In-class Exam #2 23 October 2001

Answer each question as succinctly as possible in the space provided. You may use the back if you need more space. If you use a drawing as part of your answer, be sure to also include a written explanation. Each question is worth 3 pts, unless otherwise noted, for a total of 100 points possible for this exam. Read each question carefully, and donít hesitate to ask if a question seems unclear.

1.  a. Of the following cellular parts, which one could cells NOT survive without? Explain.
(endoplasmic reticulum, golgi apparatus, nucleus, ribosomes, or vacuole) (4pts)
Any one of:
ER: without transmembrane protiens cell can not control transport in and out, without lipids can not make membrane.
GA: modified and membrane proteins critical to transport etc.
Nucleus: contains DNA needed to make proteins etc.
Ribosomes: needed to make proteins.
Vacuole: toxins that were stored in the vacuole could build up and kill the cell.

b.  What are two differences between animal and plant cells? (4pts)
Any 2 of: Plants- chloroplasts, cell wall of CELLULOSE, Plasmodesmata
            Animals- no chloroplast, ECM of PROTEIN, no plasmodesmata

c.  How are cytoplasmic ribosomes functionally different from the ones found on the endoplasmic reticulum?
Ctyoplasmic are making soluble proteins and ER ribosomes are making protiens destined to be transmembrane or secreted.

d.  Calcium concentrations are generally low inside cells, and high in the extracellular matrix. What type of transport protein is needed to maintain this situation, and what type is needed if the internal calcium concentration is to be increased? (4pts)
To maintain need pumps to move against conc. gradient. To increase internal calcium concentration need channels to allow diffusion into cell.

2.  a. According to table 1 on page six, what is the exclusion limit of the plasmodesmata?
Between 537 and 850 (Mr)

b. According to table 2 on page six, what is the exclusion limit of the plasmodesmata 5 hours after virus infection?
About 4,400 Mr

c.  How does the virus change plasmodesmata size?
Movement protein is produced that interacts and changes the size of the plasmodesmata by changing the ER that passes between the cells.

d.  Why does it take 5 hours to see the change?
Movement protein must be synthesized; gene expression takes time.

3.  a. Why are enzymes critically important to the chemical reactions occurring inside the human body?
They allow chemical reactions to take place at "normal" temperatures By lowering the energy of activation.

b.  Why is cellular respiration dependent on photosynthesis?
Photosynthesis produces sugar that is used to produce ATP in cellular respiration.

c.  Given that cellular respiration and photosynthesis are related, how do animals get energy without the ability to do photosynthesis?
They eat other organisms to obtain the nutrients/sugars.

4.  a. Why is a membrane needed for cellular respiration? (4pts)
A gradient of H ions is part of the ATP production. The membrane is needed to maintain this gradient. (Partial credit (3 out 4 pts) electron transport needs proteins in contact so held in membrane. (only partial credit because this is not absolutely critical.))

b.  What has more potential energy, ATP or ADP?
ATP

5.  a. What supplies the energy for ATP production in photosynthesis?
Either: Sunlight. H ion gradient.

b.  What are two similarities in the mechanism of ATP production in photosynthesis and cellular respiration? (4pts)
Any 2 of: have H ion gradient, electron transport, channel makes ATP

c.  How do plants use the ATP produced during photosynthesis?
It is used to produce sugar.

6.  a. If there is a double stranded linear DNA molecule and this DNA molecule is replicated, what is the outcome after replication is complete? Include in your answer how many double stranded DNA molecules are present, and are they exactly the same as the original molecule?
There are now 2 double stranded DNA molecules. They are not the same, there is a small gap at one end of each one.

b.  A protein called DNA ligase is necessary to copy about half of the DNA, but not used on the other half. Why is this? (4pts)
It is needed on the lagging strand. The lagging strand is produced in several parts that move in the opposite direction of the helicase (Okazaki fragments), these fragments must be connected together by ligase to form a completely covalently bonded molecule.

c.  In DNA replication, what is an RNA primer, why is it needed, and what makes it?
DNA polymerase can only use double stranded DNA with a single stranded part hanging off as a template. Once the DNA has been separated by the helicase it is single stranded.. Primase makes a short piece of RNA to bind to the DNA, making it double stranded so DNA pol can work.

7.  a. What are two dangers of multiple rounds of DNA replication? (4pts)
Replication errors or shortening of DNA due to gaps.

b.  How do telomeres help mitigate both of these problems? (4pts)
If the telomeres are too short, the number of replications are limited. Telomoeres protect the coding region of the DNA; they get shorter due to gaps, but the genes are not affected.

c.  Name a specific type of human cell where you would expect to find telomerase, and explain why this cell type needs telomerase.
Cells that need to constantly divide such as: stomach lining, hair, skin, or tumors or reproductive cells such as: sperm, eggs, embryos.

8.  Almostattheendase (a hypothetical protein) is made of 200 amino acids.

a.  The mRNA can be no smaller than 600 bases.

For the following questions, answer bigger, smaller, or the same.

b.  The pre-mRNA is Bigger than the mRNA. Explain.
It contains exons that are removed during processing to make the mRNA.

c.  The gene (the DNA) is Bigger than the pre-mRNA. Explain.
The gene contains the promoter and terminator sequences plus everything that will be part of the pre-mRNA.

9.  a. What is one similarity, and two differences between DNA polymerase and RNA polymerase?
Similar, any 1 of: only add nucleotides to 3' end, active in nucleus, add complementary bases
Different, any 2 of: DNA pol make DNA, RNA pol make RNA; DNA pol needs double strand to start, RNApol does not; DNA pol has start site but no end, RNA pol has start and stop site

b.  After the mRNA is made and transported out of the nucleus, what are two ways to keep it from being translated into protein? (4pts)
Degrade mRNA rapidly or inhibit ribosome binding to mRNA.

c.  What does transfer RNA (tRNA) do?
Brings amino acids to ribosome and matches correct triplet code.

d.  During the cell cycle, cyclin levels are low after division and higher right before the cell divides again. What might be one change in the gene expression of cyclin that could cause this change?
Any 1 of: Increase transcription of cyclin gene, do not degrade cyclin mRNA, more translation of cyclin mRNA, do not degraade cyclin protein as more produced.

10.  We know that chloroplasts and mitochondria have their own DNA. What is an additional fact about this DNA that supports the endosymbiotic theory? (4pts)
Any one of: DNA is circular, DNA lacks all genes needed for photosynthesis/respiration, similarity in genes between organelle and alleged bacterial precursor.
(partial credit (2 out of 4) double membrane, look like bacteria, have ribosomes, current examples of symbiosis exist)