CASES CELL BIOLOGY
TELOMERASE AND CANCER
What can happen if the telomerase gene is activated in cells that normally do not?
The shortened telomeres would become longer again, but the main problem is that damaged DNA
molecules where breaks in both strands occurred might be fixed with telomeric ends. This means
that they are not repaired properly and there might be loss of information in DNA.
Tumor cells also have an increased telomerase activity; it is essential for their survival because they
divide very rapidly, so the telomeres of the DNA contained in these cells would shorten fast and
cells would soon die without intense telomerase activity.
I-CELL DISEASE
The lysosome is an organelle where molecules are degraded by lysosomal enzymes. To ensure that
these enzymes are transported from the Golgi to the lysosome, they need to be modified in the
Golgi by attaching a mannose-6-phosphate. This modification is made by the mannose-6-phosphate
transferase. The M6P are recognised by a specific receptor in the Golgi membrane, and are then
transported to the lysosomes (or late endosomes) through vesicles.
What happens if lysosomal proteins (enzymes) do not receive the M6P?
They would be transported outside of the cell and not to the lysosome. Consequently, if lysosomes
do not receive the enzymes they need to degrade molecules, these molecules accumulate in the
cell causing problems such as I-cell disease. The lysosomes continue to increase in size as the
affected person ages, and this disturbs the functioning of cells; however, I-cell disease is lethal so
the patient dies at a young age.
What is the cause of I-cell disease?
The enzyme mannose-6-phosphate transferase is probably not working properly, so it does not add
the M6P to the lysosomal enzymes.
TAXOL
Microtubules use dynamic instability (shrinkage and growth) to search for kinetochores (where
they must attach) in the cell during prometaphase; they also need to shrink during anaphase to pull
chromosomes towards the spindle poles.
What does Taxol do?
It is a drug used for chemotherapy. It attaches to microtubules and stabilizes them, so they cannot
use their dynamic instability to attach to kinetochores and pull chromosomes at opposite poles.
Mitosis is consequently arrested or cells die, because some mitotic stages (such as anaphase and
cytokinesis) might be skipped and generate polyploid cells, or cytokinesis might happen in cells with
mis localized chromosomes, so chromosomes are not equally distributed in daughter cells and
cause aneuploidy. The abnormal number of chromosomes in aneuploid cells is detected and
apoptosis is initiated. Hence, if microtubules do not function properly in cancer cells, they also die.
TELOMERASE AND CANCER
What can happen if the telomerase gene is activated in cells that normally do not?
The shortened telomeres would become longer again, but the main problem is that damaged DNA
molecules where breaks in both strands occurred might be fixed with telomeric ends. This means
that they are not repaired properly and there might be loss of information in DNA.
Tumor cells also have an increased telomerase activity; it is essential for their survival because they
divide very rapidly, so the telomeres of the DNA contained in these cells would shorten fast and
cells would soon die without intense telomerase activity.
I-CELL DISEASE
The lysosome is an organelle where molecules are degraded by lysosomal enzymes. To ensure that
these enzymes are transported from the Golgi to the lysosome, they need to be modified in the
Golgi by attaching a mannose-6-phosphate. This modification is made by the mannose-6-phosphate
transferase. The M6P are recognised by a specific receptor in the Golgi membrane, and are then
transported to the lysosomes (or late endosomes) through vesicles.
What happens if lysosomal proteins (enzymes) do not receive the M6P?
They would be transported outside of the cell and not to the lysosome. Consequently, if lysosomes
do not receive the enzymes they need to degrade molecules, these molecules accumulate in the
cell causing problems such as I-cell disease. The lysosomes continue to increase in size as the
affected person ages, and this disturbs the functioning of cells; however, I-cell disease is lethal so
the patient dies at a young age.
What is the cause of I-cell disease?
The enzyme mannose-6-phosphate transferase is probably not working properly, so it does not add
the M6P to the lysosomal enzymes.
TAXOL
Microtubules use dynamic instability (shrinkage and growth) to search for kinetochores (where
they must attach) in the cell during prometaphase; they also need to shrink during anaphase to pull
chromosomes towards the spindle poles.
What does Taxol do?
It is a drug used for chemotherapy. It attaches to microtubules and stabilizes them, so they cannot
use their dynamic instability to attach to kinetochores and pull chromosomes at opposite poles.
Mitosis is consequently arrested or cells die, because some mitotic stages (such as anaphase and
cytokinesis) might be skipped and generate polyploid cells, or cytokinesis might happen in cells with
mis localized chromosomes, so chromosomes are not equally distributed in daughter cells and
cause aneuploidy. The abnormal number of chromosomes in aneuploid cells is detected and
apoptosis is initiated. Hence, if microtubules do not function properly in cancer cells, they also die.
