Homework 4 KEY


Homework 4 - KEY


1. ANSWER MUST BE HAND DRAWN. a. Draw out the following tetrapeptide, showing all the covalent bonds (including those in the side groups): N terminal - asp-trp-thr-lys – C terminal b. Circle all of the molecules in the backbone of the tetrapeptide that could hydrogen bond with one another to promote folding of the tetrapeptide into a secondary structure. c.


ANSWER MUST BE HAND DRAWN. a. Draw the amino acid glycine as it would appear in a very acidic aqueous solution (pH

< 2). b.

Draw squares around the two side groups that could interact with one another to form an ionic bond and promote the folding of the tetrapeptide into a stable tertiary structure.

Draw the amino acid glycine as it would appear in a very basic aqueous solution


Bio 2 Laboratory Page 1 Fall 2008


a. Draw out the disaccharide lactose. (ANSWER MUST BE HAND DRAWN.) b. Using whatever resources you need, find out what enzyme E. coli makes that enables it to break the glycosidic bond between the two monosaccharides in lactose to release glucose as an energy source.

The name of the enzyme is beta-galactosidase.


c. Why did a gene encoding this enzyme evolve in E. coli? (I.e. Why does E. coli need this enzyme while most bacteria do not?)


E. coli inhabits the digestive tracts of mammals. Young mammals subsist on milk from their mothers, which contains only lactose as a sugar. Since E. coli sometimes finds itself in the gut of a young mammal, it must be able to break down the lactose into glucose and galactose in order to survive. Therefore, it has evolved an enzyme that can do this. E. coli then uses the glucose as an energy source.

What is an aquaporin? How did the discovery of aquaporins help explain why cell membranes are so permeable (in general) to water, despite the fact that water is a polar molecule? (You will need to use the internet to research this question.)

Aquaporins are channels in the cell membrane (gated by proteins) that enable water to move freely across the membrane. Water has only limited solubility in the lipid bilayer and rapid osmosis would therefore not be possible without aquaporins.

In many biology textbooks, you’ll see the statement that DNA differs from RNA in that it is double-stranded while RNA is single-stranded. Is this really true? Explain.

No – RNA can and will fold back upon itself to form double-stranded regions. This

“intrastrand” base-pairing stabilizes the secondary structures of RNA molecules, which can be very important for their functions (e.g. tRNAs).

Bio 2 Laboratory Page 2 Fall 2008




6-8. More practice with unit conversions and common lab calculations. Try to work these problems without looking at your notes. Remember to show your entire calculation and to strike through units to receive full credit!

How many moles of glucose are in 10 mL of an 8% solution of lactose? (MW lactose = 342.3)

The average MW of a DNA base-pair is 650 and the number of nucleotides of DNA in a human cell is 6.6 billion. How many pounds would one mole of the human genome weigh? (1 pound =

454 g)

How much of a 7% solution of lactose would you need in order to make 100 mL of a lactose solution at 2 g/L?

Note that in this problem, you first have to convert the concentrations into the same units.

Then you can perform the actual calculation to determine the initial volume you will need.

Bio 2 Laboratory Page 3 Fall 2008