The ends of chromosomes contain regions referred to as telomeres.

These telomeric regions consist of telomeric repeat sequences.

The exact sequence of telomeric repeat can vary from species to species.

The repeat sequence shown here is found on telomeres of human chromosomes. Since DNA replication can only proceed in the 5' to 3' direction and requires a primer, it is not possible for a DNA polymerase to replicate the 3' ends of linear chromosomes.

If this problem were not corrected, chromosomes would shorten with each cycle of DNA replication.

The addition of telomeric repeat sequences by an enzyme complex referred to as telomerase assures that this shortening does not occur. Telomerase is a unique and interesting enzyme because, in addition to having DNA polymerase activity, it also contains an RNA sequence that provides a template for the synthesis of telomeric repeat DNA.

Part of the RNA sequence hybridizes with the single-stranded overhang on the DNA strand, leaving a single-stranded overhanging RNA sequence.

The repeat unit shown here is from Tetrahymena, which is the model organism used in telomere and telomerase research. The DNA polymerase function of telomerase then synthesizes a DNA strand complementary to the RNA found in the telomerase and subsequently translocates to the end of the newly synthesized strand, and the process repeats itself.

Many cycles of repeats can occur.

Once telomerase has completed its function, DNA primase synthesizes an RNA primer near the 3' end, and DNA polymerase fills in the vacant region.

A short region at the 3' end will remain single-stranded. The end result is that telomerase will have added many repeat sequences from a few dozen to a few hundred.

This prevents chromosome shortening.