Q1.
Does a unicellular organism like amoeba or yeast ever ‘grow old’? When it divides, it produces almost two identical copies. So, does aging happen at all?
Answer
The honest answer is: it depends on whether the division shares out the damage equally — and in yeast it does not, so a yeast cell really does grow old.
Start by defining what ageing is. Ageing is not the passing of time; it is the accumulation of damage that has not been repaired — damaged proteins, damaged membranes, errors in the DNA. So the question becomes: when a single cell divides, where does that damage go?
| Organism | How division shares the damage | Does it age? |
|---|---|---|
| Amoeba (binary fission) | Roughly equally between two nearly identical daughters | The damage is halved every generation, so it never builds up. In culture with plenty of food, amoeba keeps dividing indefinitely — no ageing is seen |
| Yeast (budding) | Unequally. The mother keeps most of the damaged proteins; the small bud starts clean | Yes. The mother can produce only about 20 – 30 buds and then stops dividing and dies |
| Bacteria such as E. coli | Almost equally, but the daughter that inherits the older end of the cell also inherits more damage | A mild form of ageing — that daughter grows a little more slowly |
Why budding yeast is the clear case: each time a yeast cell buds, it leaves a permanent bud scar on its wall. Count the scars under a microscope and you know how many times that cell has divided. After about twenty to thirty buds the cell stops dividing altogether, becomes larger and misshapen, and dies. Meanwhile every bud it produced started life "young" and went on to have its full quota. So the parent ages while the offspring is rejuvenated — and the difference is not in their genes, which are identical, but in how the contents of the cell were divided between them. This is called replicative ageing, and yeast is one of the standard laboratory organisms for studying it.
So what about the lineage? Here your instinct is right. After an amoeba divides there is no parent left over and no corpse — there are only two amoebae. In that sense the line does not age and could in principle go on for ever. What can age is a particular cell that keeps its damage instead of passing it on.
Tip — a good question to take further: if a cell can be "reset" to youth simply by dividing unequally, what exactly is being reset? That question is being actively researched, and yeast is where much of the work is done. Notice that Louis Pasteur's principle from earlier in this chapter still holds throughout: new life always comes from pre-existing life — but not always in the same condition.