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Telomerase

ACTSCI-4RJATJ

Use the information below, as well as the original passage, to answer this question.

Scientists have always had a great curiosity about cancer cells. When grown in culture dishes, cancer cells will divide indefinitely, unlike normal actively dividing cells such as fibroblasts, which eventually give out and enter apoptosis. As new research about telomeres became known, oncologists decided to re-examine cells from various abnormal growths. What they saw was stunning, the telomeres of these cells never shrank, nor did the ending DNA fibers ever fray. Whether it was the first division or the 500th, these cell telomeres retained their original base pair number and their ability to attract the protein caps necessary to keep their DNA strands from fraying.

The question that was quickly asked is what substance is responsible for keeping these abnormal cells telomere’s healthy? Four biological molecules known to exist in actively dividing cells were identified as possible candidates. A study of the concentrations of these molecules were undertaken to see if one could possibly be the telomeres “magic bullet”.

The data table below shows the results of a study using five different types of cells from human donors grown in the same culture medium. Chemical assays were used to measure the concentration of each molecule in each cell type.

$$ \text{Concentration in ug/ml} $$

Cell Type Rate of Cell Division DNA Polymerase Housekeeping Protein Telomerase Spindle Microbtube Protein
Liver Medium 1.4 2.5 0.4 0.6
Neuron Very low 0.2 2.5 0.1 0.1
Fibrolast High 1.7 2.5 1.2 0.9
Bone Sarcoma Very High 2.1 2.5 6.1 0.9
Breast Carcinoma Very High 2.2 2.5 6.1 0.9

Telomeres are DNA sequences that repeat at the end of chromosomes. They protect chromosomes by attracting protein caps. These caps bind to the chromosomes and keep the long, thin threads of DNA from fraying. Usually the sequences are about 200 base pairs (bp) long and repeat 400-500 times at the end of each chromosome. When a cell is going to divide it enters a part of its life cycle called the synthesis phase. During this time a protein molecule called DNA polymerase will make a copy of each chromosome so that each new cell after division will have its own set of nucleotide bases. However DNA polymerase can never quite make it to the very end of each chromosome, so that after each cell division, the telomeres have shortened.

Eventually, the telomere sequences are too short to attract the protein caps, and without the proper protection, the delicate DNA threads fray, and the cell can no longer make the proteins it needs to divide, the cell enters a phase known as apoptosis, or programmed cell death.

Telomeres have attracted a great deal of research and much has been discovered their role in cell division. One of the first experiments was designed to elucidate exactly how much telomere DNA was needed in order for a cell to be healthy enough to divide. The researcher involved used cultures of human skin cells, called fibroblasts, to test the hypothesis. Skin cells were used because they are known to grow and divide actively for a number of generations. Cells were cultured in normal growth media (a cell sample is put in a petri dish and fed, the cells then divide on their own) and the number of cell divisions until apoptosis were measured. The length and condition of the telomeres were measured and recorded after the 10th division and then after apoptosis for each culture. The data is shown below:

Culture Plate # # of Cell Divisions until Apoptosis Telomere length after 10th division (bp) Telomere length after 20th division (bp) Telomere length just before apoptosis
1 23 8000 6000 3300
2 22 7500 5500 3450
3 24 8400 6400 3200
4 23 8000 6000 3300
5 26 9000 7000 3200

A researcher makes a logical assumption that treating osteoarthritic bone cells with telomerase will lead to repair of the telomeres in the bone cells and improved growth. An experiment is run to test this idea and the results for the first few generations are encouraging. The bone cell telomeres stay healthier for longer. However, after five generations the telomerase is inactivated and the bone cells progress once again towards apoptosis.

What may the researcher be thinking as to the reason why the telomerase stops working?

A

The telomerase was destroyed by DNA polymerase or Housekeeping protein.

B

The oxidative stress the bone cells are under may negatively affect the telomerase.

C

Cells of the immune system directly destroy the telomeres, which then destroys the telomerase.

D

Telomerase needs healthy telomeres in order to function properly and this was destroyed during the synthesis phase of the cell cycle.