Download MCDB 1030 – Spring 2005

MCDB 1030 – Spring 2005
Homework Set 2
Translation, Mutations, and Natural Selection
1. What are three types of RNA molecules? What roles do they serve?
mRNA: carries nucleotide coding information in a single-stranded form to the ribosome
where translation of the code into amino acid sequence occurs.
tRNA: small nucleic acid “adapter” molecule that has both a specific anticodon sequence
that complements a codon sequence in mRNA and an attachment site for the amino acid
specified by that codon
rRNA: catalytic molecule that forms part of the structure of the cytoplasmic ribosome.
2. a) Starting at the “start codon” in the following mRNA, what are the corresponding
anticodons in the tRNA for each triplet code? (A real mRNA would be much longer!)
5’ CGA-AUG-CAC-CGA-AGG-UAU-GAG-GGA-UGC-UAA 3’
UAC GUG GCU UCC AUA CUC CCU ACG AUU
b) Show the DNA that would encode this mRNA.
3’ GCT-TAC-GTG-GCT-TCC-ATA-CTC-CCT-ACG-ATT 5’
5’ CGA-ATG-CAC-CGA-AGG-TAT-GAG-GGA-TGC-TAA 3’
c) What polypeptide is specified by this mRNA?
Methionine or f-methionine – histidine – arginine – arginine – tyrosine – glutamic acid –
glycine –cysteine
3. What is a silent mutation? Give an example and explain using the mRNA shown above.
A change in the DNA sequence that does not lead to a change in the amino acid sequence.
e.g. the amino acid histidine is CAC. If the CAC in the DNA changed to CAT, the
resulting mRNA would be CAU. Since this codon still encodes histidine, there would be no
change in the protein.
4. What is a missense mutation? Give an example and explain using the mRNA shown above.
A change in the DNA sequence that does lead to a change in the amino acid sequence.
e.g. histidine is coded by mRNA sequence CAC. If the CAC in the DNA changed to CAG,
the resulting mRNA would have CAG, and the histidine would be replaced in the amino
acid sequence by glutamine.
5. What is a nonsense mutation? Give an example and explain using the mRNA shown above.
A change in the DNA sequence that leads to termination (early stop) in amino acid
sequence.
e.g. tyrosine is coded by mRNA sequence UAU. If the TAT in the DNA changed to TAG,
the mRNA would change from UAU to UAG, which is a stop codon.
6. What are the three most important things you learned from reading Ch. 11 of Guns, Germs,
and Steel?
7. List 3 important differences between bacterial and eukaryotic cells.
Any three of the following
i.
ii.
iii.
iv.
v.
eukaryotic cells have nuclei; bacteria don’t
bacteria have a cell wall containing proteoglycan; some eukaryotes don’t – if
they have a cell wall, it is composed of a different type of polysaccharide
bacteria are generally much smaller than eukaryotes
eukaryotes have a much more complicated cytoplasm, which contains multiple
membrane-bound organelles
other possibilities from Table 3.5
8. Examine Figure 3.13 and describe the major differences between the cell walls of Gram
negative and Gram positive bacteria.
Gram negative bacteria have a single membrane surrounded by a thick layer of
peptidoglycan.
Gram positive bacteria have a thin layer of peptidoglycan sandwiched between two
membranes.
9. What is the difference between endotoxins and exotoxins?
Endotoxins are lipid molecules released from the membranes of dead Gram-negative
bacteria that cause fever, inflammation, shock, vasodilation (dilation of the blood vessels),
and blood clotting. Exotoxins are protein toxins that are secreted from living pathogens.