The end-replication problem

Explanation

Replication enzymes can replace an internal RNA primer because an adjacent DNA fragment supplies an extendable 3-prime end. At the end of a linear chromosome, removing the terminal lagging-strand primer can leave no such upstream DNA end. DNA polymerase cannot start from nothing to replace it.

The resulting daughter DNA can therefore be shorter at one end. Telomeres are repetitive DNA-protein structures at chromosome ends that help protect coding regions and prevent ends being treated as ordinary breaks. They do not mean that DNA polymerase suddenly reverses its synthesis direction.

The problem arises from linearity and the enzyme's need for a primer. A circular bacterial chromosome has no equivalent terminal end, although it still needs primers and careful completion of replication. Do not claim all circular DNA replication is primer-free.

Telomerase can extend telomeric DNA in certain cells, using an internal RNA template, allowing conventional enzymes to complete more of the complementary strand. This provides explanatory context; the central required reasoning is why a terminal primer creates a problem. Telomere biology is not a simple one-number clock predicting a person's lifespan.

In most body cells telomerase is not active, so telomeres shorten a little with each division. When they become critically short, the cell stops dividing or undergoes apoptosis. Apoptosis is programmed cell death, in which the cell dismantles itself in a controlled way. This limits how many times most body cells can divide.