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Exam 7: Dna: Structure and Replication

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Question 21

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Imagine it is the year 2050.Because the Martian landers discovered that liquid water was present on Mars,another probe was sent to look for signs of life.Cells containing double-stranded DNA were found,and using an automated variant of the Meselson-Stahl experiment,the original DNA was labeled with ¹⁴N and then transferred to ¹⁵N (note that this is the reverse of the original M-S experiment).In the low temperatures of Mars,the replication machinery works slower,so each cell division takes 60 hours. Assume you do not know if replication machinery uses a dispersive,conservative,or semiconservative approach.Predict what the centrifuge tube patterns should be at 60,120,180,and 240 hours.

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The amount of DNA increases wi...

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Question 22

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The Meselson-Stahl experiment made clear predictions regarding experimental outcomes if dispersive,conservative,or semiconservative DNA replication was occurring in their Escherichia coli cells.To begin the experiment,cells were grown for a period of time in media containing __________ to label the chromosome.

A) heavy nitrogen,¹⁵N
B) radioactive phosphate,³²P
C) chemically modified nitrogenous bases
D) only sulfur and no nitrogen
E) None of the answer options are correct.

F) A) and B)
G) C) and D)

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Question 23

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A-form DNA has very little external exposure of its nucleotide bases as compared to the B-form.B-DNA is the biologically significant form.Why do you think this is so?

Primase and telomerase enzymes are both considered types of:

Initiation of replication occurs at an "origin of replication" site that typically includes an AT-rich region.Initiation benefits from these AT-rich regions because:

Telomeres have been a recent focus in experiments regarding both aging and cloning.For example,it has been shown that in the absence of telomerase,the chromosome becomes shorter and shorter after each cell division.When chromosomes reach a certain length,the cell may cease to divide and die.Hence,maintaining telomerase action could be a "fountain of youth" for a cell. Excess telomerase activity is associated with cancer cells,providing unlimited numbers of cell divisions (immortality)to these cells and presenting a dangerous threat to the organism. Find a recent news or research article on telomeres,and discuss what the experiment tells you about the role of telomerase in the cell.Could activation of this enzyme in humans be beneficial with regard to age-related disease? Hint: "http://www.wired.com/wiredscience/2010/11/mouse-aging-reversal/ (and similar articles)

The Meselson-Stahl experiment made clear predictions regarding experimental outcomes if dispersive,conservative,or semiconservative DNA replication was occurring in their Escherichia coli cells.This experiment enables the detection of "new" and "old" DNA by assessing the _______________ of DNA molecules in the cells.

What structural feature of DNA suggests a possible method of its replication?

In eukaryotic DNA replication,re-association of histones with newly formed DNA is accomplished by:

Evidence of transformation of R strain cells by biomolecules from S strain cells includes:

The spontaneous (though infrequent) change of a nitrogenous base to a slightly distinct form is called:

Alfred Hershey and Martha Chase examined transformation using bacteriophage (bacterial DNA virus) and bacterial cells.If phage are labeled with radioactive sulfur and allowed to infect bacterial cells,the radioactive sulfur will be localized to:

Mendel identified evidence for the location of genes on separate structures (chromosomes) through his:

The number of proteins in the prokaryotic replisome is 13,while the number in a eukaryotic cell is 27.What are important protein components of the replisome in general? Why might a eukaryotic cell have more components?

Oswald Avery and colleagues strengthened scientific support that DNA was the transforming factor by replicating the Griffith experiment with some important differences in experimental design.They found that the enzyme ______________ was effective at destroying the transforming capacity of S strain biomolecules.

An epidemic disease affecting sheep grazing near the hot springs of Thermopolis,Wyoming,was found to be due to a new virus.This virus could infect sheep kidney cells cultured in vitro.The virus appeared to contain four chemically defined biomolecules,which we will call W,X,Y,and Z.Investigators wished to determine which of these components carried the genetic information of the virus. An experiment was conducted in which one of the four components was radioactively labeled in different batches of kidney cells cultured and infected with the virus.Thus,four batches of labeled virus were generated (labeled W,X,Y,or Z). Radioactive virus from each of these batches was allowed to attach to nonradioactive kidney cells.The cells were centrifuged down to remove the unattached viruses.The cells were then briefly exposed to a vigorous agitation to release viral particles on the cell surface,and again centrifuged.The supernatant (containing shaved-off viral parts)and the pelleted infected cells were examined for radioactivity,with the following results: % Radioactivity $\begin{array} { | l | l | l | l | l | } \hline \text { Labeled component: } & \mathrm { W } & \mathrm { X } &\mathrm { Y } & \mathrm { Z } \\\text { Supermatarit } & { 100 }&0&0&0\\\text { Pelleted cells } & 0 & 100 & 100 & 100 \\\hline\end{array}$ a)On the basis of these results,which of these components does NOT carry genetic information? Why? Other experiments revealed that one of these purified biomolecules was contaminating sheep DNA,and the two remaining components did appear to be derived from a virus.The nitrogenous base compositions were as follows: $\begin{array} { | l | l | l | l | } \hline \text { Labeled component: } & \mathrm { X } & \mathrm { Y } & \mathrm { Z } \\\% \mathrm {~A}& 21 & 26 & 28 \\\% \mathrm { G } & 21 & 32 & 22 \\\% \mathrm {C } & 32 & 21 & 22 \\\% \mathrm { T } & 0 & 21 & 28 \\\% \mathrm {U} & 26 & 0 & 0 \\\hline\end{array}$ b)What can you say about the type of nucleic acid found in each of these three components? c)Can you determine which of these purified biomolecules is most likely sheep DNA and which are more likely to belong to the virus? Explain your response.

In the classic experiment by Griffith,evidence of the action of a hereditary biomolecule was identified by:

Based on its association with chromosomes,this was once considered the hereditary molecule before the discovery of DNA.

When comparing the three key models of DNA replication,the model that included the separation of the two strands of the original DNA (template) and using those strands as templates to synthesize two new DNA strands is called:

In 1959,Arthur Kornberg isolated DNA polymerase activity from Escherichia coli cells,revealing its ability to copy DNA by mixing ___________________ in a tube and measuring synthesis of a DNA strand.