Protects the ends of the chromosomes; when they degrade, the cell dies.

• A. Telomere
• B. DNA polymerase III
• C. Ribosome
• D. mRNA

Answer: (A)

Telomeres are the protective caps at the ends of linear chromosomes. They consist of repetitive DNA sequences and special binding proteins that shield chromosome ends from being mistaken for DNA breaks. Each time a cell divides, the end-replication problem causes telomeres to shorten; in germ and some stem cells, an enzyme called telomerase can extend them, but in most somatic cells telomerase is inactive and telomeres gradually erode. When telomeres reach a critically short length, the cell recognizes unrepaired chromosome ends as damage, triggering DNA damage responses that halt the cell cycle or trigger cell death. That’s why degradation of telomeres ultimately leads to cellular demise. The other options don’t protect chromosome ends: ribosomes synthesize proteins, mRNA carries genetic information from DNA, and DNA polymerase III mainly replicates DNA but does not guard chromosome termini.