15.1 Selective Breeding

Selective Breeding

• Selective breeding is a method of breeding that allows only those organisms with desired characteristics to produce the next generation

  • Selective breeding takes advantage of naturally occurring genetic variation

  • Humans use selective breeding to pass wanted traits on to the next generation of organisms

• The two most common methods of selective breeding are hybridization and inbreeding

  • Hybridization is a breeding technique that involves crossing dissimilar individuals to bring together the best traits of both organisms

    • The offspring made by such crosses are called hybrids, and they are often hardier than either parent

  • Inbreeding is the continued breeding of individuals with similar characteristics to maintain the desired characteristics of a kind of organism

    • Although it may seem like a good way to keep certain characteristics, inbreeding increases the probability that organisms may inherit alleles that lead to genetic disorders

Increasing Variation

• When scientists change the genetic makeup of an organism, they are using biotechnology, the process of manipulating the organisms, cells, or molecules to produce specific products

• Breeders can increase the genetic variation in a population by introducing mutations, which are the ultimate source of biological diversity

  • Mutations are changes in DNA that parents can pass to offspring and they often happen on their own

  • Breeders can also make mutations happen using radiation and chemicals

    • Because they are small, millions of bacteria can be treated with radiation or chemicals at the same time to develop useful kinds of bacteria

    • Drugs that keep chromosomes from separating during meiosis have been very useful in plant breeding

      • Plants grown from these cells are called polyploid because they have many sets of chromosomes; polyploidy can quickly make new kinds of plants that are larger and stronger than their diploid relatives

15.2 Recombinant DNA

Copying DNA

• Scientists can now transfer genes for particular traits from one organism to another by taking DNA from cells, cutting them into pieces using restriction enzymes, and separating the pieces by size

• Biologist Douglas Prasher discovered a way to find genes; by making a piece of RNA that would match that DNA sequence, Prasher made the DNA “magnet” to find the GFP gene

• Once they find a gene, biologists often need to make many copies of it

  • Polymerase chain reaction is a technique that allows biologists to make many copies of a gene

    • The first step in using the polymerase chain reaction method to copy a gene is to heat a piece of DNA, which separates its two strands

    • Then, as the DNA cools, primers bind to the single strands

    • Next, DNA polymerase starts copying the region between the primers; these copies can serve as templates to make more copies

Changing DNA

• Changing genes for a practical purpose is called genetic engineering

• Recombinant-DNA technology—joining together DNA from two or more sources— makes it possible to change the genetic composition of living organisms

  • Recombinant DNA molecules are made up of DNA from different sources

  • Restriction enzymes cut DNA at specific sequences, making “sticky ends,” or, single-stranded bases of DNA at the end of a piece of DNA

  • DNA ligase allows two single-stranded pieces that are complementary to each other to stick together

• In addition to their own large chromosomes, some bacteria have small circular DNA molecules called plasmids located in the cytoplasm

• Plasmids are often used in recombinant DNA studies

  • Scientists insert a desired piece of DNA into a plasmid

  • Then they use transformation to add the recombinant plasmid to the bacteria

  • The bacteria will then copy the new DNA along with the rest of the cell’s genome

Transgenic Organisms

• Transgenic is a term used to refer to an organism that contains genes from other organisms

  • They can be made by putting recombinant DNA into the genome of a host

• Genetic engineers can make transgenic plants, animals, and microorganisms

  • Transgenic organisms can be produced by the insertion of recombinant DNA into the genome of a host organism

• A clone is a member of a population of genetically identical cells produced from a single cell

  • That individual, then, is genetically identical to the organism from which the cell was taken

  • Multicellular organisms are much more difficult to clone than bacteria and other microorganisms

  • However, in 1997, Scottish scientist Ian Wilmut announced that he had made a sheep, called Dolly, by cloning

15.3 Applications of Genetic Engineering

Agriculture and Industry

• Researchers have used genetic engineering to try to improve the products we get from plants and animals

• Since 1996, genetically modified (GM) plants have become an important part of our food supply

  • For example, one type of modification uses bacterial genes that make a protein called Bt toxin that is harmless to humans but kills insects who eat it

    • Plants with the Bt gene do not have to be sprayed with pesticides and make more food per acre than unmodified plants, too

• Transgenic animals are also becoming more important to our food supply

  • For example, many dairy farms now raise cows that have been injected with hormones made by recombinant-DNA techniques that help the cows make more milk

  • Pigs can be genetically modified so they make leaner meat and cleaner wastes

  • Using growth-hormone genes, scientists have made transgenic salmon that grow much more quickly than wild salmon, making the fish easier to farm

• Ideally, genetic modification could lead to better, less expensive, and more nutritious food as well as less harmful manufacturing processes

Health and Medicine

• Today, recombinant-DNA technology is the source of some of the most important and exciting advances in the prevention and treatment of disease

  • One interesting development in transgenic technology is golden rice, which contains increased amounts of provitamin A

    • Since a lack of provitamin A causes infant blindness and other problems, there is hope that provitamin A-rich golden rice will help prevent these problems

  • Some scientists are making transgenic plants and animals that make human antibodies to fight disease

    • Several laboratories have made transgenic sheep and pigs that make important human proteins in their milk that can be taken from the milk and used as medicines in people

• Transgenic animals are often used as test subjects in medical research

• Recombinant DNA can be used to make important proteins that can save human lives

  • Products now made in genetically engineered bacteria include insulin to treat diabetes, blood-clotting factors for hemophiliacs, and human growth hormone

• Gene therapy is a process of changing a gene to treat a medical disease or disorder; an absent or faulty gene is replaced by a normal working gene

  • Genes are added to human cells using a scientist-made virus that cannot reproduce and hurt people

  • Then they put DNA that has the working gene into their modified virus and infect the patient’s cells with the virus carrying the gene

  • With luck, the virus will insert the healthy gene into the cells that need it and correct the problem

• Genetic testing is now available for diagnosing hundreds of different disorders and is often used by couples to find out if they are carrying an allele for a genetic disorder

  • Genetic tests use a piece of DNA that is complementary to a defective allele to find that allele

  • Other genetic tests search for changes in cutting sites of restriction enzymes

• DNA microarray technology lets scientists learn the activity levels of thousands of genes at once

  • A DNA microarray is the glass slide or silicon chip that carries thousands of different kinds of single-stranded DNA fragments arranged in a grid

Personal Identification

• DNA fingerprinting is a tool used by biologists to determine whether two samples of genetic material are from the same person

  • DNA samples can be taken from blood or any other tissue

  • Then, restriction enzymes cut the DNA into pieces

  • Next, gel electrophoresis separates the pieces by size

  • A DNA probe finds the fragments that have highly variable regions; this is shown as a series of DNA bands on the gel

  • If enough enzymes and probes are used, the pattern of bands from one person is unlike anyone else’s pattern of bands

• Forensics is the scientific study of crime scene evidence

  • DNA fingerprinting has helped solve crimes, convict criminals, overturn guilty verdicts, and save more than 110 persons who were sentenced to death for crimes they didn’t commit

  • DNA forensics is used in wildlife conservation as well to stop the ivory trade

• DNA fingerprinting allows us to figure out who the biological father of a child is by making it easy to find alleles carried by the child that do not match those of the mother

15.4 Ethics and Impacts of Biotechnology

Profits and Privacy

• Biotechnology raises questions concerning privacy issues

  • Do you have exclusive rights to your DNA?

  • Should you, like patent holders, be able to keep your genetic information confidential?

• The Genetic Information Nondiscrimination Act was signed into law in 2008 and protects Americans against discrimination based on their genetic information

Safety of Transgenics

• GM crops are controversial, as people argue about the safety of using them

  • Careful studies of such foods have provided no scientific support for concerns about their safety, and it does seem that foods made from GM plants are safe to eat

  • Even if GM food itself presents no hazards, there are many serious concerns about the unintended consequences that a shift to GM farming and ranching may have on agriculture

Ethics of the New Biology

• In a democratic nation, everyone is responsible for making sure that the tools science has given us are used wisely

  • Just because we have the technology to modify an organism’s characteristics, are we justified in doing so?