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Lac Operon: State Of Transcription

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The lac operon encodes a set of genes for the metabolism of lactose. There are three genes encode by the lac operon: lac Z gene, lac Y gene, and lac A gene. (1 )
The lac Z gene translate into B-galactosidase. (1)
The lac Y gene translate into lactose permease. (1)
The lac A gene translate into lactose transacetylase. (1)

These three genes are the structural genes of the lac operon. (1,2) The direction of the structural gene on the lac operon is Z-Y-A. (1, 2)

The lac I gene is close to the structural genes, and it encodes for the lac repressor. (1, 2) The function of the lac repressor is that it can bind to the sequence of DNA called the operator (lac O), and stop transcription of the structural genes Z-Y-A. (1, 2) The repressor has another function in that it can bind to the inducer, which is a metabolite of lactose called allolactose. (1, 2) When the repressor bind to the inducer allolactose or lactose, it becomes inactive and cannot bind to the operator. Therefore, transcription of the structural genes B-galactosidase to metabolize lactose is induced when there is no repressor. (2) This induction of transcription only occur in the present of lactose, so the inducer is lactose.

On the other hand, if the inducer allolactose or lactose is absent, then it cannot bind to repressor. The repressor will stay active and bind to the operator. Binding of repressor to the operator will prevent transcription of lac Z-Y-A. (1) As a result, there is no B-galactosidase production because lactose is absent. Thus, the lac operon is an inducible system of regulation for lactose. (2)

Now let say that if the repressor gene lac I is deleted or mutated ( lac I-). In a mutated lac I- cell, there is no repressor so the transcription of the structural gene is "always on" regardless of whether there is an inducer present or not. (1) This unregulated expression of the structural genes as a result of the mutation in the repressor gene lac I- is known as "constitutive expression". (1)

This diploid genotype mean that a cell has one lac operon with a normal gene I and one lac operon with mutated lac I- gene. (1) The state of transcription in the presence of lactose is that lactose will bind to the repressor produced by the normal lac I gene and inactivate it. (1) Transcription of the structural genes will happen normally for both the wildtype lac I and mutant lac I-. The "good" or wildtype lac I gene will produce enough repressors to compensate for the bad mutated repressor lac I- on the other lac operon. (1) The normal repressor will travel to the operator on the other mutated lac I operon and bind to it. As a result, the expression of both lac operons will be normally control. (1)

In the absence of the lactose, the "good" lac I gene will encode enough repressors to bind to both of the (lac O) operators on each lac operons. So each of the lac operons (wildtype and mutated) will control expression normally. (1) So the state of transcription in the presence and absence of lactose in cell with the diploid genotype of wildtype lac I/ mutant lac I- is that transcription will happen normally because the normal lac I will encode enough repressor to compensate for the mutant lac I-. Thus, the expression of both operons will be control normally. As mentioned, the function of lac I is that it encodes for the repressor protein that binds to the operator on the lac operon; and repress transcription of the structural genes Z-Y-A. The function of lac I- is that it is a mutation in the lac I gene, which prevent production of repressor. Lac I- does not produce repressor protein. As a result, the lac operon will always be on and transcription of gene Z-Y-A is always on. The function of lac I- is to constitutively expressed the lac Z-Y-A on the lac operon. (1)

References:
1. http://www.cliffsnotes.com/study_guide/The-Lactose-OperonA-Look-at-Regulation.topicArticleId-24594,articleId-24575.html
2. Brock et al. Biology of Microorganisms. 7th edition.

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Solution Preview

The lac operon encodes a set of genes for the metabolism of lactose. There are three genes encode by the lac operon: lac Z gene, lac Y gene, and lac A gene. (1 )
The lac Z gene translate into B-galactosidase. (1)
The lac Y gene translate into lactose permease. (1)
The lac A gene translate into lactose transacetylase. (1)

These three genes are the structural genes of the lac operon. (1,2) The direction of the structural gene on the lac operon is Z-Y-A. (1, 2)

The lac I gene is close to the structural genes, and it encodes for the lac repressor. (1, 2) The function of the lac repressor is that it can bind to the sequence of DNA called the operator (lac O), and stop transcription of the structural genes Z-Y-A. (1, 2) The repressor has another function in that it can bind to the inducer, which is a metabolite of lactose called allolactose. (1, 2) When the repressor bind to the inducer allolactose or lactose, it becomes inactive and cannot bind to the operator. Therefore, transcription of the structural genes B-galactosidase to ...

Solution Summary

The lac operon encodes a set of genes for the metabolism of lactose. There are three genes encode by the lac operon: lac Z gene, lac Y gene, and lac A gene. (1 ) The lac Z gene translate into B-galactosidase. (1) The lac Y gene translate into lactose permease. (1) The lac A gene translate into lactose transacetylase. (1) The lac I gene is close to the structural genes, and it encodes for the lac repressor. (1, 2) The function of the lac repressor is that it can bind to the sequence of DNA called the operator (lac O), and stop transcription of the structural genes Z-Y-A. (1, 2) The repressor has another function in that it can bind to the inducer, which is a metabolite of lactose called allolactose. (1, 2) When the repressor bind to the inducer allolactose or lactose, it becomes inactive and cannot bind to the operator. Therefore, transcription of the structural genes B-galactosidase to metabolize lactose is induced when there is no repressor. (2) This induction of transcription only occur in the present of lactose, so the inducer is lactose.
On the other hand, if the inducer allolactose or lactose is absent, then it cannot bind to repressor. The repressor will stay active and bind to the operator. Binding of repressor to the operator will prevent transcription of lac Z-Y-A. (1) As a result, there is no B-galactosidase production because lactose is absent. Thus, the lac operon is an inducible system of regulation for lactose. (2)

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Genetics: Multiple Choice Questions

Some questions are easier than others but I would like to double check they are right please. Would appreciate if someone could help, I need to submit this by Monday.

1. Eye color is an X-linked gene and red is dominant to white in Drosophila. If I mate a red eyes male with a white eyes female and I mate a white eyed male and a purebred red eyed female, the progeny from both crosses will be the same.
a. T b. F

2. Mary met her partner Bobby at a support group for families dealing with colorblindness. Mary and Bobby are unaffected but both have one bro who is affected. They have no knowledge of other family members. Which of the following is true?
a. There is a 100% probability that they will have an affected child
b. They might both be heterozygotes
c. Since neither one of them is affected they are not at rick of having an affected child
d. Bobby will not pass a mutatnt allele to either sons or daugters

3. Mendel did not see the phenomenon of gene linkage in his experiments because:
a. He used plants in his crosses
b. He studied alleles/genes that were far apart on the same chromo
c. He studies allels/genes that were in coupling
d. He studied alleles/genes that were dominant
e. All above

4. During this phase of cell cycle the sister chromatids are already separated
a. Meta II
b. G2 phase
c. Ana O
d. Prophase
e. Cytokinesis

5. If a chi- sq test od principle of segregation is "statistically significant" this means:
a. P-value for test is > 5%
b. Data shows a good fit to 1st principle
c. Calculated x2 statistic is < 0.05
d. Calculated x2 statistic must be smaller than critical-sq value corresponding to p=0.05
e. None

6. Genes A & B are 12 centimorgans apart. Suppose F1 is created by crossing AAbb and aaBB . If the F1 were testcrossed with a doubly recessive aabb, what % of offspring wld have recombinant phenotypes?
a. 1.2%
b. 2.4%
c. 6%
d. 12%
e. 24%

7. In a test-cross, the recombinant rate b/w genes l and b is 0.03. A third gene r had a recombination rate of 0.12 with b and a recombination rate of 0.15 with l. What is the order of the 3 genes on a chromosome?
a. l r b
b. r l b
c. r b l
d. b r l
e. cannot be determined

8. Uracil is a ______, adenosine is a ______
a. Base/nucleoside
b. Nucleotide/nucleoside
c. Nucleotide/base
d. Nucleoside/base

9. An accurate exmplanantion of pleiotrophy
a. Interaction of 2 genes to give a phenotype
b. Recombination
c. Meiosis
d. Mitosis
e. None

10. After screening a population of 3000 people for a given dominant genetic defect. 64 has the defect and 5 of these has 1 affected parent. What is the mutation frequency for this gene population?
a. 60/3000
b. 65/4000
c. 5/6000
d. 60/6000
e. none

11. compare the (wt) and mutant (mt) DNA seqs. What best describes the mutation.
wt (ATCGGATGTTA)
mt (GTCGTATGTTA)
a. Transversion/transition
b. Transition/ Transversion
c. Transition/ transition
d. Deamination/ transition
e. None

12. HOW ARE THE HFR AND F' STRAINS RELATED?
a. Hfr strain donates DNA to F- strain , converting to F' strain
b. When Hfr strain is used for transduction its called an F' strain
c. Hfr strains are converted into F' strains when the F factor and some flanking bacterial DNA exits the bacterial chromosome
d. An F' strain is a Hfr straing carrying an R plasmid
e. After DNA is transferred from Hfr strain to F- strain, the Hfr becomes a F' strain

13. How can viruses cause cancer?
a. They produce oxygen
b. Carry toxins
c. Always undergo lytic cycle
d. Can cause insertional mutagenesis
e. All of above

14. If 2 diff strains of bacteria undergo gene exchange when mixed, how wld you test for transformation?
a. Expose to strains to a virus
b. Expose to strains to DNA only
c. Repeat Griffiths exp
d. Repeat Meselson/Stahl exp
e. 2 of above

15. 1. Which of the following is true?
a. The anticodon of the tRNA and amino acid have no relationship
b. Any tRNA can be charged with any amino acid
c. tRNA binds to the codon in DNA
d. The anticodon must first get activated before charging of the tRNA
e. None of these are true

16. 2. In translation the mRNA must be activated by AMP.
a. True
b. False

17. 3. The Oc mutation in the lac operon is a recessive mutation.
a. True
b. False

18. 4. The C-value paradox means:
a. large genome-more genes
b. small genome-more genes
c. large genome-fewer genes
d. small genome-fewer genes
e. none of the above

19. Xeroderm Pigmentosum is associated with a mutant DNA repair sys
a. True
b. False

20. To be capable of transformation, bacteria must be in a physiological state called
a. Hfr
b. F+
c. F
d. F"
e. None

21. If the homozygouse recessive genotype occues with freq 0.04 in a population. What is the freq of recessive allele according to HW eq?
a. 0.2
b. 0.04
c. 0.0016
d. 0.08
e. unclear

22. An example of a missense mutation would be the insertion of a number of base/nucleotides which is not a multiple of 3
a. True
b. False

23. A mutation in a proto-oncogene is a dominant mutate
a. True
b. False

24. In a given pop of dogs, curly hair (h) recessive to wildtype straight hair (h+). The population of curly hair odgs is 50 and straight haired dogs is 150. What is the frequency of heterozygotes in the population according to HW eq
a. 50%/0.5
b. 25%/0.25
c. 75%/0.75
d. 10%/0.1
e. none

25. Tautomerization is an example of spontaneous mutation
a. True
b. False

26. Generation of AP site occurs during direct repair
a. True
b. False

27. Genetic drift is associated with large populations
a. True
b. False

28. What would one use PCR for
a. Obtain lrg qty of protein
b. Obtain lrg qty of DNA
c. Obtain lrg qty of RNA
d. 2 of above
e. none

29. Which of the following stops replication of DNA in euks?
a. End of chromosomes
b. Release factors
c. RHO protein
d. DNA ligase

30. Which of the following affects the rate of transcription of lac operon
a. CAP
b. RNA Pol III
c. Poly A Tails
d. Primase
e. None

31. The E.coli genomes codes for more genes than human genome
a. True
b. False

32. Prokarytotes rRNA genes are found in
a. Ribosomes
b. Nucleus
c. Nucleolus
d. Neucleoid
e. Cell wall

33. The nucleosome is
a. Found in nucleus
b. Composed of RNA
c. Composed of DNA and RNA
d. Found in cell way
e. None

34. Correct units of measuring DNA are ______ and ______ while units for NRA are
a. bp, nm/ bases
b. turns, nm / bp
c. S values, bases/ bp
d. A and B
e. A, B and C

35. A mutation that changes a codon from Trp into a stop codon will always be an example of
a. Framshift mutation
b. Missense mutation
c. Silent mutation
d. Neutral mutation
e. Nonsense mutation

36. How is the inducer of the lac op formed?
a. Actions of Lac Y
b. Activity of Lac A protein
c. Presence of mutant repressor protein
d. Presence of CAP
e. Activity of Lac Z protein

37. If glucose and lactose are present in the bacterial media the lac operon shld be turned on
a. True
b. False

38. The Oc mutation in the lac op is a recessive mutation
a. True
b. False

39. In Euks, transcription control is necessary because
a. Diff genes are present in diff cell types
b. Diff genes are transcribed in diff cell types
c. All genes are transcribe in all cell types
d. 2 of above
e. none

40. C-paradox means
a. Large genome- fewer genes
b. Small genome- fewer genes
c. Large genome- more genes
d. Small genome- more genes

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