GENETICS

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Genetics

I

 

INTRODUCTION

DNA Strands

Nucleic acids are complex molecules produced by living cells and are essential to all living organisms. These acids govern the body’s development and specific characteristics by providing hereditary information and triggering the production of proteins within the body. This computer-generated model shows two strands of deoxyribonucleic acid (DNA) and the double-helical structure typical of this class of nucleic acids.

Ken Eward/Photo Researchers, Inc.

Genetics, study of the function and behavior of genes. Genes are bits of biochemical instructions found inside the cells of every organism from bacteria to humans. Offspring receive a mixture of genetic information from both parents. This process contributes to the great variation of traits that we see in nature, such as the color of a flower’s petals, the markings on a butterfly’s wings, or such human behavioral traits as personality or musical talent. Geneticists seek to understand how the information encoded in genes is used and controlled by cells and how it is transmitted from one generation to the next. Geneticists also study how tiny variations in genes can disrupt an organism’s development or cause disease. Increasingly, modern genetics involves genetic engineering, a technique used by scientists to manipulate genes. Genetic engineering has produced many advances in medicine and industry, but the potential for abuse of this technique has also presented society with many ethical and legal controversies.

Genetic information is encoded and transmitted from generation to generation in deoxyribonucleic acid (DNA). DNA is a coiled molecule organized into structures called chromosomes within cells. Segments along the length of a DNA molecule form genes. Genes direct the synthesis of proteins, the molecular laborers that carry out all life-supporting activities in the cell. Although all humans share the same set of genes, individuals can inherit different forms of a given gene, making each person genetically unique.

Chromosome

This photomicrograph shows a specialized type of giant chromosomes called polytene chromosomes. Polytene chromosomes occur in many species of two-winged flies. They are formed when the strands of DNA within normal chromosomes undergo numerous rounds of replication without separating from one another.

London Scientific Films/Oxford Scientific Films

Since the earliest days of plant and animal domestication, around 10,000 years ago, humans have understood that characteristic traits of parents could be transmitted to their offspring. The first to speculate about how this process worked were Greek scholars around the 4th century bc, who promoted theories based on conjecture or superstition. Some of these theories remained in favor for several centuries. The scientific study of genetics did not begin until the late 19th century. In experiments with garden peas, Austrian monk Gregor Mendel described the patterns of inheritance, observing that traits were inherited as separate units. These units are now known as genes. Mendel’s work formed the foundation for later scientific achievements that heralded the era of modern genetics.

II

 

THE IMPORTANCE OF GENETICS

Gene Therapy

Gene therapy may one day be used to treat, cure, or prevent a variety of genetic disorders. In gene therapy, a type of virus known as a retrovirus is used to replace a defective gene in cells with a new, healthy gene. Scientists insert the healthy gene into the ribonucleic acid (RNA) of the retroviruses. These retroviruses are mixed with cells taken from a patient and cultured in a laboratory. The retroviruses insert the healthy gene in to the deoxyribonucleic acid (DNA) of the cells. The cells with the new, healthy gene are then injected back into the patient.

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The modern science of genetics influences many aspects of daily life, from the food we eat to how we identify criminals or treat diseases. In agriculture, genetic advances enable scientists to alter a plant or animal to make it more useful. For instance, some food crops, such as oranges, potatoes, wheat, and rice, have been genetically altered to withstand insect pests, resulting in a higher crop yield. Tomatoes and apples have been modified so that they resist discoloration or bruising on their way to market, enhancing their appeal on supermarket shelves. The genetic makeup of cows has been modified to increase their milk production, and cattle raised for beef have been altered so that they grow faster.

Genetic technologies have also helped convict criminals. DNA recovered from semen, blood, skin cells, or hair found at a crime scene can be analyzed in a laboratory and compared with the DNA of a suspect. An individual’s DNA is as unique as a set of fingerprints, and a DNA match can be used in a courtroom as evidence connecting a person to a crime.

Genetics has revolutionized the way industries produce certain substances, many of which formerly required costly and arduous manufacturing methods. In medicine, scientists can genetically alter bacteria so that they mass-produce specific proteins, such as insulin used by people with diabetes mellitus or human growth hormone used by children who suffer from growth disorders.

DNA Fingerprinting

DNA fingerprinting can help investigators identify the suspect in a crime. The horizontal pattern of lines represents a person's genetic makeup. In the sample shown, suspect S2 matches the evidence, blood sample E(vs).

Leonard Lessin/Peter Arnold, Inc.

In other medical applications, genetic technologies have been instrumental in the development of gene therapy. In this still-experimental form of treatment, scientists try to cure disease by replacing malfunctioning genes with healthy ones. Gene therapy has shown promise in treating some devastating conditions, including some forms of cancer and cystic fibrosis. Genetically engineered vaccines are being tested for possible use against the human immunodeficiency virus (HIV), the virus that causes acquired immunodeficiency syndrome (AIDS).

The field of human genetics has been energized in recent years by the Human Genome Project, an international collaboration of scientists, governments, and drug companies from around the world. Scientists working on this project have developed detailed maps that identify the chromosomal locations of the estimated 20,000 to 25,000 human genes. The vast databases emerging from the project help scientists study previously unknown genes as well as many genes all at once to examine how gene activity can cause disease. Scientists expect that the project will lead to the development of new drugs targeted to specific genetic disorders.

Despite the benefits derived from genetic advancements, some observers have voiced concerns that genetically engineered organisms could harm people or the environment. Others fear that new genetic technologies may enable scientists to modify genes that affect characteristics other than those responsible for disease. They warn that determining who has undesirable genetic characteristics may lead to discriminatory practices. Others are concerned about the common misperception that a person’s genes determine all aspects of a person’s life, including health and behavior. This misperception leads people to blame their genetic makeup for problems, leaving no room for the influence of free will, personal responsibility, or hope for change. These and other challenging issues place geneticists at the crossroads of science and social responsibility, where they work to promote understanding of genetic advances and prevent the abuse of them.

III

 

PRINCIPLES OF GENETICS

Fruit Fly Chromosomes

The chromosomes of the fruit fly, Drosophila melanogaster, lend themselves well to genetic experimentation. There are only 4 pairs—one of which, marked here with Xs and Ys, determines the fly’s sex—versus the human complement of 23 pairs. In addition, the fly chromosomes themselves are large. Thomas Hunt Morgan and his associates based their theory of heredity on studies using Drosophila. They found that chromosomes were passed from parent to offspring in a way that Gregor Mendel ascribed to genes. They proposed, correctly, that genes in fact occupy specific physical locations on chromosomes.

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The site where genes work is the cell. Some organisms, such as paramecia or amoebas, are made up of a single cell. Other organisms are made of many kinds of cells, each having a different function. For instance, a tree contains some cells that form the root system and other cells that form leaves. Each cell’s function within an organism is determined by the genetic information encoded in DNA.

In animals, plants, and other eukaryotes (organisms whose cells contain a nucleus), DNA resides within membrane-bound structures in the cell. These structures include the nucleus, the energy-producing mitochondria, and, in plants, the chloroplasts (structures where photosynthesis takes place). In prokaryotes, one-celled organisms and bacteria that lack internal membrane-bound structures, DNA floats freely within the cell body.

A

 

Cell Division and Reproduction

Animal Cell

An animal cell typically contains several types of membrane-bound organs, or organelles. The nucleus directs activities of the cell and carries genetic information from generation to generation. The mitochondria generate energy for the cell. Proteins are manufactured by ribosomes, which are bound to the rough endoplasmic reticulum or float free in the cytoplasm. The Golgi apparatus modifies, packages, and distributes proteins while lysosomes store enzymes for digesting food. The entire cell is wrapped in a lipid membrane that selectively permits materials to pass in and out of the cytoplasm.

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Organisms could not grow or function properly if the genetic information encoded in DNA was not passed from cell to cell. DNA is packaged into structures called chromosomes within a cell. Every chromosome in a cell contains many genes, and each gene is located at a particular site, or locus, on the chromosome. Chromosomes vary in size and shape and usually occur in matched pairs called homologues. The number of homologous chromosomes in a cell depends upon the organism—for example, most cells in the human body contain 23 pairs of chromosomes, while most cells of the fruit fly Drosophila contain 4 pairs.

Within all organisms, cells divide to produce new cells, each of which requires the genetic information found in DNA. Yet simply splitting the DNA of a dividing cell between two new cells would lead to disaster—the two new cells would have different instructions and each subsequent generation of cells would have less and less genetic information to work with. Imagine how chaotic it would be to rip an architectural blueprint in two, give each half to different contractors, and tell them to construct identical buildings. Just as each contractor would require a full copy of the blueprint to construct a complete building, each new cell needs a complete copy of an organism’s genetic information to function properly.

Organisms use two types of cell division to ensure that DNA is passed down from cell to cell during reproduction. Simple one-celled organisms and other organisms that reproduce asexually—that is, without the joining of cells from two different organisms—reproduce by a process called mitosis. During mitosis a cell doubles its DNA before dividing into two cells and distributing the DNA evenly to each resulting cell. Organisms that reproduce sexually use a different type of cell division. These organisms produce special cells called gametes, or egg and sperm. In the cell division known as meiosis, the chromosomes in a gamete cell are reduced by half. During sexual reproduction, an egg and sperm unite to form a zygote, in which the full number of chromosomes is restored.