The end of a chromosome, which consists of tandemly repeated short sequences of DNA that perform the function of ensuring that each cycle of DNA replication has been completed. Each time a cell divides some sequences of the telomere are lost; eventually (after 60–100 divisions in an average cell) the cell dies (the telomere theory of ageing (see senescence) is based on this phenomenon). Thus for a limited period telomeres provide protection for the genes located at the ends of the chromosome. Also, proteins associated with telomeric DNA prevent the unequal ends of the replicated DNA strands from activating repair pathways that might cause the cell cycle to stall or even lead to death of the cell. Human telomeres consist of the nucleotide sequence TTAGGG repeated hundreds to thousands of times, and some 50–200 nucleotides are lost during each replication cycle. In yeasts, protists, and the stem cells and germ cells of higher organisms, the chromosomes are kept at their appropriate lengths by the action of telomerase, which catalyses the addition of lost telomeric sequences. Telomerase is an enzyme consisting of RNA and protein that is inactive in normal cells of higher organisms. Its presence in tumours is linked to the uncontrolled multiplication of cancer cells.