Why wouldn't the reproductive cells be old?

[pagetwo]

Millions and millions of years passed, generations have been dead and other generations were born. Yet we are all divided from the original reproductive cells of our ancestors! Why don't we see new arrivals of aged bodies? Why can reproductive cells permanently be young?

[lucaspa]

Millions and millions of years passed, generations have been dead and other generations were born. Yet we are all divided from the original reproductive cells of our ancestors! Why don't we see new arrivals of aged bodies? Why can reproductive cells permanently be young?

 

There is a misconception here: We are NOT "divided from the original reproductive cells of our ancestors". We are descended from our ancestors but we are not directly divided from the original reproductive cells. You started with a false premise and then reached a false conclusion.

 

The reproductive cells are a specialized type of cell in our body. They are descended from "normal" somatic cells and undergo meiosis, not "mitosis". In other words, the progenitor reproductive cell does not "divide", but rather separates the chromosomes so that the two cells have only half the complement of DNA.

 

During reproduction, two of these cells (one from each parent) combine to make a new individual. That new individual has a unique and new genome of DNA - half from each parent. But since each chromosome of each parent could split, that makes a huge number of combinations.

 

Let's try to make this simple: just 1 chromosome per parent. Remember, each chromosome has 2 chains of DNA (the double helix). Call them A and B. Mom also has 2 chains, call them 1 and 2.

 

So Dad makes sperm with A and sperm with B.

Mom makes eggs with 1 and eggs with 2.

 

So, we can have the following combinations:

Sperm A and egg 1 for A1

Sperm A and egg 2 for A2

Sperm B and egg 1 for B1

Sperm B and egg 2 for B2

 

Four possible individuals with just 1 chromosome. It is 2^2n with n = the nubmer of chromosomes. Now, just apply the 45 non sex chromosomes in humans and you have 2^90 possible combinations! A huge number.

 

It's not that reproductive cells are either "young" or "old", but that they combine to make a new individual. And that individual will be "young" because it's first cell just came into existence.

[Mr Skeptic]

Aging is not something unavoidable for all living things, it is something that we acquired along the way. Single-celled creatures (and human cancers) do not age, rather they split into two daughter cells that are both young. A similar aspect has by necessity been preserved in our reproductive cells. Among other things, they have enzymes which add telomers back to the DNA.

[SkepticLance]

To lucaspa

 

Everything you said is correct and accurate, but you just evaded the whole question. And the question is a valid one, and an intelligent one.

 

First : It is difficult to answer that question, since there are a number of theories as to why we age in the first place. If we do not know exactly why we age, the question is literally impossible to answer.

 

One possible answer relates to mitochondria. One theory of ageing is that it is caused by accumulated genetic errors in the mitochondria in our cells. Since mitochondria are the source of cellular energy, when some become useless, the cells no longer have access to as much energy, and become less useful - a good definition of ageing.

 

Mitochondria are passed on via the maternal line - there are none in sperm. The cells that give rise to ova essentially segregate out the defective mitochondria - meaning that what is left is mitochondria without genetic defect - 'young' mitochondria. As the zygote divides, the 'young' mitochondria divide with it, and the new life form is born with 'young' and hence energy producing mitochondria.

 

Of course, that is just one theory. There are as many theories to answer the question as there are theories of ageing.