• Question: Do different cells have different Hayflick limits? What does this depend on?

    Asked by cecylia to Louise, Michaela, Steve on 24 Jun 2010 in Categories: .
    • Photo: Louise Johnson

      Louise Johnson answered on 24 Jun 2010:

      Interesting question!

      The Hayflick limit is the number of times a cell can divide before it dies. Normal cells from a human foetus, if you grow them in a dish in the lab, will divide about 50 times, but as people get older their cells don’t have as many replications left. So cells of the same type, but from people of different ages, have different Hayflick limits.

      Cells of different types also have different limits – stem cells, whose job it is to make new cells to replace those that die, last much longer than others. Germ cells – the cells which make eggs or sperm – should be able to keep dividing indefinitely. And many cancer cells never stop growing.

      A lot of this variability is probably to do with an enzyme called telomerase. Telomeres are the ends of chromosomes, and because of a quirk in the way chromosomes are replicated, they get shorter every cell division, unless the enzyme is around to patch them up again. Without telomerase, eventually the cell starts to lose genes it needs to stay alive. The gene for telomerase is switched off in most of our body cells, but cancer cells often mutate to switch it back on again.

      A really cool book about this is “The Immortal Life of Henrietta Lacks” by Rebecca Skloot – it’s about the woman whose cancer cells became used in labs all over the world. She never knew how much she contributed to science.

    • Photo: Steven Kiddle

      Steven Kiddle answered on 24 Jun 2010:

      Yes, some cells such as stem cells and cancer cells have a much larger (or infinite) hayflick limit. It depends on the telomere, a large section at the end of a chromosome that decreases in size when the DNA is copied. The hayflick limit is reached when the telomere has disappeared and normal DNA starts to be lost.

      A protein expressed in stem and cancer cells called telomerase can rebuild the telomere to prevent the hayflick limit killing the cells descendents.

    • Photo: Michaela Livingstone

      Michaela Livingstone answered on 24 Jun 2010:

      Technically cells tend to have a hayflick limit of 40-60 cell divisions. But some cells never reach their limit. Nerve cells for example do not divide, so technically never reach their theoretical limit. Other kinds of cells don’t have limits at all. Certain cancer cells for example never stop dividing, which makes them really dangerous. Generally it depends upon the length of a chromosome’s telomeres, which are repeating sequences found at the ends of chromosomes. After each division they become shorter, eventually they become so short that genes start being deleted after each replication – at this point the cells normally die (since it’s reached it’s hayflick limit). But certain cell can express a protein called Telomerase which can replace the lost telomeres. These cells are then called “immortallised”.

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