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Unit 1:
Ch. 1
Ch. 2
Ch. 3
Interlude A
Unit 2:
Ch. 4
Ch. 5
Ch. 6
Ch. 7
Ch. 8
Ch. 9
Interlude B
Unit 3:
Ch. 10
Ch. 11
Ch. 12
Ch. 13
Ch. 14
Ch. 15
Interlude C
Unit 4:
Ch. 16
Ch. 17
Ch. 18
Ch. 19
Interlude D
Unit 5:
Ch. 20
Ch. 21
Ch. 22
Ch. 23
Ch. 24
Ch. 25
Ch. 26
Ch. 27
Ch. 28
Ch. 29
Ch. 30
Interlude E
Unit 6:
Ch. 31
Ch. 32
Interlude F
Unit 7:
Ch. 33
Ch. 34
Ch. 35
Ch. 36
Ch. 37
Ch. 38
Interlude G

» Getting Started » A Guide to the Reading » Tying it all together

Getting Started

Below are a few questions to consider prior to reading Chapter 15. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. How was Alba, the white rabbit, made to glow in the dark?
  2. What is the natural function of restriction enzymes?
  3. Why must DNA be heated during the course of a DNA hybridization experiment?
  4. What would you expect to find in a “DNA library”?
  5. What are the advantages of using the Polymerase Chain Reaction (PCR) for gene cloning?
  6. Why would DNA fingerprinting not be useful to distinguish between identical twins?
  7. What percentage of the world’s soybeans, corn, cotton, and canola crops were “genetically modified” in 2002?
  8. How did gene therapy help Ashanthi DeSilva to lead a normal life?

A Guide to the Reading

When exploring the content in Chapter 15 for the first time, the following concepts typically give students the most difficulty. For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas.

Restriction Enzymes and Gel Electrophoresis

Restriction enzymes are capable of breaking the covalent bonds which hold the long, individual strands of DNA together.  What makes restriction enzymes useful for the analysis of DNA is the specificity they display in selecting a region in which to cut the DNA molecule.  Each restriction enzyme has a “target sequence”, a specific sequence of nucleotides typically 10 base pairs or fewer in length, which must be present in order for the enzyme to be able to attach to the DNA molecule and “cut” the two strands.  This target sequence varies from enzyme to enzyme (see Figure 15.1).  Since identical copies of a DNA molecule (i.e. chromosome) would have the same set of target sequences, a restriction enzyme would cut these copies in the exact same location.  However, if the copies of DNA under analysis were different in their nucleotide sequences, a single restriction enzyme would recognize and cut only those DNA molecules with the correct target sequence, resulting in DNA fragments of differing sizes.  In a test tube, it would be impossible to visualize these DNA fragments.  Therefore, scientists have devised a way to separate these DNA fragments based on size using gel electrophoresis.  In gel electrophoresis, the DNA samples are loaded onto a slab of a Jell-o like matrix and subjected to an electrical charge.  The DNA fragments, which are negatively charged, are attracted to the positively charged end of the gel slab and slowly migrate in that direction.  Larger fragments move more slowly since they have a more difficult time making their way through the matrix, while smaller fragments move more rapidly.  Fragments of the same size tend to move as a group, accumulating in a region called a band.  By analyzing these separated bands, scientists can determine whether or not a sample of DNA contained one or more target sequences for a particular restriction enzyme.  This technique is particularly useful as a “DNA Fingerprinting” tool.

For more information on this concept, be sure to focus on:

  • In Section 15.1, Several key enzymes are used in DNA technology
  • In Section 15.1, Gel electrophoresis sorts DNA fragments by size
  • Figure 15.1, Restriction Enzymes Cut DNA at Specific Places
  • Figure 15.2, Gel Electrophoresis
  • Figure 15.3, Identifying the Sickle-Cell Allele with Restriction Enzymes and Gel Electrophoresis

Polymerase Chain Reaction

The Polymerase Chain Reaction (PCR) is a technique which utilizes the enzyme DNA polymerase in a test tube to produce millions of copies of a piece of DNA in a short period of time.  The key to understanding PCR is realizing that a portion of the sequence of DNA which a scientist wishes to copy (or amplify) must already be known.  This is because the DN polymerase enzyme is only capable of extending a DNA strand; it cannot build a DNA strand from scratch.  Therefore, PCR requires the use of primers (see Figure 15.8), which are short segments of DNA that match the two ends of the sequence of DNA targeted for copying.  In a typical PCR reaction, the two strands of template DNA are separated using heat.  By cooling the reaction a little, the primer DNA strands are allowed to bind to their complementary sequences located on the target strands.  DNA polymerase can then begin to “extend” the primer using the original DNA as a template.  This results in two identical copies of the target DNA.  This cycle is repeated 20-30 times, each time doubling the number of copies of the target DNA in the test tube, until billions of copies of the DNA sequence of interest are obtained.

For more information on this concept, be sure to focus on:

  • In Section 15.2, The polymerase chain reaction can clone DNA very rapidly
  • Figure 15.8, The Polymerase Chain Reaction


As presented in the chapter, cloning can mean several things.  In DNA cloning, a single copy of a gene is isolated and inserted into some type of vector for easy copying and subsequent transfer to another organism.  On a larger scale, the cloning of entire organisms is referred to as “reproductive cloning” while the cloning of tissues or organs is termed “therapeutic cloning.”  If you are familiar with the story of Dolly the sheep, you are aware of the process of reproductive cloning where an offspring that is an exact genetic duplicate of an existing individual is produced.  The process involves the transfer of a diploid nucleus isolated from the cell of an individual into an egg cell where the nucleus has been removed.  In this case, the imported diploid nucleus provides the entire genetic blueprint for the development of the egg into an embryo, a fetus, and ultimately, a newborn.  Since the DNA present in the newborn comes entirely from an existing individual, the newborn is genetically identical to the DNA donor and therefore represents a clone of this individual.  In therapeutic cloning, stem cells are harvested from the developing embryo.  In this case, these genetically identical stem cells may then be implanted or transferred to the DNA donor to treat a particular disorder, such as a degenerative neurological disease, or to restore the normal function of a tissue.  The key to both these types of cloning procedures is the fact that the resulting stem cells or organisms are all genetic duplicates of the original donor. 

For more information on this concept, be sure to focus on:

  • Section 15.2, Working with DNA: DNA Cloning
  • Chapter 15 - Science Toolkit, Human Cloning

Tying it all together

Several concepts presented in this chapter build upon concepts presented in previous chapters and are also revisited and discussed in greater detail in subsequent chapters, including:

DNA Structure and Replication

  • Chapter 12 – Section 12.3, How DNA Is Replicated

Single Gene Mutations and Disease

  • Chapter 13 – Section 13.6, The Effect of Mutations on Protein Synthesis

The Immune System

  • Chapter 28, Defense Against Disease

Stem Cells and Development

  • Chapter 29, Reproduction and Development

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