Dr. Reichlerís Bio 212   Name:_________KEY_________________
Exam #3  6 July 2001

Answer each question as succinctly as possible.  You may use the back if you need more space than is provided below the question.  Each question is worth 6 pts, unless otherwise noted, for a total of 100 points.  Read each question carefully, and donít hesitate to ask if a question seems unclear.  You may use drawings to answer some of these questions, but be sure to include a written description in addition to the drawing.

1. a.  If you were studying an organism, without knowing the sequence of the DNA, what evidence could disprove the hypothesis that this organism is haploid?
It undergoes meiosis.

b. If you were studying an organism, without knowing the sequence of the DNA, what evidence could disprove the hypothesis that this organism is diploid?
There are an odd number of chromosomes.  Or the cell joins with another to form a zygote.

2. a.  In the absence of functional microtubules, what parts of mitosis would and would not take place?
Would:  chromosomes would condense, nuclear membrane would dissolve.
Would not:  no movement of chromosomes, no separation of chromosomes, no cell division

b. If you are watching a cell undergo mitosis, how would you know when mitosis had ended?
The cell divides, and the chromosomes decondense.

c. What two parts (not specific genes) of a chromosome are absolutely necessary for a cell to successfully complete mitosis?
Origin of replication, centromere
3. a.  If you were watching a cell proceed through meiosis, what would be the first indication that the cell is not going through mitosis?
Chromosomes gather as tetrads.

b. At the end of meiosis, what does one know definitively about the cells?
They are haploid.

4. a.  What is one signal that might serve as a checkpoint for a cell to enter S-phase?
Any one of:  cell size, quantity of nutrients, external signal

b. Briefly describe the interaction between cyclin dependent kinase (CDK) and cyclin in regulating mitosis.
CDK maintains a constant level throughout the cell cycle.  Cyclin decreases after mitosis, and then slowly builds up more and more.  When there is sufficient cyclin, it binds to CDK forming MPF that signals the cell to enter mitosis via phosporylation.

5. If genotypes give rise to phenotypes, how come identical twins (who share identical genes) are not exactly the same?
Environmental factors mediate between genes and traits.

6. Internetisosis is a recessive genetic disease of humans.  The dominant gene is labeled "I" and the recessive gene "i".  People who are afflicted with this disease get fidgety and nervous if they do not spend 20 hours per day surfing the internet.

a. What is the genotype of someone with Internetisosis?  (4pts)

b. If Internetisosis is a sex-linked trait and the genotypes of the parents are XI:Xi and XI:Y, what are the chances that a son will be born who has the disease?  What are the chances that a daughter will be born who has the disease?
Son:  50%
Daughter:  0%

c. If Internetisosis is dominant and has incomplete dominance, what might be a phenotypic difference between someone with II and Ii genotypes?
Only need 10 hours of internet.

d. In this example Internetisosis is subject to genomic imprinting.  The dominant allele for Internetisosis is methylated in males.  If the genotypes of a couple are male: Ii and female: Ii, what are the possible genotypes and phenotypes of their offspring?
Genotypes:  II, Ii, Ii, ii
Phenotypes:  50% have Internetisosis, 50% don't.

e. If the gene for Internetisosis is carried on the mitochondrial DNA, How would this effect its inheritance?
Only can be inherited from mother.
7. a.  What is recombination, and when does it occur?
Exchanging of parts of chromosomes that occurs during meiosis.

b. Describe the mechanism for recombination?
Enzymes cut the DNA of a homologous pair of chromosomes, exchange the DNA, and religate the cuts.