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Cell Cycle

ACTSCI-PON0KR

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

Based on the description in the passage, which event most likely happens after the synthesis phase of the cell cycle?

A

Cells with healthy telomeres will go through apoptosis.

B

DNA polymerase will add bases as nucleotides begin to copy themselves.

C

The cell will have two copies of DNA and then be able to divide.

D

DNA polymerase will repair frayed telomeres at the very end of the chromosomes.