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In prokaryotic cells, gene regulation usually occurs at
the level of transcription. Examples are operons—the
trp operon and the lac operon. The trp operon is a repressible operon because the repressor coded by a regulator gene must bind with a corepressor (i.e.,
tryptophan) before the complex can bind to the operator
and stop protein synthesis.In the lac operon, a repressor
protein ordinarily binds the operator so that RNA polymerase is unable to bind to the promoter and transcription is therefore unable to take place. When lactose is
present, it binds to the repressor, and then this combination is unable to bind to the operator. The lac operon
is an inducible operon.
Eukaryotic cells have five levels of control (gene
regulation): chromatin structure, transcriptional, posttranscriptional, translational, and posttranslational. Chromatin packing is used as a way to keep genes turned off.
Transcriptional control includes the use of transcription
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activators and factors. Posttranscriptional control includes mRNA processing and the speed with which mRNA
leaves the nucleus. Translational control pertains to the
life expectancy of mRNA molecules, which can vary;
some mRNAs may need modification before they can be
translated. Some of these modifications may be signals from
microRNA. Posttranslational control includes activation
of the protein product and degradation of a protein.
Mutations are changes in DNA nucleotide base sequences. Types of mutations include point and frameshift
mutations. Mutations can be spontaneous or caused by an
environmental mutagen. Carcinogens are mutagens that
cause cancer.
Cancer is due to a series of genetic mutations among
regulatory genes that control the cell cycle. These mutations
activate oncogenes or deactivate tumor suppressor genes,
resulting in uncontrolled cell division that leads to a
tumor.
Study the text section by section as you answer the questions that follow.
• Regulator genes control the expression of genes that code for a protein product.
1. a. Label the following diagram of an operon, using the alphabetized list of terms.
active repressor
mRNA
operator
promoter
regulator gene
structural genes
transcription is prevented
126
h. Which of these codes for a repressor?
i. To which of these does RNA polymerase bind?
j. To which of these does the repressor bind?
k. Which of these codes for enzymes of the pathway?
2. Cross out all portions of the following diagram that are not in use if the trp operon is turned
regulator gene
operator
inactive }
on.
structural genes
enzymes
repressor
tryptophan
3. Cross out all portions of the following diagram that are not in use if the lac operon is turned off.
regulator gene
(----"----operator
4
structural genes
,.
active }
repressor
+
inducer
1
enzymes
4. Put this sequence of events in order to describe how E. coli ensures that the lac operon is maximally turned on
when glucose is absent.
a.
Cyclic AMP builds up.
b.
Now RNA polymerase is better able to bind to the promoter.
c.
Cyclic AMP binds to catabolite activator protein (CAP).
d.
The complex attaches to a CAP binding site next to the lac promoter.
5. a. With the trp operon, are the structural genes ordinarily turned on or off?
b. Why is the trp operon a repressible operon?
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• The control of gene expression can occur at all stages, from transcription to the activity of proteins in the
eukaryotic cell.
• Transcriptional control is more significant than posttranscriptional control.
6. Complete the following table:
Level of Control of Gene Activity
1.
2.
3.
4.
5.
Affects the Activity Of
7. Indicate whether these statements about euchromatin and heterochromatin are true (T) or false (F).
Decompacted euchromatin is inactive.
a.
Looped euchromatin is actively being transcribed.
b.
c.
Highly compacted and condensed heterochromatin is inactive.
Heterochromatin is actively being transcribed.
d.
8. a. Which statement in question 7 is supported by knowledge of Barr bodies?
b. Why?
9. a. Which statement in question 7 is supported by knowledge of lampbrush chromosomes?
11_ Why?
10. Consider the following diagram:
T7anscription activators
bind to enhancer.
promoter
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DNA
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gene
0"
1
enhancer
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mediator
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proteins
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jilt Transcription factors
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Transcription factors
bind to promoter.
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RNA polymerase
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RNA polymerase
binds.
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mRNA transcription begins.
a. What are transcription factors?
b. Where do transcription factors bind?
c. Where do transcription activators bind?
d. What conformational change occurs before transcription begins?
11 Question 10 pertains to what level of genetic control in eukaryotes?
12 A DNA sequence that can move between chromosomes is called a
13. Place the appropriate letters next to each statement.
PTC—posttranscriptional control
a.
b.
c.
d.
TL—translational control
PTL—posttranslational control
The product of a metabolic pathway binds to an enzyme that speeds the first reaction of the pathway.
mRNA persists for different lengths of time in cells.
Introns are processed into smaller signals called microRNAs.
Patterns of mRNA splicing differ.
• Mutations occur when the nucleotide base sequence of DNA changes.
• Mutations can lead to proteins that do not function or do not function properly.
• Mutations of regulatory genes are now known to cause cancer.
14. The original base sequence is UACUACUAC.
a. Name the mutation that reads UAUACUACU.
b. Name the mutation that reads UACUAGUAC.
c. Which of these two types of mutations causes sickle cell disease?
15. The mutation of a a.
b.
.
to an oncogene and/or a tumor suppressor gene can lead to a
When a tumor is at its place of origin, it is said to be C.
cancer usually leads from a localized tumor to a d '
e.
. The development of
tumor, found at some distance from the
tumor.
The p53 gene is a f.
gene. The p53 protein acts as a g.
expression of genes whose products are h '
inhibitors. i.
factor and can turn on
, programmed cell
death, can also be stimulated by the p53 gene.
16. Place the appropriate letters next to each description. Two answers are required for each statement.
P—proto-oncogenes 0—oncogenes T—tumor suppressor genes MT—mutated tumor suppressor gene
a.
b.
c.
cell division is always promoted
normal genes in cells that regulate cell division
mutated genes that cause cancer
17 a. Aside from replication errors, what affects the rate of mutation?
b. How is DNA protected against mutations due to environmental mutagens?
18. a. What is a carcinogenic mutagen?
b. Name two environmental factors that are carcinogenic.
129
KEYWC:›RC0 CROSSWCORC1
Review key terms by completing this crossword puzzle, using the following alphabetized list of terms:
Barr body
carcinogen
corepressor
frameshift
genetic
hi stone
inducer
operator
point
promoter
repressor
transposon
Across
1 Molecule that binds to a repressor, allowing the repressor to bind to an operator in a repressible operon
3 In an operon, a sequence of DNA where RNA polymerase binds prior to transcription
5 Mutation of a gene in which the insertion or deletion of at least one base changes the corresponding
mRNA
7 Altered gene mutation whose sequence of bases
differs from the previous sequence
8 DNA sequence capable of randomly moving from
one site to another in the genome
9 Mutation of a gene in which there is a change of
one base only in the sequence of bases
10 Protein molecule responsible for packing chromatin
11 Molecule that brings about activity of an operon
by joining with a repressor and preventing it from
binding to the operator
Down
1 Environmental agent that causes mutations, leading
to the development of cancer
2 In an operon, the sequence of DNA to which the
repressor protein binds
4 Dark-staining body in the nuclei of female mammals
that contains a condensed, inactive X chromosome
(two words)
6 In an operon, a protein molecule that binds to an
operator, preventing RNA polymerase from binding to the promoter site