CHAPTER 19 SOLUTIONS MANUAL QUESTIONS
AND CORRECT ANSWERS RATED +
QUESTION
What feature is commonly seen in the sequences recognized by type II restriction enzymes?
CORRECT ANSWER
The recognition sequences are palindromic and 4–8 base pairs long.
QUESTION
What normal role do restriction enzymes play in bacteria? How do bacteria protect their own
DNA from the action of restriction enzymes?
CORRECT ANSWER
Restriction enzymes cut foreign DNA, such as viral DNA, into fragments. Bacteria protect their
own DNA by modifying bases, usually by methylation, at the recognition sites.
QUESTION
Explain how gel electrophoresis is used to separate DNA fragments of different lengths.
CORRECT ANSWER
Gel electrophoresis uses an electric field to drive DNA molecules through a gel that acts as a
molecular sieve. DNA fragments are loaded into wells, and because DNA is negatively charged, it
migrates toward the positive electrode. Shorter DNA fragments move faster through the gel
matrix than longer fragments.
QUESTION
After DNA fragments have been separated by gel electrophoresis, how can they be visualized?
CORRECT ANSWER
DNA can be visualized by staining with fluorescent dyes such as ethidium bromide, which
fluoresce under ultraviolet light, or by attaching radioactive or chemical labels to the DNA
before electrophoresis.
, QUESTION
What is the purpose of Southern blotting? How is it carried out?
CORRECT ANSWER
Southern blotting is used to detect specific DNA fragments complementary to a labeled probe.
DNA is digested with restriction enzymes, separated by gel electrophoresis, denatured, and
transferred to a membrane. A labeled probe hybridizes to complementary DNA, which is then
detected.
QUESTION
Give three important characteristics of cloning vectors.
CORRECT ANSWER
1. An origin of replication
2. A selectable marker such as antibiotic resistance
3. One or more unique restriction sites for DNA insertion
QUESTION
How can an antibiotic-resistance gene and the lacZ gene be used to determine which cells
contain a particular plasmid?
CORRECT ANSWER
Cells are grown on antibiotic-containing media to select for plasmid uptake. Blue/white
screening using lacZ and X-gal distinguishes plasmids with inserts (white colonies) from those
without inserts (blue colonies).
QUESTION
Briefly explain how the polymerase chain reaction (PCR) is used to amplify a specific DNA
sequence.
CORRECT ANSWER
DNA is denatured, primers anneal to target sequences, and a thermostable DNA polymerase
extends the primers. Repeated cycles double the amount of target DNA.
AND CORRECT ANSWERS RATED +
QUESTION
What feature is commonly seen in the sequences recognized by type II restriction enzymes?
CORRECT ANSWER
The recognition sequences are palindromic and 4–8 base pairs long.
QUESTION
What normal role do restriction enzymes play in bacteria? How do bacteria protect their own
DNA from the action of restriction enzymes?
CORRECT ANSWER
Restriction enzymes cut foreign DNA, such as viral DNA, into fragments. Bacteria protect their
own DNA by modifying bases, usually by methylation, at the recognition sites.
QUESTION
Explain how gel electrophoresis is used to separate DNA fragments of different lengths.
CORRECT ANSWER
Gel electrophoresis uses an electric field to drive DNA molecules through a gel that acts as a
molecular sieve. DNA fragments are loaded into wells, and because DNA is negatively charged, it
migrates toward the positive electrode. Shorter DNA fragments move faster through the gel
matrix than longer fragments.
QUESTION
After DNA fragments have been separated by gel electrophoresis, how can they be visualized?
CORRECT ANSWER
DNA can be visualized by staining with fluorescent dyes such as ethidium bromide, which
fluoresce under ultraviolet light, or by attaching radioactive or chemical labels to the DNA
before electrophoresis.
, QUESTION
What is the purpose of Southern blotting? How is it carried out?
CORRECT ANSWER
Southern blotting is used to detect specific DNA fragments complementary to a labeled probe.
DNA is digested with restriction enzymes, separated by gel electrophoresis, denatured, and
transferred to a membrane. A labeled probe hybridizes to complementary DNA, which is then
detected.
QUESTION
Give three important characteristics of cloning vectors.
CORRECT ANSWER
1. An origin of replication
2. A selectable marker such as antibiotic resistance
3. One or more unique restriction sites for DNA insertion
QUESTION
How can an antibiotic-resistance gene and the lacZ gene be used to determine which cells
contain a particular plasmid?
CORRECT ANSWER
Cells are grown on antibiotic-containing media to select for plasmid uptake. Blue/white
screening using lacZ and X-gal distinguishes plasmids with inserts (white colonies) from those
without inserts (blue colonies).
QUESTION
Briefly explain how the polymerase chain reaction (PCR) is used to amplify a specific DNA
sequence.
CORRECT ANSWER
DNA is denatured, primers anneal to target sequences, and a thermostable DNA polymerase
extends the primers. Repeated cycles double the amount of target DNA.
