Chapter 4 Homework

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Biochemist Erwin Chargaff was the first to note that, in DNA, [A] = [T] and [G] = [C], equalities now called Chargraff's rule. Using this rule, determine the percentages of all the bases in DNA that is 20% thymine.
20% T
20% A
30% G
30% C
A single strand of RNA is 20% U. What can you redict about the percentages of the remaining bases?
20% A
20% U
30% G
30% T
The composition (in mole-fraction units) of one of the strands of a double-helical
DNA molecule is [A] = 0.30 and [G] =0.24. (a) What can
you say about [T] and [C] for the same strand? (b) What
can you say about [A], [G], [T], and [C] of the complementary strand?
If set equal these number would be equivalent to 108. Set the denominator to 100.
0.278=A
0.278=T
0.444=G
0.444=C

The complimentary strand will have the exact same ratio.
Why does heat denature, or melt, DNA in a solution?
Heat does this by breaking the hydrogen bond between the bases. This can also be done by adding acid or alkali to ioniz the bases.
What does it mean to say that the DNA chain in a double helix have opposite polarity?
This is a very simple concept to understand. Essentially the DNA molecule has a 5' and a 3' end. The 5' part end with a phosphorous group that has a negative charge. The 3' end has a positive charge because of the hydroxyl group.
If the forces-hydrogen bonds and stacking
forces-holding a helix together are weak, why is it difficult to disrupt a double helix?
The DNA molecule is stabilized by 4 things:
1) base stacking
2) hydrophobic effect
3) Van der Waals effect that is substantial in a large amount of interaction
4) Rigid back bone
DNA in the form of a double helix must be associated with cations, usually Mg. Why is this requirement the case?
This stabilizes the charges of the DNA molecule. The destabilizing part of DNA is the charge repulsion. Increasing salt concentrations (cations such as Na or Mg) mask these repulsions and increase stability.
Not quite from A to Z. Describe the three forms that a double helix can assume.
The three forms that helical DNA can assume is circular, relaxed molecule, and super coiling.
The DNA of a deletion mutant of l bacteriophage
has a length of 15 micrometers instead of 17 mm. How
many base pairs are missing from this mutant?
So remember that 1 Angstrom is equal to 0.001 micrometers. So 2 micrometers are missing which translates to 2000 angstroms missing from the DNA molecule. Each base pair takes about 3.4 A of space so 2000/3.4 = 588 BP are missing.
(a) Suppose that you want to radioactively label DNA but not RNA in dividing and growing bacterial cells. Which radioactive molecule would you add to the culture medium? (b) Suppose that you want to prepare
DNA in which the backbone phosphorus atoms are
uniformly labeled with 32P. Which precursors should be
added to a solution containing DNA polymerase and primed template DNA? Specify the position of radioactive atoms in these precursors.
Titrated thymine or titrated thymidine. b) I would make dATP, dGTP, dCTP, and TTP all labled with 32P and his would be on the Phosphorous.
What is a retrovirus and how does information
flow for a retrovirus differ from that for the infected
cell?
A retrovirus is a virus that uses RT to create a DNA version of it's own DNA then implants it into our DNA to create mRNA which will replicate it's own DNA then create proteins for the capsid.
Suppose that you want to assay reverse transcriptase activity. If polyriboadenylate is the template in the assay, what should you use as the primer? Which radioactive nucleotide should you use to follow chain elongation?
You should use a DNA primer. The thymine should be labeled since it is only in DNA.
Reverse transcriptase has ribonuclease activity as well as polymerase activity. What is the role of its ribonuclease activity?
The ribonuclease would degrade the RNA, something vital if the DNA should impant into the host's DNA.
Spontaneous deamination of
cytosine bases in DNA takes place at low but measurable
frequency. Cytosine is converted into uracil by loss of its amino group. After this conversion, which base pair occupies this position in each of the daughter strands resulting from one round of replication? Two rounds of replication?
One of the strands will result in a U:A pair being produced, the other strand will produce a C:G pair.
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