What happens to telomeres with each replication cycle?

• A. They encode proteins required for replication
• B. They are formed only during S phase
• C. They remain the same length during replication
• D. They shorten with each replication

Answer: (D)

Telomeres shorten with each replication because the machinery that copies DNA cannot fully replicate the very ends of linear chromosomes. During each round of division, a small portion of telomeric DNA is lost in the final steps of synthesizing the ends, so the telomeres progressively get shorter. This shortening helps limit cell divisions and is linked to aging in many somatic cells where telomerase, the enzyme that can lengthen telomeres, is not active. Telomeres do not encode replication proteins, they aren’t formed only during S phase, and their length is not maintained in most cells without telomerase; in cells with active telomerase, they can be extended, but in typical somatic cells they still shorten with replication.