1. Chapter 18 215 Regulation of Gene Expression
Operon: promoter, operator and structural genes
Promoter: where RNA polymerase binds
Operator: repressor protein binds to stop gene transcription
Structural gene: codes for polypeptide
Activator: switches on an operon

2. 215 Regulator gene: codes for a repressor protein
Constitutive enzyme: is always synthesized in the presence or absence of an inducer
Inducible enzyme: synthesized only in the presence of an inducer
Cistron: a transcript of several genes which can be translated into several polypeptides
Polycistronic: a mRNA which is transcribed from DNA with several genes

3. 216 Substrate Induction System (Jacob-Monod Model of Gene Induction):
In the presence of substrate, the genes are activated to produce enzymes which break down the substrate.
Beta-galactosidase: splits lactose into glucose and galactose; coded by lac Z gene.
Permease: transports lactose into the cell; coded by lac Y gene.
Transacetylase: mechanism is unknown; coded by lac A gene.

4. (a) Lactose absent, repressor active, operon off
Fig. 18-4
Regulatory
gene
Promoter
Operator
DNA
lacI
lacZ No
RNA
made 3
mRNA
RNA
polymerase 5
Active
repressor
Protein
(a) Lactose absent, repressor active, operon off
lac operon
DNA
lacI
lacZ
lacY
lacA
RNA
polymerase
Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes
For the Cell Biology Video Cartoon Rendering of the lac Repressor from E. coli, go to Animation and Video Files. 3
mRNA
mRNA 5 5
-Galactosidase
Permease
Protein
Transacetylase
Allolactose
(inducer)
Inactive
repressor
(b) Lactose present, repressor inactive, operon on

5. (a) Lactose absent, repressor active, operon off
Fig. 18-4a
Regulatory
gene
Promoter
Operator
DNA
lacI
lacZ No
RNA
made 3
mRNA
RNA
polymerase 5
Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes
Active
repressor
Protein
(a) Lactose absent, repressor active, operon off

6. (b) Lactose present, repressor inactive, operon on
Fig. 18-4b
lac operon
DNA
lacI
lacZ
lacY
lacA
RNA
polymerase 3
mRNA
mRNA 5 5
-Galactosidase
Permease
Transacetylase
Protein
Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes
Allolactose
(inducer)
Inactive
repressor
(b) Lactose present, repressor inactive, operon on

7. Regulator gene directs the synthesis of repressor protein
Regulator gene directs the synthesis of repressor protein. The repressor protein binds to the operator region.
The binding physically blocks the RNA polymerase from binding to the promoter region. So, transcription cannot be carried out, and no enzymes can be synthesized. The genes are turned off.
If substrate (lactose) is presence, it binds with the repressor protein. The binding causes a conformational change in the repressor protein.

8. 216 4. The repressor protein dissociates from the operator region.
5. Since the operator region is not blocked, the RNA polymerase can then bind to the promoter regions (P1 and P2). It initiates the transcription of the structural genes (lac Z, lac Y and lac A) to produce mRNAs. The genes are turned on.
6. The mRNAs carry the instruction to the ribosome where the enzymes are synthesized. Lactose can then be metabolized.

9. 217 End-Product Corepression System:
In this system, the genes are turned on all the time as the repressor protein produced by regulator gene is not active. It cannot bind to the operator region.
The RNA polymerase binds to the promoter region and initiates transcription of DNA to produce mRNA.
The mRNA carries instruction to the ribosome to synthesize the enzyme. The gene is turned on.

10. Polypeptide subunits that make up enzymes for tryptophan synthesis
Fig. 18-3a
trp operon
Promoter
Promoter
Genes of operon
DNA
trpR
trpE
trpD
trpC
trpB
trpA
Regulatory
gene
Operator
Start codon
Stop codon 3
mRNA 5
mRNA
RNA
polymerase 5 E D C B A
Protein
Inactive
repressor
Polypeptide subunits that make up
enzymes for tryptophan synthesis
Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes
(a) Tryptophan absent, repressor inactive, operon on

11. 217 4. The enzyme breaks down the substrate into end product.
5. The end product binds to the repressor protein. The binding causes a conformational change in the repressor protein to become active.
6. The active repressor protein binds to the operator region to block the RNA polymerase from binding to the promoter region. The gene is turned off.

12. 217-218 cAMP-CAP Activation System:
cAMP: cyclic adenosine monophosphate
CAP: catabolic gene activator protein
If glucose is abundant, the operon for lactose remains shut off. The cell preferentially metabolizes glucose. As the glucose concentration goes down, the concentration of cAMP increases. The cAMP binds with an activator protein (CAP), and the cAMP-CAP complex then binds to the promoter region of the operon. The binding of this complex facilitates the binding of RNA polymerase to the promoter. The structural genes are transcribed, and the enzymes for lactose metabolism are produced.

13. 218 Eukaryotic gene expression can be regulated at any stage:
Differential gene expression:
Human genome has an estimated 22,333 genes. A human cell expresses about 20% of its genes at any given time. The differences in cell types are due to differential gene expression, not due to different genes. Only 1.5% of human genome codes for proteins, the rest codes for RNA products.

14. 218 Regulation of Chromatin Structure:
Functions of chromatin organization:
1. to pack the DNA inside a nucleus
2. to control the gene expression by DNA
methylation and histone acetylation.

15. (a) Histone tails protrude outward from a nucleosome
Fig. 18-7
Histone
tails
Amino
acids
available
for chemical
modification
DNA
double helix
(a) Histone tails protrude outward from a
nucleosome
Figure 18.7 A simple model of histone tails and the effect of histone acetylation
Unacetylated histones
Acetylated histones
(b) Acetylation of histone tails promotes loose
chromatin structure that permits transcription

16. 218
DNA methylation: addition of methyl groups (-CH3) to the bases of DNA to inactivate the gene. In each female, one of the two X chromosomes is compacted to form a Barr body (a heterochromatin). The DNA in Barr body is heavily methylated.

17. 218
Histone acetylation: the attachment of acetyl groups (-COCH3) to the positively charged lysines in histone tails. It will cause the histone tails of a nucleosome to change the shape, loosening the grip to the DNA. This allows an easy access to the genes in the acetylated region by transcription proteins to initiate gene transcription. The gene is turned on as the gene is being transcribed.
Histone deacetylation: the removal of acetyl groups

18. 219 Organization of a Typical Eukaryotic Gene:
Eukaryotic genes are interrupted by introns. The primary transcript (pre-mRNA) contains regions of introns and exons. The intron regions are excised and the exon regions are spliced to form a final transcript (mature mRNA).

19. 220
RNA Processing:
In eukaryotes, the initial transcripts must be processed before they can act as mRNA, tRNA or rRNA. A mRNA transcript must be tagged with 7-methylguanosine at the 5’ end, and with adenine at 3’ end.
mRNA Degradation:
In prokaryotes, the mRNAs are degraded by enzymes within a few minutes. The mRNA in eukaryotes can last for hours, days or even weeks.

20. 221 Noncoding RNAs (ncRNA):
Only about 1.5% of the human genome codes for proteins. A significant amount of the genome codes for non-protein-coding RNAs (noncoding RNAs or ncRNAs).

21.
MicroRNA (miRNA) is a small single-stranded RNA formed from longer RNA precursors. They fold back to form a double-stranded hairpin held by hydrogen bonds. An enzyme called Dicer cuts it into short double-stranded fragments from the primary miRNA transcript. One of the strands is degraded.

22. 222
The other strand (miRNA) binds to a large protein to form a complex, which can degrade the target mRNA or block its translation. Small interfering RNAs (siRNAs) are similar in size and function as miRNAs. They turn off the genes with the same sequence.

23. 226
Cancer results from genetic changes that affect cell cycle control:
Tumor: benign, malignant (cancer)
Metastasis

24. 226 Characteristics of Cancer Cells:
Have abnormal number of chromosomes (HeLa cell: 70-80) or have chromosomes with altered sequences.
Spherical shape due to lack of microfibrils.
Loss of anchorage dependence.
Lack of contact inhibition property.
Absence of cellular affinities, resulting in metastasis.

25. 226 Cell coats bear abnormal antigens.
Consume more glucose than normal cells. They metabolize glucose at a high rate and excrete much lactic acid and proteolytic enyzmes, which may alter the cell surface of their own or that of the normal cells.

26. 226 Carcinogens: Oncogenes:
Proto-oncogenes: These are the genes related to the viral genes. They code for proteins that regulate cell growth, cell division, and cell adhesion. They can be converted to oncogenes by mutation.
Tumor-suppressor genes help prevent uncontrolled cell growth, repair damaged DNA, control cell adhesion, and serve as the components of the signaling pathways that inhibit the cell cycle. A change in these genes may also cause cancer.

27. EFFECTS OF MUTATIONS Fig. 18-22
Colon
EFFECTS OF MUTATIONS
Loss of tumor-
suppressor gene
APC (or other) 1
Activation of
ras oncogene 2
Loss of
tumor-suppressor
gene p53 4
Colon wall
Loss of
tumor-suppressor
gene DCC 3
Additional
mutations 5
Figure A multistep model for the development of colorectal cancer
Normal colon
epithelial cells
Small benign
growth (polyp)
Larger benign
growth (adenoma)
Malignant tumor
(carcinoma)

28. 227
The p53 gene codes for a transcription factor (p53 protein) that promotes the synthesis of growth-inhibiting proteins. The gene is named for the 53,000-dalton molecular weight of its protein product. The gene is activated by DNA damage caused by radiation or toxic chemicals. It is often called the guardian angel of the genome.

29. 227
The p53 protein often activates p21 gene whose product binds to cyclin-dependent kinases to halt the cell cycle, thus allowing time for DNA repair.
The p53 protein also activates “suicide” genes whose proteins cause apoptosis (programmed cell death). Mutation of p53 gene can lead to cancer.

30. 227 The Multistep Model of Cancer Development:
Cancer is often caused by multiple mutations. Higher cancer rate is correlated with age because of the accumulation of mutations.

31. 227
At DNA level, there must be about half a dozen changes taking place in order for a cell to become cancerous. The changes include the appearance of at least one active oncogene, and the mutation of several tumor-suppressor genes. The mutations knock out both alleles to block tumor suppression. In many malignant tumors, the gene for telomerase is activated. The enzyme prevents the erosion of the ends of the chromosomes and thus allows the cell to divide indefinitely.

32. 227
About 15% of human cancer is caused by viruses. The insertion of viral nucleic acid may disrupt a tumor-suppressor gene or convert a proto-oncogene to an oncogene.

33. 228
In 5% to 10% of breast cancer, there is evidence of a strong inherited predisposition. More than half of the inherited breast cancers are associated with mutation of BRCA1 (BReast CAncer) genes on chromosome 17. The other half of breast cancers are associated with mutation of BRCA2 gene on chromosome 13.
Mutations of either gene can result in breast cancer and ovarian cancer. Both genes serve as tumor suppressor genes whose products are involved in DNA repair.