter Sutton and German Theodor Boveri, independently discovered that genetic characteristics were transferred from parent to offspring through chromosomes, threadlike structures found in the cellular nucleus. In the 1940s, scientists discovered that DNA was the primary building block of chromosomes, ending a long-standing debate on the basic engine of inheritance. The structure and function of DNA was first observed in 1953 by British scientists James Watson and Francis Crick. Watson and Crick discovered that DNA was a double helix, a shape like a twisted spiral staircase. It replicated itself by unwinding its individual strand into two sections, each of which formed a new double helix of DNA.

Overview

The genetic instructions that give the human body its unique traits and allow it to function are included in the DNA molecule. Each molecule is made of two connected, twisted strands. Each of these strands is comprised of four chemical units called nucleotide bases—adenine (A), thymine (T), guanine (G), and cytosine (C). The bases on opposite sides of a DNA strand always pair with a specific counterpart on the other. For instance, adenine always pairs with thymine, and guanine always pairs with cytosine. The combination, or sequence, of these base pairs of nucleotides is the genetic code that acts as the body's instruction manual. The human genome has more than three billion combinations of base pairs in its DNA. The different nucleotides are held together by hydrogen bonds, a connection formed between particles at the atomic level.

The DNA strands are tightly wound together in chromosomes found in the cellular nucleus. All the cells in the body contain the same set of encoded DNA instructions in twenty-three pairs of chromosomes. The body "reads" the base-pair combinations from sections of DNA to create genes. Genes carry specific instructions on how to make proteins, organic molecules that perform many functions in the body. For example, a gene called OCA2 is responsible for making a protein that helps cells produce the pigment melanin in the iris of the eye. People with more melanin in their irises have brown eyes, while those with lesser amounts have blue or green eyes. The human genome consists of about thirty thousand specific genes, each capable of making an average of three proteins. Each chromosome contains hundreds of thousands of genes. If a DNA strand is like a library, the genes would be the books.

In the 1980s, the National Institutes of Health and the United States Department of Energy began coordinating a project to identify and categorize all the genes in the human body. The Human Genome Project (HGP) officially began in 1990 and included researchers from the United States, the United Kingdom, France, Germany, Japan, and China. Originally planned as a fifteen-year project, the HGP finished two years ahead of schedule at a cost of about $2.7 billion. The project was not only able to estimate the number of genes in the body but was also able determine the exact order of the base pairs in a molecule of DNA.

As a result of the HGP, researchers have discovered more than 1,800 genes that can lead to specific diseases and diagnose possible inherited conditions in a matter of days. Because no two human genomes are exactly alike, doctors can examine a person's genetic makeup to determine if a targeted treatment would be more effective. The HGP has also led to advances in genetic testing that have proven useful for determining risk factors for a disease or aiding law enforcement in gathering DNA evidence. As a follow-up to the HGP, scientists have proposed creating a synthetic human genome. The controversial proposal would not create a new human or a new form of life. Proponents say it could be used to reduce the considerable costs of sequencing human DNA and help develop cells resistant to radiation, viral infections, and cancer.

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