Wild type E. coli metabolizes the sugar lactose by expressing the enzyme ß-galactosidase. You have isolated a mutant that you call lac1–, which cannot synthesize ß-galactosidase and cannot grow on lactose (Lac–). During an condition isolate a mutation that constitutively expresses abnormally high levels of ßgalactosidase, which you designate lacc. Preliminary P1 transduction experiments indicate that lacc  is linked to the Tn5 insertion.  To map lacc  relative to lac1– you set up two reciprocal crosses. In the first cross you grow P1 on a strain that carries the Tn5 insertion and the lac1– mutation. You then use this lysate to infect a lacc mutant and select for Kanr. From 100 Kanr transductants examined, 20 are Lac–, 76 express ß-galactosidase constitutively and 4 show normal ß-galactosidase expression. In the second cross you grow P1 on a strain that carries the Tn5 insertion and the lacc mutation. You then use this lysate to infect a lac1– mutant, and select for Kanr. From 100 Kanr transductants examined, 81 are Lac– and 19  express ß-galactosidase constitutively. So what will be the correct order of  the Tn5 insertion and the lac1– and lacc  mutations. Express any measured distances as cotransduction frequencies.

#Question id: 2917

#Unit 2. Cellular Organization

With respect to the interconversion between open and condensed chromatin shown below: Which of the directions (i) or (ii) would you anticipate would be the consequence of the following types of chromatin modification?

#Question id: 2918

#Unit 2. Cellular Organization

Some DNA sequences in our cells have high frequencies of methylated cytosines (hypermethylation); some others have a low frequency of methylated cytosines (hypomethylation). Which of the two methylation states best describes the kind of sequences listed below? 

 I. Satellite DNA in pericentromeric heterochromatin

II. Promoters 

III. Dispersed transposon repeats

IV. CpG islands 

V. Enhancers

#Question id: 2919

#Unit 2. Cellular Organization

Topoisomerases play the essential role of unlinking the DNA molecules to allow them to separate into the two daughter cells for cell division. Which of the following statements regarding the mechanism of topoisomerases is INCORRECT?

#Question id: 2920

#Unit 2. Cellular Organization

Which of the following statements is INCORRECT with respect to the organization and content of the human genome?

#Question id: 4256

#Unit 2. Cellular Organization

With respect to CpG islands in our genomic DNA, which, if any, of the following descriptions do not apply?

1) frequently occurring (there are about 30,000 in the human genome).

2) long DNA sequences (typically from 10 kb to 100 kb in length).

3) low CG dinucleotide frequency.

4) frequently associated with transcriptional start sites.

#Question id: 19984

#Unit 2. Cellular Organization