Download Answers to Mastering Concepts Questions

Answers to Mastering Concepts Questions
Chapter 12
12.1
1. A gene is a recipe (in DNA nucleotide bases) for making a polypeptide or a protein (of amino
acids).
2. Transcription writes DNA’s genetic information into a complementary mRNA strand.
Translation uses the sequence of mRNA nucleotides to form a chain of amino acids.
3. Messenger RNA, transfer RNA, and ribosomal RNA.
12.2
1. The steps of transcription are initiation, elongation of the RNA molecule, and termination.
2. RNA polymerase uses the DNA template to bind additional RNA bases into the growing chain
of RNA being transcribed.
3. The promoter signals the start of a gene, and the terminator signals the end of a gene.
4. Before it leaves the nucleus of a eukaryotic cell, mRNA is altered in the following ways:
- a cap is added to the 5’ end of the mRNA molecule;
- a poly A tail is added to the 3’ end;
- introns are removed and exons are spliced together.
12.3
1. Researchers knew that life uses four nucleotides and 20 amino acids. They reasoned that the
genetic code could not reflect 1-base or 2-base “words,” because neither could encode enough
amino acids. A triplet code (3-base “words”) could potentially encode 64 amino acids, which is
more than enough for the 20 amino acids found in biological proteins.
They deciphered the genetic code by adding synthetic mRNA molecules to test tubes containing
all the ingredients needed for translation. They analyzed the sequences of the resulting
polypeptides to determine which codons correspond to which amino acids.
2. The near-universality of the genetic code points to a common origin for life.
3. The steps of translation are initiation, elongation of the polypeptide, and termination.
4. A polypeptide folds into its finished shape because of interactions between different portions of
the same protein, plus interactions with water. Enzymes bond certain regions of a protein
together, while chaperone proteins stabilize partially folded regions.
12.4
1. The steps in protein synthesis that require energy are transcription, translation and the
synthesis of the biochemicals needed for these processes, including nucleotides, tRNA, rRNA,
enzymes, and other molecules.
2. Protein production costs a lot of energy; the regulation of gene expression avoids the
production of unnecessary proteins and therefore saves energy.
3. A repressor protein binds to an operator and prevents the genes in the operon from being
transcribed.
4. In signal transduction, transcription factors facilitate the binding of RNA polymerase at the
promoter.
5. Eukaryotic cells control gene expression by:
- methylating DNA so that it is more tightly bound and cannot be accessed by transcription
factors and RNA polymerase;
- removing different introns from the same mRNA (which can therefore encode multiple
proteins);
- preventing mRNA from leaving the nucleus of a cell;
- degrading mRNA shortly after it has formed;
- having some proteins that must be further processed to become functional;
- preventing a protein from moving to the Golgi apparatus.
12.5
1. In a point mutations, one DNA base is substituted for another. Point mutations include
missense mutations (which change a triplet base so it specifies a different amino acid) and
nonsense mutations (which change an amino acid-encoding codon into a stop codon). Mutations
that involve insertion or deletion of nucleotides are called frameshift mutations. Expanding repeat
mutations increase the number of copies of three-or four-nucleotide sequences over several
generations. This causes extra amino acids to be inserted into a protein, deforming it.
2. Mutations are caused by DNA replication errors, errors in crossing over during meiosis,
chromosome inversions and translocations, exposure to chemicals or radiation.
3. A germline mutation is one that occurs in a cell that will give rise to a sperm or an egg cell. A
somatic mutation occurs within a non-germline body cell.
4. Mutations are used to learn how genes normally function and to develop new varieties of crop
plants. Mutations can also be used to trace the evolution of viruses and other infectious agents.
12.6
1. The human genome maximizes its protein-encoding informational content by being able to
produce many more proteins than there are genes that encode them. One way it does this is by
removing different combinations of introns from mRNA molecules, producing multiple mRNA
transcripts that translate into different proteins.
2. Some of the 98.5% of the human genome that does not code for protein encodes rRNA, tRNA,
and regulatory sequences that control gene expression. It also contains pseudogenes that may
be remnants of non-functional DNA that encoded proteins in our ancestors. It also contains
transposons (transposable elements) that jumped from bacteria and viruses to humans. It also
contains tandem repeats of DNA sequences in telomeres, centromeres, and on the Y
chromosome.
12.7
1. Transgenic organisms produce drugs and other useful chemicals, degrade pollutants,
incorporate pesticides in their tissues, are models for medical research, and secrete human
proteins.
2. The steps in creating a recombinant plasmid include:
- using restriction enzymes to cutting out the gene sequence from donor DNA;
- cutting the plasmid with the same restriction enzymes;
- allowing the donor sequence to combine with the plasmid DNA.
3. Bacteria, plant, and animal cells are induced to take up recombinant DNA by exposure to
electricity, shooting it into cells with gene guns, inserting it into liposomes, and inserting it as
plasmids into bacteria that enter plant cells and inject the plasmids.
4. One problem with transferring eukaryotic genes into bacterial cells is that the bacterial cells do
not splice introns out of mRNA, so the encoded proteins are not produced correctly in the
bacterial cells. Reverse transcriptase solves this by making a DNA copy of mRNA extracted from
a eukaryotic cell. Because the introns have already been removed from mRNA, the resulting
DNA can be inserted into a bacterial cell, where it will encode the correct protein.
12.8
1. Gene therapy uses an infectious agent such as a virus to convey a repaired gene into the cells
containing faulty genes. It is difficult to accomplish because viruses normally cause an immune
system response, which will destroy the viruses with therapeutic genes. A massive immune
system response also may overwhelm and kill the individual undergoing treatment.
2. Antisense RNA silences genes by adding an artificial complementary strand of RNA to mRNA,
making it a double strand. Ribosomes cannot translate double stranded mRNA. Gene knockouts
silence genes by replacing a normal copy of a gene with a disabled version that will not be
transcribed.
3. DNA microarrays can be used to quickly determine whether a particular gene is present in a
cell or whether a cell makes a particular protein. In more practical terms, they can tell how a
cancer patient will respond to a cancer drug and whether the drug will be effective against the
cancer. DNA microarrays also can be used to predict how effective an antibiotic will be against a
particular strain of bacteria.
4. An organism’s genome is all of the DNA it contains; an organism’s proteome is all of the
proteins it produces.