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03-131 Genes, Drugs & Disease
Problem set 1
August 22, 2015
Problem Set 1: Answer Key
1. (4 pts, 5 min) What would happen if ribosomes were inhibited in cells? What could the cell not do?
It could not synthesize proteins.
2. (4 pts, 5 min) What would happen if a eukaryotic cell was missing the golgi apparatus? What could the
cell not do?
It could not export proteins out of the cell.
3. (4 pts, 10 min) A small virus like particle is isolated that is able to reproduce without the need to infect
host cells. Name two fundamental properties that this particle must have that a virus does not.
It must be able to do the following without reliance on a host cell (only two required):
i) replicate its genetic material
ii) undergo protein synthesis using ribosomes.
iii) utilize energy
4. (5 pts, 5 min) Viruses that have double stranded RNA as their genetic information do not require
conversion of the RNA to DNA for reproduction. What activity must be provided by the virus that is not
found in the host cell?
 Eukaryotic cells cannot copy RNA to RNA (they copy DNA to RNA).
 The virus must copy its RNA to RNA in order to form new particles.
 Therefore the virus must also provide and enzyme that copies RNA to RNA (in the same
fashion, HIV provides an enzyme that copies RNA to DNA).
5. (5 pts, 5 min) What is the normal charge (most stable state) for a Mg ion? Be sure to justify your
answer by discussing the distribution of electrons in shells.
The electronic configuration of Mg is illustrated on the right, and
written as 1s22s22p63s2. The third shell contains only 2 electrons,
but it can hold a total of 8 in the 3s and 3p orbitals. To reach the
same configuration as the inert gases, it could either gain 6
electrons to become similar to argon, making Mg a -6 ion.
Alternatively, it could lose 2 electrons to become similar to Ne,
making Mg a +2 ion. The latter is more stable, so Mg is a +2
ion.
6. (8 pts, 10 min)
i) Add the correct hydrogen atoms to the drawing on the right,
briefly justify your answer. See diagram, atoms in green.
chiral
center
ii) Identify any/all chiral centers in this molecule. There is one (as
indicated) note that this carbon is bonded to four different
groups. The atoms above the dotted line are different than
the atoms below the dotted line. The four different groups
are circled with dotted green lines.
iii) Does this molecule contain an aromatic group? If so, indicate by
circling the group. (not required to answer, but it is circled in
red)
7*. (5 pts, 10 min) The chemical structures of the drugs Dexedrine
and Benzedrine are the same, and shown on the right. Both of these drugs have the identical physical
properties, such as melting point, molecular weight, etc. The dose of Dexedrine is 5mg/day, but the
dose of Benzedrine is 10 mg/day. Additional side effects are seen when patients take Benzedrine.
03-131 Genes, Drugs & Disease
Problem set 1
August 22, 2015
Why is twice as much Benzedrine required for the same biological effect and why are there reduced
side effects with Dexedrine? [Hint: Is there a chiral center in this compound?]
This drug contains a chiral center, a carbon that is attached to four
different groups (aromatic, hydrogen, amino, -CH3). Therefore there
are two enantiomers of the drug. One enantiomer is biologically active
and the other is not. Dexedrine consists entirely of the enantiomer
that is active, while Benzedrine consists of a racemic mixture (50:50)
of both enantiomers, so twice as much is required for the same effect.
Chiral
Important points/skill you should master from this problem set:
1. Fundamental properties of viruses, what enzymes they must provide to replicate.
2. Role of cellular components: ribosomes-protein synthesis, golgi-export.
3. Predict the charge on atomic ions. Important ions for this course are Cl-, Na+, K+,
Mg2+, Ca2+.
4. Complete the structure of a compound by adding hydrogen atoms.
5. Identify chiral centers in molecules – carbon bound to four different groups.
6. The presence of a chiral center on a drug raises the possibility that one of the
enantiomers is biologically inactive (in the case of Dexedrine/Benzedrine) or worse –
it is harmful (thalidomide, discussed in lecture).