Question

which statement below best explains why cells from advanced malignant tumors are often found to contain damaged chromosomes as well as abnormal numbers of chromosomes?
tumor cells may complete s phase more than once per cell cycle.
tumor cells with abnormal chromosomes are no longer regulated by contact inhibition.
tumor cells grow and divide more rapidly than normal cells, which results in chromosome abnormalities.
tumor cells are no longer regulated by the checkpoints that normally prevent completion of the cell cycle by cells with chromosome abnormalities.

Explanation:

Cell cycle checkpoints are crucial for ensuring proper chromosome number and integrity. In normal cells, if there are chromosome abnormalities, the checkpoints halt the cell cycle to allow for repair or apoptosis. Tumor cells, however, often have mutations that disable these checkpoints. This means that even if there are damaged chromosomes or an abnormal number of chromosomes (due to errors in processes like DNA replication or mitosis), the cell cycle is not stopped. As a result, these abnormal cells can continue to divide and accumulate more chromosomal issues.

Option 1: Completing S - phase more than once per cell cycle is not a common or typical cause of the described chromosomal abnormalities in tumor cells.
Option 2: Contact inhibition is related to cell - to - cell contact and cell division (cells stop dividing when they come into contact with other cells). It is not directly related to the cause of chromosomal abnormalities.
Option 3: While tumor cells do grow and divide rapidly, the rapid growth itself is not the direct cause of the chromosomal abnormalities. The lack of proper checkpoint regulation is more fundamental.

Answer:

Tumor cells are no longer regulated by the checkpoints that normally prevent completion of the cell cycle by cells with chromosome abnormalities.