Think of the “age” of DNA in two different ways:
Question you’re really asking | What matters biologically |
---|---|
Lineage age – How long ago was the template for this DNA copied? | A history-of-copies problem. You trace backwards through the maternal line to the cell where that stretch of DNA last sat silent, waiting to be copied again. |
Molecule age – How old is any particular strand right now? | A chemistry problem. DNA strands are broken down and recopied every time the cell replicates them, so individual molecules are usually days to months old. |
1 Where your mitochondrial DNA first “started its clock” for you
- Your mother’s oocytes (egg precursors) were formed while she was still a 5- to 6-month-old fetus inside your grandmother’s womb.
- The mitochondria in those oocytes, complete with their circular mtDNA, were there at that moment.
- Your mother was 21 when you were born. So the template lineage of the mtDNA that ended up in the zygote that became you had already spent:
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≈ 0.75 year inside your grandmother during pregnancy + 21 years inside your mother (from her birth to your conception) -------------------------------------------------------------- ≈ 21 ¾ years older than you at the instant of fertilisation
So in the same lineage sense you used for your nuclear DNA, your mitochondrial DNA is about 22 years older than you (a few months older than its nuclear roommate).
2 But the actual strands aren’t 22 years old
Mitochondria keep dividing inside the ovary while the egg matures, and they copy their DNA each time. After fertilisation, mtDNA continues to replicate every few cell divisions as the embryo grows. Each individual mtDNA molecule in your cells today is probably weeks to months old, not decades. What’s ancient is the information chain, not the chemical strands themselves.
3 Why the maternal line makes it “one generation older”
Because sperm mitochondria are actively destroyed after fertilisation, the entire mitochondrial population in every cell of your body descends from that single oocyte. Each generation pushes the lineage one step back: your mtDNA’s ancestry runs through your mother, grandmother, great-grandmother, and so on without intermixing with paternal mitochondria.
4 A neat comparison
Component in the zygote that became you | “Lineage age” at your birth |
---|---|
Paternal nuclear DNA (from a sperm produced ~2 months before conception) | ~2 months older |
Maternal nuclear DNA (from an oocyte stalled since your mother’s fetal life) | ~21 years older |
Mitochondrial DNA (in that same oocyte) | ~22 years older |