Chem 30B KEY

3rd hour exam Seager Chpts 8 - 11

May 14, 2001 107 points

A. Fill in the blanks:

[2] 1. The DNA sequence CCGCTATTAGCT codes for a peptide containing 4 (give a number) amino acid residues.

[2] 2. Before a cell can split into two daughter cells it must first replicate its DNA.

[2] 3. Anticodons are located on what specific kind of molecules? t-RNA

[2] 4. When a triglyceride is hydrolyzed what functional group is broken down ?

ester

[2] 5. The RNA that carries the information which determines the primary structure of a polypeptide chain is called m-RNA

[2] 6. A newly discovered hormone is found to be fat soluble. It is most likely a member of the specific family of chemicals called steroids .

[2] 7. Hydrolysis of a protein involves a change in which level of protein structure?

Primary

[2] 8. The most common lipid in our diet is triglyceride .

[2] 9. The only covalent bond (linkage) in the 3∞ structure of a protein is disulfide link

[2] 10. A beta pleated sheet is held together by hydrogen bonding

[2] 11. The most important difference between a coenzyme and a prosthetic group is

The prosthetic group is held much more tightly.

[4] 12. The flow of information in molecular biology (sometimes known as the "central dogma") can be represented by DNA ---> RNA ---> Protein

[4] 13. The codon UGU is complementary to the triplet ACA on DNA and to the anticodon ACA.

14. [2] a. The principal kind of chemical compound in biological membranes is

phospholipid .

[2] b. The other two kinds of compounds also present in these membranes are

proteins & cholesterol /34

Part B 1. [5] Explain why many amino acids have more than one codon.

A codon is a set of 3 bases. There are 64 possible combinations of 3 bases, so there are 64 codons. But there are only 20 amino acids, so more than one codon must correspond to each amino acid.

2. [6] Small carboxylic acids and amines are liquids at room temperature, but even the smallest amino acids are solids. Explain what aspect of amino acid structure is responsible for this difference.

Amino acids are predominantly in zwitterion, anion or cation forms. The opposite chages of ions attract each other strongly and make pure amino acids soldids

3. [6] Draw the structure of a basic amino acid in a neutral solution (pH 7), clearly showing its charges.

4. [8] Draw the structure of a soap molecule and briefly explain its cleansing action.

Soaps emulsify greasy dirt. The hydrophobic tail dissolves in the grease while the hydrophilic (ionic) head remains in the water.

5. [8] Describe the interaction of an allosteric enzyme and its modulator. Draw and label a simple sketch to support your description.

The modulator can either speed up or decrease the activity of the enzyme.

6. [9] Which one of the forces holding together the tertiary (3∞) structure of a protein is the main one involved in each of the following situations? Briefly explain each choice.

a. The activity of an enzyme is measured at pH 6 and at pH 8. The measured activity is much greater at 8 than at 6.

Salt bridges

b. A reducing agent is used as part of a permanent wave .

Disulfide links

c. A protein is denatured by putting it into a non-polar solvent like hexane.

Hyrdophobic/hyrophilic interactions

7. List and briefly describe three substantially different chemical structures in which hydrogen bonding plays a significant role. Draw a simple diagram or chemical structure to illustrate each example.

MANY POSSIBLE ANSWERS

Complementary base pairing: DNA/DNA, DNA/RNA, RNA/RNA

2∞ protein structure: Alpha helix and beta pleated sheet, backbone H bonding

3∞ protein structure: side chain H bonding

Solubility of sugars in water

Boiling points of water or alcohols

8. [10] Explain in detail why and how a small change in the pH of a solution can cause a major change in the activity of an enzyme.

Adding acid or base will change the salt bridges in the protein structure. Adding acid (lowering pH) increases the + charges or decreases the ≠ charges. Adding base (raising pH) will do the opposite. Since the salt bridges are one of the forces holding together 3∞ and 4∞ structures, changing them changes the shape of the enzyme which changes its activity.

9. [6] Draw a simple sketch (diagram) of an RNA nucleotide. Label each part.

/16