Download Unit VII Study Guide KEY

Unit VII Study Guide KEY
I. Vocabulary Check
1. D
2. Q
3. GG
4. W
5. S
6. H
7. A
8. Z
9. CC
10. M
11. E
12. HH
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O
F
B
N
U
EE
I
AA
J
DD
K
L
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Y
C
FF
P
G
II
BB
R
T
V
X
II. Complete the following paragraph about gene expression in prokaryotes.
There are important similarities and differences in gene expression of eukaryotes versus prokaryotes. In transcription in all cells, the
enzyme, _RNA polymerase______ unzips the DNA, moving in a _3’__ to _5’__ direction. Nucleotides are moved in according to
_Chargaff’s_____ rules and _mRNA___ is synthesized in a _5’___ to _3’___ direction. While similar in principle, there are also
differences between prokaryotic transcription and eukaryotic transcription. First, the _processing_______ step which involves
_removing introns___, _constructing a 5’ cap______, and _a poly A tail_______ is absent. Secondly, the absence of a __nuclear
envelope_____ in prokaryotic cells __decreases_______ the total amount of time required for transcription and translation combined.
And a third important difference is the organization of genes in the prokaryotic chromosome. Multiple genes required for a single
metabolic pathway are organized together in groups known as _operons______. In addition to the genes coding for necessary
_enzymes_____ for a particular metabolic pathway, this includes the _promoter_______, the binding site for the enzyme needed for
transcription, and the _operator____, the on-off switch for the gene cluster. The operator is “on” unless it is blocked by a
_repressor_____, a _protein_______ molecule coded for by the _regulatory__________ gene, which is separate from the rest of the
_operon___. This gene is _constitutive________, meaning it is always expressed; but the _repressor________ protein it produces
may be synthesized in a _functional__________ form, meaning transcription will be blocked, or in a _dysfunctional__________ form,
meaning transcription can proceed. The _lac____ operon is a(n) _inducible______ operon, meaning it is normally off. Its repressor is
synthesized in a _functional______ form, meaning it will fit into the _operator_____ and block transcription. However, when
_lactose_____ is present, it binds to the repressor and renders it _dysfunctional___, allowing transcription to proceed. A(n)
_repressible____ operon, on the other hand, is normally on. Therefore, its repressor is synthesized in a _dysfunctional_______ form.
An example is the _trp____ operon. When _tryptophan_____ is present in the cell due to over-production or availability from
surrounding environment, it binds to the repressor and changes its shape rendering it _functional___, and transcription is blocked.
Repressors are negative controls; there are also forms of positive control known as _activators_______. An example is found in the
lac operon. A bacterium’s first choice for energy metabolism is _glucose_______, so when it is present in the cell along with lactose,
lactose metabolism is inhibited. The enzyme, _RNA polymerase______ has a _low___ affinity for the lac operon promoter. When
_glucose____ concentration is high, the concentration of another molecule, cAMP, is _low____. As _glucose____ concentration
decreases, cAMP concentration _increases_______, eventually resulting in activation of _CAP____. This activator binds to the DNA
and enhances the affinity of _RNA polymerase______ for the promoter. Assuming lactose is available, and the repressor is
_dysfunctional______, the _enzymes____ required for lactose metabolism are synthesized.
In prokaryotes, transcription and translation can occur virtually simultaneously. Translation takes place at the _ribosomes_________
which are different from those found in eukaryotic cells in that they are _70_S and they are _smaller_____ in size. The mRNA
transcript slides through the _small____ subunit until the start codon, _AUG____, is reached. A _t___RNA carrying the amino acid,
_methionine (met)___, arrives and at this point, the _large____ subunit joins the complex. Translation continues, a __t___RNA arrives
at the _A_ site, delivering the amino acid specified by the transcript’s instructions. Base-pairing takes place between the
_anticcodon____ of _t__RNA and the _codon___ of the transcript to insure proper protein assembly. Although the first two positions
must base pair exactly, there is variation in the third position, a phenomenon known as _wobble____. After the RNA arrives at the
_A__ site, a _peptide____ forms between its amino acid and the existing polypeptide chain. That bond is formed through the process
of _dehydration synthesis _(AKA condensation)______. The RNA complete with its polypeptide chain is then moved to the _P____
site. Another RNA arrives with its amino acid; the growing polypeptide chain is transferred, followed by the departure of the now
“empty” RNA through the _E___ site. The empty RNA is returned to the cytoplasm where it is “restocked” with another amino acid
through the action of the enzyme, _aminoacyl- tRNA synthetase______. Translation of the transcript continues until a _stop_ codon
(either _UAA__, _UAG__, or _UGA___) is reached. The polypeptide chain then folds into its natural conformation, which may include
secondary structure created by _hydrogen bonding between the amino group of one amino acid and the carboxyl group of another ___,
tertiary structure created by _interactions (ionic bonds, covalent bonds, disulfide bridges, hydrogen bonds, van der Waals forces)
between R groups____, and quaternary structure created by _more than one polypeptide chain_____________________. Often, a
single mRNA is translated many times in consecutive fashion by _polyribosomes___________.
III. Complete the following paragraph about gene cloning.
The agent used for gene cloning is often a _plasmid_____, a self-replicating piece of bacterial DNA. The first step in this process is to
create _recombinant___ DNA; that is, to insert the foreign DNA into the bacterial DNA. This is done by exposing the DNA containing
the gene of interest and the bacterial DNA to the same _restriction______ enzyme. Typically, an enzyme is chosen that will create
_sticky_____ ends. Following enzyme exposure and the addition of the glue, __DNA ligase____, there are three possible outcomes.
Some of the bacterial DNA pieces will simply re-join in their original conformation; that will also occur with the DNA containing the gene
of interest. However, some of the fragments of DNA will join together such that the gene of interest becomes incorporated into the
bacterial _plasmid_____. In order to identify which _plasmids______ contain the gene of interest, a _reporter______ gene must be
used. An example is the lacZ gene which codes for an enzyme needed in lactose metabolism. Colonies growing on agar containing
lactose that can break it down are blue; those that cannot are white. This represents a “_loss___ of function” gene; the site where the
_restriction___ enzyme initially cleaves the bacterial DNA is in the middle of this gene. If insertion of the foreign gene is successful, the
lacZgene is interrupted and no longer functional. Following _restriction___ enzyme exposure, bacteria are then treated to trigger the
uptake of the circular bacterial chromosome in a process known as _transformation_____. A second _reporter_________ gene is
used; however, this represents a “_gain____ of function” gene. The circular piece of DNA contains a gene for ampicillin resistance. If
bacteria take up the circular chromosome, they will be _resistant____ to ampicillin. Following treatment, the bacteria are plated on
agar containing ampicillin and lactose. Only bacteria that took up the _plasmid___ will grow. In order to isolate bacteria containing
_plasmids_____ incorporated with the gene of interest, only _white__-colored colonies are selected, indicating the _inability____ to
metabolize lactose. In summary, any colony that grows on the selective medium _contains a plasmid___, and any colony on that
medium that is colored white _contains the plasmid containing recombinant DNA_______.