Q1. The functional lac repressor is a homoteramer. Still, the X-ray structure of lac repressor bound to the lac operator sequence centered at +11 reveals that the two half sites that constitute the complete operator sequence with two-fold rotational symmetry are each bound by a monomeric subunit of the repressor. If repressor tetramer uses only two of its subunits to bind to an operator sequence, why is tetramer formation so important to the control of lac operon gene expression?

Q2. The counter current exchange respiratory system of fish, which of the next would not be expected?

1. Blood flows through the gills from anterior to posterior. 
2. Water travels over the gills from anterior to posterior. 
3. In the back (posterior) portion of the gill, amount of carbon dioxide in the gills is higher than the water. 
4. In the front (anterior) portion of the gill, amount of oxygen in the gills is lower than the water.