Download Answers to Mastering Concepts Questions

Answers to Mastering Concepts Questions
Chapter 7
7.1
1. Griffith’s research established that a then-unknown molecule in a lethal strain of bacteria could
transform nonlethal bacteria, making them able to kill mice. Avery and his colleagues added
enzymes that destroyed either proteins or DNA to the mixtures that Griffith used in his
experiments. In Avery’s experiments, mice died only from bacterial solutions mixed with enzymes
that destroyed proteins. Mice did not die from bacterial solutions mixed with enzymes that
destroyed DNA. These experiments showed that DNA, not protein, changed bacteria from
nonlethal to lethal.
2. The Hershey-Chase “blender experiments” used radioactive sulfur to label the protein coats of
one batch of bacteriophages and used radioactive phosphorus to label the DNA of another batch
of bacteriophages. Both batches of viruses were allowed to infect bacteria. Then the solutions
were separately blended at high speeds to separate viral protein coats from bacterial cells.
Radioactively labeled bacteria were only found in the batches that had been infected by phages
with radioactively labeled DNA. The protein-labeled phages did not transmit radioactivity to the
bacteria they had infected. These experiments confirmed that DNA, not protein, is the genetic
material.
7.2
1. The components of a DNA molecule are nucleotides. These are composed of a deoxyribose
sugar bonded to a phosphate and a nucleotide base (adenine, thymine, cytosine, or guanine).
The three-dimensional structure of DNA is a double helix, which resembles a twisted ladder.
2. The evidence included Rosalind Franklin’s X-ray diffraction photo of a crystal of DNA, plus
Erwin Chargaff’s work that showed that DNA contains equal amounts of adenine and thymine and
equal amounts of cytosine and guanine.
3. A gene is a DNA sequence of nucleotide bases that are transcribed to a complementary string
of RNA bases. The RNA is translated at a ribosome, where amino acids are assembled into a
protein according to the “recipe” encoded in the sequence of nucleotides in the RNA molecule.
7.3
1. DNA replicates before a cell divides to give a complete set of genetic instructions to each
daughter cell.
2. Semiconservative replication means that only one strand of a DNA molecule is newly-formed.
The other strand is original DNA from the parent cell.
3. The steps in DNA replication are: the helicase enzyme unwinds and separates a doublestranded DNA molecule; the primase enzyme forms a short stretch of complementary RNA on
each DNA template; DNA polymerase adds DNA nucleotides to the end of the RNA primer,
proofreading and correcting errors as it goes; RNA primers are removed; on the lagging strand,
the enzyme ligase joins fragments of DNA.
4. A mutation is any change in a cell’s DNA sequence. Mutations are important because they
increase genetic diversity by creating new alleles. Mutations may be harmful, neutral, or
beneficial.
5. DNA repair is necessary because a mutation can cause errors that can be harmful or even
lethal.
7.4
1. A target DNA sequence, primers, nucleotides, and Taq DNA polymerase interact to increase
the number of copies of the target DNA. These components are combined in a small tube, and
the temperature is raised. This causes the strands of DNA to separate. When the temperature is
lowered, the primers attach to the separated DNA strands. Taq DNA polymerase uses the
nucleotides to replicate the original DNA. The process is repeated for many rounds; the number
of copies of the target DNA doubles with each round.
2. PCR is useful because it can multiply the DNA in a small amount of tissue into a sample large
enough to be analyzed in further research. PCR is used in forensics to establish guilt or
innocence, and it is also used to investigate genetic relationships and identify human remains.
PCR is an important research tool in agricultural research, veterinary medicine, environmental
science, and human health care.
7.5
1. The Sanger method produces DNA fragments that are complementary to the DNA being
sequenced. The process adds “terminator nucleotides,” normal nucleotides, radioactively labeled
nucleotides, and replication enzymes to solutions of DNA and allows replication to proceed. DNA
polymerase stops replication when it encounters a “terminator nucleotide.” Researchers put these
fragments into electrophoresis gels that separate them by size. The labeled nucleotides are read
off the gel and the original sequence is the complement of the sequence that appears on the gel.
2. A DNA microchip is a small glass square that has short DNA fragments of known sequence
attached to it. Complementary fragments of nucleic acids stick to the fragments that are attached
to the chip.
7.6
1. DNA profiling detects genetic differences among individuals. There are many different profiling
techniques, but all focus on regions of the genome that are known to vary within the population.
2. After researchers have analyzed DNA from two sources at several restriction sites, they
consult databases to determine how frequently each allele appears in the population. Then the
frequencies of occurrence are multiplied to produce the probability of this combination of alleles
occurring in a sample.
3. Because each individual inherits mitochondria only from his/her mother, analysis of the DNA in
mitochondria gives different information from analysis of nuclear DNA (which comes from both
parents). For example, mitochondrial DNA can establish a relationship between mother and child,
but it can’t differentiate between siblings.