In compliance with the Dartmouth honor principle, all the work you hand in on this exam is to be your own. Please remember to be precise in your wording – scientific descriptions rely on accurate use of specific terms. Also, try to keep your answers concise. If you can say something briefly there is no need to create a lengthy answer just to fill up space. Use the space provided and the point values for each question as indicators of the amount of detail your answer should contain. If you really need extra space for an answer, turn the page over and continue your answer on the back of the same piece of paper. To facilitate grading, we separate the exam by page; if your answer is on a different sheet of paper we will not see it. Please put your name on each page now.

If something is not clear to you, please ask me during the exam. That is why I stay in the room. Good luck. - Prof. Gross

 

1.   What is meant by genotype and phenotype? (4 points) Which of these did Mendel study? (1 point)

Genotype is the set of genes an organism possesses. Phenotype is the set of measurable or recognizable traits that result from the genotype. Mendel studied phenotype.

 

2.   Give an example of a trait that does not follow Mendel's laws and explain why it does not. (5 points)

several examples are possible but they include the following (only one answer needed):

multigenic/polygenic traits that are the result of many genes contributing to the trait - such as height, shape of nose, etc.

incomplete dominance is the result of each allele making a product that contributes to the observed trait so that if only one allele is present, the trait is incomplete - such as in flowers that are red because they have two red alleles. If only one allele is present, half as much red pigment is made and the flower is pink

codominance results when each allele is expressed and that expression does not have an influence on the other allele. An example is the ABO blood types - each allele can make either A antigen, or B antigen, or no antigen. Red blood cells can therefore be AA, AB, AO, BB, BO, or OO.

sex linked traits are associated with gender - not a Mendelian inheritance

3.   What happens during S-phase of the cell cycle? (2 points)

The main event is chromosome duplication, which includes DNA replication.

 

4.   What is a nucleosome? (2 points) What function does it serve? (2 points)

A nucleosome is a particle consisting of 8 histone molecules and about 150 bp of DNA wrapped around the outside of the histone core. It is important because it allows the DNA to be compacted in the nucleus into a more orderly arrangement.

5.   In what way does the quality of ear wax relate to incidence of breast cancer? Explain your answer. (6 points)

Wet ear wax correlates with a higher incidence of breast cancer than does dry ear wax. Ear wax is produced by the apocrine gland system which includes the mammary glands as well. Apparently, the different alleles in the apocrine gland system that lead to ear wax 'quality' also influence susceptibility to breast cancer.

 

6.   Explain the process of crossing over during meiosis. (4 points)  Why is it important? (2 points)

Crossing over is a process in which corresponding parts of chromosome pairs (sister chromatids) can swap genetic information by having their DNAs cut and reconnected. It is important because it leads to more genetic diversity.

7.   Explain how chloroplasts are thought to have evolved in primitive eukaryotic cells hundreds of millions of years ago. What evidence supports this theory? (7 points)

Once upon a time, a primitive eukaryotic cell engulfed a prokaryotic cell much like today's blue-green algae. This prokaryote had the ability to conduct photosynthesis. Initially, there was a symbiotic relationship and the host cell provided nutrients to the engulfed prokaryote while the engulfed prokaryote provided food for the host eukaryotic cell through photosynthesis. Many years passed and over thousands of generations the arranged became fixed in place as the prokaryote lost some of its DNA while developing the ability to duplicate itself within the host cell. Current evidence in support of this is that the chloroplast contains its own DNA that is replicated independent of the host nuclear chromosomal DNA. The chloroplast codes for some of its own proteins (e.g. ribosomal proteins and enzymes involved in photosynthesis). The chloroplast also has a double membrane around it, with the inner membrane resembling that of a prokaryote with the outer membrane resembling that of the host cell's plasma membrane - exactly the arrangement one would expect if the evolution was as described.

 

8.   Describe the experiment done by Avery and his colleagues to demonstrate that DNA is the genetic material. (6 points)

R and S pneumonia bacteria; S is lethal to mice, R is not. Heat killing S makes them non-lethal. Mixing heat killed S with R, and then injecting into mice is lethal. The R strain picked up something form the cell debris of the killed S strain. Treating the killed S with DNAase eliminated the Òtransforming' ability of the extract, but proteinase did not - the 'transforming factor' therefore had to be DNA.

 

 

9.   Diagram and label how rho-factor functions in prokaryotic transcription. (5 points) In words, rho factor interacts with the core RNA polymerase during transcription and changes its shape. The new shape allows the core-rho complext to terminate transcription at the termination site.

 

10. What are telomeres? (2 points) Why are they important to the life of the cell? (2 points)

Telomeres are the ends of chromosomal DNAs. They consist off 100s to 1000s of repeats of a short DNA sequence (typically 6 nts). They are important to life because after each DNA replication, the length of the telomeres gets shorter. After a certain number of cell divisions the telomeres are too short and the cell will no longer divide.

 

11. Define apoptosis (2 points) and explain why it is important. (3 points)

Apoptosis is programmed cell death. It is important because it is an essential part of normal embryological development and is important in other processes like wound healing in individuals.

 

 

12.  We discussed the disorder phenylketonuria (PKU), which leads to the inability to metabolize phenylalanine, and therefore to higher phenylalanine levels in the bloodstream. Testing of newborns for high phenylalanine levels can result in them being placed on special (low phenylalanine) diets to avoid most of the complications of the disorder. When this approach was taken back in the 1970s it lead to some unexpected consequences years later. Explain what happened later on and how you think the situation should be dealt with. [note that there is no particular Òright' answer, but you are expected to explain your thoughts clearly.] (15 points)

      Women with PKU reached reproductive age and started having children (previously, they were in mental institutions and did not have children). Because of the elevated phenylalanine levels in their blood, the fetuses suffered damage and the children of these women had some serious difficulties. There are many possible answers to what to do about this situationÉ ranging from forced sterilization of the women to nothing being done, and almost every idea in between these two. Any of the choices really come down to the role of the government (or the population as a whole) balanced against the rights of the PKU women and their families.

  

 

13. Meselson and Stahl performed experiments to determine if DNA replication was conservative, dispersive, or semiconservative. State which of these hypotheses turned out to be correct (2 points) and simply explain what the process is. (2 points)

Replication is semiconservative. This means that the original DNA double helix splits into two strands, with each strand remaining intact and serving as a template for the synthesis of a new DNA strand.

 

 

 

14. Using B and b, give the likely genotype of the individuals in the figure at the left: (6 points)               A-1: BB               A-2: bb               B-1: Bb               C-1: Bb               D-1: bb               D-3: Bb or BB

 

 

15.  What are nuclear pores and what is their function? (3 points)

A nuclear pore is a structure on the surface of the nucleus that spans the nuclear envelope. It has a distinct structure (it is not just a hole) that serves to regulate what can pass into and out of the nucleus.

 

16. Allelic diversity is illustrated by HLA antigens. Explain how HLA antigens are involved in organ rejection after transplantation surgery. (4 points)

Allelic diversity refers to the fact that there are many different alleles of the HLA antigens in the population as a whole. However, each individual can have at most two different alleles (we are only diploid). This means that the chances of two individuals (e.g. a donor and a recipient for organ transplant) are not very likely to have the same pair of HLA-A and HLA-B antigens. Having mismatched HLA antigens means that the transplanted organ will be rejected as a Òforeign' body and will be attacked by the recipient's immune system.

 

17. What do lysosomes do? (2 points)

Lysosomes contain many digestive enzymes and can digest material (food) taken in by the cell.

 

18. Base composition of DNA is important. Would you expect to see a high or a low AT content in bacteria that live in hot springs? Explain your answer. (3 points)

Bacteria that live in hot springs need to keep their DNA stable. This would mean that they would want to have high GC content and low AT content because the AT basepairs only have 2 hydrogen bonds while the GC basepairs have 3. More bonds between the basepairs mean a more stable structure.

19. What is the role of a topoisomerase? (2 points)

A topoisomerase is an enzyme that changes the topology of the DNA. In particular, it can cut and rejoin DNA during replication to relieve the torsional stress caused by unwinding the double helix that occurs in replication.

 

 

20. A pulse-chase experiment allows scientists to track the fate of molecules within cells.

a. Explain how this works. (3 points)

A cell is treated with a labeled precursor (typically a nucleotide or an amino acid) for a short time. It is incorporated into a product by the cell (typically DNA or protein). The cell is then removed for the medium containing the labeled precursor and allowed to grow in non-labeled medium. Over time it is then possible to follow the labeled material in the cell and determine its behavior (fate).

 

b. The figure at the left shows a time course during a 'chase' period. Before these images were taken, the cells were labeled with radioactive leucine during the 'pulse.' The black dots show the presence of radioactivity. Explain what is happening in these images. (3 points) Radioactive proteins were synthesized incorporating the leucine during the pulse period. Thus, the chase shown in the figure is following the fate of proteins synthesized during the pulse. During the chase the protein (the radioactivity) moves from the endoplasmic reticulum through the Golgi into vesicles and then out of the cell.

 

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