EMQ 14. Theme: the roles of specific proteins.

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Each of the proteins below plays an important role in at least one of the cell processes in the list. For each of the cell processes listed, identify the relevant protein.

A Actin
B Cytochrome c
C DNA ligase
D Glucose 6-phosphate phosphatase
E Phosphofructokinase
F Poly-A polymerase, an enzyme that generates a long, simple chain of adenosine residues.
G Tubulin
H V(J)D recombinase

1.Cutting and splicing of DNA.
Your Answer:
Answer: H, V(J)D recombinase. In developing B cells, the chromosomal loci coding for the light and heavy chains of antibodies are cut and spliced as part of the process that generates a large number of different Fab regions. See book page 318.
2. DNA replication.
Your Answer:
Answer: C, DNA ligase. DNA ligase joins the short Okazaki fragments that are synthesized on the lagging strand during DNA replication. See book page 69.
3. Generation of the hydrogen ion electrochemical gradient across the inner mitochondrial membrane.
Your Answer:
Answer: B, cytochrome c. Cytochrome c is reduced by complex III of the electron transport chain and in turn is used by complex IV to reduce molecular oxygen to water. Complexes III and IV both pump hydrogen ions up their electrochemical gradient out of the mitochondrial matrix. See book page 197.
4. Gluconeogenesis by the liver, which synthesizes glucose from amino acids and other non-carbohydrate molecules and releases it into the extracellular medium to be carried around the body by the blood.
Your Answer:
Answer: D, glucose 6-phosphate phosphatase. Only simple glucose can be carried by the glucose transporter, which cannot transport phosphorylated sugars. Therefore the glucose-6-phosphate generated by gluconeogenesis must be dephosphorylated in order to be exported from the cell. See book page 216.
5. Glycolysis: the breakdown of glucose to yield pyruvate.
Your Answer:
Answer: E, phosphofructokinase. Phosphofructokinase catalyzes the first committed step of glycolysis: once phosphorylated to fructose-1,6-bisphosphate, the sugar is committed to being broken down as far as pyruvate. See book page 209.
6. Mechanical support of cilia and flagella.
Your Answer:
Answer: G, tubulin. Cilia and flagella are supported by the 9+2 axoneme of microtubules. See book page 286.
7. RNA synthesis and processing.
Your Answer:
Answer: F, poly-A polymerase. This enzyme generates the poly-A tail at the 3’ end of processed eukaryotic mRNAs. See book page 91.
8. Segregation of chromosomes during cell division.
Your Answer:
Answer: G, tubulin. The mitotic (and meiotic) spindle are made of tubulin. See book page 298.
9. The initiation of apoptosis.
Your Answer:
Answer: B, cytochrome c. If released from mitochondria into the cytosol, cytochrome c activates caspase 9 which in turn cleaves and activates effector caspases. See book page 310.
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