The Hayflick limit proposes that somatic cells, the cells of the body, cannot divide indefinitely. The Hayflick limit remains a foundation in the theory of natural cancer defenses, as well as studies of aging.
In the case of cancer theory, cancer can be thought of a system colony of rogue cells that have begun to divide out of control. The Hayflick limit proposes that, under normal, that is healthy conditions, when control is lost, the Hayflick limit will impose a natural limit on the number of times a cell may divide.
In terms of aging theory and study, or gerontology, the Hayflick limit seems to imply that humans, as well as all organisms, have a natural lifespan, that is imposed by, among other things, the Hayflick limit. The process of life seems to require a continuous supply of new cells, and when senescent cells cannot be replaced, the organism will expire.
The underlying mechanism for imposing the Hayflick limit, or programmed senescence, was discovered in the laboratory by Elizabeth Blackburn who, in conjunction with then graduate student Carol Greider, isolated a protein complex that was responsible for extending the down stream end of chromosomal DNA after replication.
The end of the chromosome is called the telomere, and the enzyme which extends the telomere is referred to as telomerase, or telomere reverse transcriptase (TERT). TERT contains a small piece of RNA which is used as a template to ad nucleotides to the telomere. The hexonucleotide, or 6 base piece of RNA, provides a template for adding bead like repeats to the end of the chromosome, thus extending the telomere. Conversely, the telomere can be defined as this section of highly repeated DNA located on the ends of each chromosome.
During the process of DNA duplication, the polymerase complex cannot completely copy the template strand to the duplicate strand. As such, with each cell division, and DNA replication, a small number of nucleotides as lost. The Hayflick can then be calculated as the length of the section of repeated DNA sequences on the end of a strand of DNA known as the telomere divided by the amount of DNA lost from the telomere on each replication. The answer is about 50.
Of course, biology never seems to offer hard and fast rules. The cell populations of organisms can be broadly divided into stem cells and somatic cells. In general terms, the cells of the body, or somatic cells are differentiated and do not express telomerase. Thus, it is to these cells that the Hayflick limit applies. Stem cells and germ line cells on the other hand, may express telomerase, and they may defeat the Hayflick limit.
In a pathological cases, somatic cells may express telomerase, and may defeat the Hayflick limit. Specifically, about 90 of all cancers express telomerase. It is most likely these immortalized cell lines, misplaced and out of control, that drive ever more detailed research into molecular nature of cancer.