the structures in the nucleus that serve as a “mitotic clock” are
a. kinases b. hormones c. telomeres d. ribosomes e. timomeres.
The Correct Answer and Explanation is:
The correct answer is c. telomeres.
Explanation:
Telomeres are specialized structures at the ends of chromosomes that play a key role in regulating cell division. These repetitive DNA sequences serve as a protective cap, preventing the loss of important genetic information during DNA replication. Over time, with each cell division, telomeres shorten, which has led to the concept of the “mitotic clock.” This phenomenon is closely related to cellular aging and the limited number of divisions a cell can undergo before it reaches a state called “senescence,” where it can no longer divide.
How Telomeres Work:
Every time a cell divides, the DNA is replicated. However, during replication, the enzymes that duplicate the DNA cannot completely replicate the very ends of the chromosomes. As a result, telomeres, which are non-coding regions at the ends of chromosomes, shorten progressively with each division. This shortening process is a built-in mechanism that limits the number of times a cell can divide. When telomeres become too short, the cell can no longer divide properly and enters a state of cell arrest or undergoes apoptosis (programmed cell death).
The gradual shortening of telomeres acts as a “mitotic clock” by determining the lifespan of a cell and its ability to divide. This mechanism is important for maintaining genomic stability and preventing uncontrolled cell division, which could lead to cancer. In some cases, telomerase, an enzyme that can add repetitive DNA sequences to telomeres, is active in stem cells and cancer cells, allowing them to avoid the normal limits on cell division.
Other Answer Choices:
- a. Kinases are enzymes that regulate various cellular processes, including cell division, but they do not serve as a “mitotic clock.”
- b. Hormones are signaling molecules that regulate bodily functions, but they are not involved in the mitotic clock.
- d. Ribosomes are involved in protein synthesis, not in regulating cell division.
- e. Timomeres seems to be a typographical error, as this term does not exist in biology.
In conclusion, telomeres are the critical structures in the nucleus that function as a “mitotic clock,” playing a key role in determining the lifespan of cells and their ability to divide.