Question

Linear eukaryotic chromosomes with telomeres may be protected by two ways of avoiding attempts by the cell to recognize the telomere as broken DNA in need of repair. Briefly describe the two protective measures, including using a relevant descriptive vocabulary term for each of the ways.

Answer 1

Telomeres are repetitive nucleotide sequences at each end of chromosomes. Their function is to protect the ends of the chromosomes from deterioration or fusion to other chromosomes during cell division. With every cell division, telomeres shorten. In healthy cells, telomeres lose up to 300 bp of DNA per cell division.

However, because the ends are protected by telomeres, the only part of the chromosome that is lost, is the telomere, and the DNA is left undamaged. Without telomeres, important DNA would be lost every time a cell divides (usually about 50 to 70 times)

Telomere shortening:

Telomere is protected from a cell's DNA repair systems because they have single-stranded overhangs, which "look like" damaged DNA. The overhang at the lagging strand end of the chromosome is due to incomplete end replication. The overhang at the leading strand end of the chromosome is actually generated by enzymes that cut away part of the DNA.

The repeats that make up a telomere are eaten away slowly over many division cycles, providing a buffer that protects the internal chromosome regions bearing the genes (at least, for some period of time). Telomere shortening has been connected to the aging of cells, and the progressive loss of telomeres may explain why cells can only divide a certain number of times.

Telomerase:solves end replication of DNA problem

Cells have the ability to reverse telomere shortening by expressing telomerase, an enzyme that extends the telomeres of chromosomes. Telomerase is an RNA-dependent DNA polymerase, meaning an enzyme that can make DNA using RNA as a template.