Biology Exam

BIOLOGY 202 FIRST MIDTERM EXAMINATION, page 1/7 FEB 12, 2009

ã Write your name here, and on

EVERY page of this exam. Name: ______________________________________________

________________________________________________________________________________________________________________

You may use the entire 80 minute exam period, but should Page Question Points

probably spend at least half of this time thinking rather than 1-2 1 20 ________

writing. Be sure to read the exam carefully, making certain 3-4 2 20 ________

that you answer all parts of each question. 5-6 3 20 ________

Answers should be kept to the space provided, using extra 7 4 12 ________

space on the back of the page only if absolutely needed. 8-9 5 16 ________

Please keep your answers brief and to the point, as you may 10 6 12 ________

be penalized for excessively rambling, irrelevant comments! ________

Most questions should be answered in no more than 2-3

sentences.

Total: 100 ________

1. (20 points)

a) Describe the hydrogen bonding that stabilizes all -helices.

This bond involves the C=O oxygen of one amino acid and the N—H hydrogen of a second amino acid,with the latter being 4 amino acids C-terminal relative to the former.

b) Does this hydrogen bonding described in a) occur within a helical chain, or between adjacent helical chains?

within

c) In reference to your answer in a), describe why a helical polypeptide is characterized by what is known as a helical dipole?

The small negative charge that occurs due to resonance on the C=O oxygen is normally compensated by the corresponding positive charge on the N—H nitrogen — this compensation is due to the hydrogen bonding described in a). However for the 4 C-terminal amino acids of an −helix, there is no

corresponding N—H to hydrogen bond to, and hence there is a negative charge at this end of the helix. Likewise, there is a positive charge at the N-terminal end of the helix.

d) For the corresponding hydrogen bonding that...