BIOL 114
Biology Dept
Kenyon College
Homework 1

1.  Explain how cell division differs between bacteria and eukaryotic cells.

Bacteria and eukaryotic cells both have double-stranded DNA and replicate bidirectionally from the origin of replication.  However, they differ in that:

  • Bacterial DNA is usually one circular chromosome.  Eukaryotes have multiple linear chromosomes, in homologous (non-identical) pairs.
  • Bacteria have only one origin (ori) site.  Eukaryotic chromosomes have multiple ori sites.
  • When bacteria are growing fast, they start the next round of replication before the first finishes.  Cell division goes on simultaneously with DNA replication.  In eukaryotes, the first round of replication is completed in S phase of interphase before starting cell division in mitosis.
  • Bacterial DNA is bound to certain DNA-binding proteins, but eukaryotic DNA is more extensively bound around histone complexes.  During mitosis, eukaryotic DNA is extensively supercoiled and condensed; bacterial DNA is not condensed.
2.  The Sheik of Qatar gives you a priceless India Blue peacock in honor of your contributions to wildlife conservation.  Upon inspection, you notice that the peacock has three white feathers.  Propose a hypothesis, and test your hypothesis by interbreeding cocks and hens.  If your hypothesis is correct, what results will you get for two generations?

The simplest possibility is incomplete dominance.  If you cross the peacock with India Blue peahens, half their offspring will be straight India Blue and half will be mixed (a few white feathers).  This may be hard to tell in the hens, however, as the white feathers would be less noticeable.  But if you do find a mixed peahen, and breed it with a mixed peacock, 1/4 of offspring will be pure white.

3. (corrected)  In Flowers, explain the inheritance pattern of all flower petal colors.  How can a cross between a pink flower and a blue flower produce four different colors?

This works like A, B, O, blood type.  Pink > Orange, Blue > Orange, Pink is codominant with Blue.

To get the cross to work, it helps to first cross Blue with Orange, and Pink with Orange.  Then you know that all the Blue and Pink offspring each are "hiding" an orange allele.
Then cross the Blue(orange) and Pink(orange) together.  The result is 1:1:1:1 Turquoise, Pink, Blue, Orange.

4.  The phenotype of fused finger joints is dominant, yet the trait occurs in children whose parents are both normal.  How can this be explained?  Upon testing, suppose one of the parents turns out to have fused toes.  How can this be explained?

The fused finger joint trait shows only partial penetrance.  Therefore, although it is dominant (only one copy of the allele is needed) the trait does not always appear.

The trait also shows pleiotropy (more than one effect).  Thus the trait can include fused toe joints as well as fingers.  Both aspects of the trait show partial penetrance.  In the case of toes, we have the added real-life complication of self-reporting: The parent reports normal because the toes cause no difficulty, though from a clinical standpoint the trait is expressed.

5. What is the "histone code"? How might the histone code be involved in the phenomenon of question 4?

The histone code involves chemical modifications to DNA-associated histones (or to DNA itself, as in the case of methylation.) Acetylation of histone usually turns a gene on (enables transcription to make messenger RNA). Methylation of histone (or of DNA itself) usually turns a gene off. Phosphorylation may either enhance or repress gene expression. Histone modifications might differ between individuals who share the same allele sequence, such as the allele associated with an extra finger. It is possible that the penetrance or expressivity of an allele could be affected by histone modification.