Brinedew

Unpublished draft

Replicative Credit

Replicative credit is the site’s term for the investment a cell lineage makes in high-fidelity DNA replication and repair, which pays off in the accuracy of every later division.

The investment

Copying a genome is expensive to do accurately. A lineage that maintains proofreading polymerases, mismatch repair, and damage surveillance (ATM, ATR) spends resources on fidelity that it could have spent on growth. The return on that spending is cumulative: every division made with an accurate copy preserves the value of the previous ones, the way interest accrues on principal. A lineage that cuts corners on fidelity banks divisions now and pays in mutations later.

Why the credit runs out

The investment logic is tied to the force of selection. Early in life, an organism’s descendants inherit the full value of high-fidelity replication, and mechanisms that keep the mutation supply low protect both the body and the germline. Late in life, the returns flow to lineages that are evolutionarily dead ends: a somatic lineage that stays accurate for forty years gains the organism little once reproduction is complete, and a lineage that defects gains at the body’s expense. Maintenance systems that were investments early become costs late, and the theory of antagonistic pleiotropy predicts their early-life benefits and late-life costs will be balanced by selection.

Cancer enters when a cell loses a fidelity system — a mismatch repair gene, a checkpoint — and moves into a regime where divisions generate diversity instead of copies, which is the raw material of tumor progression (clonal evolution).