Dr. Reichlerís Bio 212    Name:___________KEY_____________
Exam #2  26 June 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.  What are telomeres, and why are they needed?
Repetetive, non-coding DNA at the ends of linear DNA strands.  DNA polymerase leaves gaps when the DNA is replicated, and so the DNA gets shorter each replication.  The telomeres serve as a buffer so that the DNA can get shorter without losing genetic information.

b. What does telomerase do, and how (in general) does it do this?
Adds telomeres.  Telomerase is a protein with an RNA component.  It uses the RNA as template to make the telomere.

2. Most cells of your body have identical DNA, so what differentiates two different types of cells?  (5 pts)
differential gene expression

3. a.  Humans can only perceive part of the light spectrum.  What restricts our ability to perceive light outside these limits?  (5 pts)
Lack of receptors for these wavelengths.

b. What is one reason that signals go through multiple steps between perception and response?
Any one of:  amplify signal, signal specificity

4. Briefly describe how a gene could be up-regulated (increase the gene product) by the following methods:
Answer 3 of the 5, each one is worth 6 pts for a total of 18 pts for question #4.
a. genomic control
Any one of:  Acetylate near the gene or Demethylate the gene

b. transcriptional control
Any one of:  add an enhancer, remove a repressor, presence of necessary transcription factors

c. post-transcriptional control
Decrease mRNA degredation

d. translational control
Increase binding of ribosome to mRNA

e. post-translational control
Any one of:  decrease protein degradation, make active protein from inactive protein

5. For the following questions choose which of the gene expression control points would be best for the situation stated and explain why you made that choice.  Some of these have multiple possible correct answers.  The WHY is the critical part of your answer.  Some of these situations are hypothetical.
Answer 3 of the 4, each one is worth 6 pts for a total of 18 pts for question #5.

a. A plant-produced protein called "Plomoaway" detoxifies lead from inside plant cells by binding to the lead.  But Plomoaway, when not bound to lead, is itself toxic to the plant.  The plant cells can only perceive the lead once it enters the cell.
Make protein in inactive form and then activate it when lead is present.  Need protein quickly to counter-act lead toxicity, but can not keep protein in active form due  to bad effects of protein itself.

b. A protein called "DNAmover" that is only needed during cell division.
Any one of:  Make mRNA but only translate when cell enters division.  Make protein in inactive form and activate during cell division.  Gene expression decreases during cell division, so need to make mRNA or protein prior to cell division.
 Acetylate the histones and package the gene so it is available during cell division.  The reason gene expression is less during cell division is the higher level of DNA packing.  If the DNA for this gene is less packed, it can be expressed.

c. A protein called "Thumbsucker" that is needed in toddlers but never in adults.
Any one of:  Methylate or deacetylate the gene to turn it off at the genomic level.  The gene is not needed for much of the time, so it can be deactivated.
 Add a repressor to keep it from being expressed.  The gene is not needed, so keep it from being expressed.

d. Ribosomal RNA (the RNA component of ribosomes) that is needed at fairly constant levels throughout the life of a cell.
Any one of:  Control at level of transcription.  Keep a constant level by transcribing constantly.
 Decrease RNA degradation.  By keeping RNA around for a long time, it will be present throughout life of cell.

6. a.  When the type of mutation known as a substitution occurs within the DNA coding region of a protein, it may have no effect on the amino acid sequence of that protein.  How is this possible?
If the mutation is at the third base of a triplet code, changing that base may not change the amino acid sequence because some amino acids are coded for by multiple triplet codes.

b. How can the single insertion or deletion of a DNA base completely incapacitate the function of a protein?
By causing a frameshift mutation.  The reading frame is now out of order, which causes the rest of the sequence to be read incorrectly.

c. List three causes of genetic mutations.
Any three of:  replication errors, viruses, UV light, ionizing radiation, X-rays, chemicals

7. a.  Genes with what general function might be up-regulated in a cancerous cell?
Any one of:  telomerase, RAS, genes that stimulate cell division

b. Genes with what general function might be down-regulated in a cancerous cell?
Any one of:  P53, genes that inhibit cell division, genes that control contact inhibition

c. Where would you expect to find the mutation for a gene that is up-regulated in a cancer cell?
In the promoter/upstream/enhancer/repressor region