Question: Do chromosomes replace themselves?

[Salik Imran]

Dear all,

I have been hearing abut how the body (gradually) replaces dead tissue, and replaces it with new tissue. Apparently, the whole body finishes replacing itself every 7 years! So this is my question. Do chromosomes replace themselves like the body cells do? (I think I already know the answer, but I want an in-depth explanation please:).

Please get back to me on this.

Yours sincerely,

Salik Imran, 13.

[joigus]

Cells are generally replaced --except most neurons-- because they are the functional units. Chromosomes aren't. Chromosomes are packages of genetic material in gametes, which are haploid cells --some kind of random selection of half the organism's genes. Gametes in males are very much expendable. In females, they are more costly, but still. Gametes are cells for export, so to speak --I mean sex--, so they don't need to be replaced.

When cells release stress signals --chemicals that inform about something not being quite right in them-- they are disposed of, so you can imagine that the genetic material inside is at least as expendable.

I hope that helped. This is kind of my lowdown of the story. An expert will give you a more accurate picture.

Edit: Welcome to the forums, Salik.

Edited January 1, 2021 by joigus

[Salik Imran]

Wow, that is surprising! Thanks mate.

Edited January 2, 2021 by Salik Imran

[druS]

Surely, when a cell replaces itself and the cell dies, the chromosomes within that cell have been “replaced”. At exactly the same rate that cells are replaced, so are chromosomes. Surely.

[pfuggs]

On 1/1/2021 at 3:14 PM, joigus said:

Cells are generally replaced --except most neurons-- because they are the functional units. Chromosomes aren't. Chromosomes are packages of genetic material in gametes, which are haploid cells --some kind of random selection of half the organism's genes. Gametes in males are very much expendable. In females, they are more costly, but still. Gametes are cells for export, so to speak --I mean sex--, so they don't need to be replaced.

When cells release stress signals --chemicals that inform about something not being quite right in them-- they are disposed of, so you can imagine that the genetic material inside is at least as expendable.

I hope that helped. This is kind of my lowdown of the story. An expert will give you a more accurate picture.

Edit: Welcome to the forums, Salik.

When you say "functional units" is that referring to how neurons need to be connected to carry signals?

[joigus]

3 minutes ago, pfuggs said:

When you say "functional units" is that referring to how neurons need to be connected to carry signals?

That goes for neurons. But I meant it --more in general-- in the sense that the cell --every cell, including neurons-- is the basic unit that carries out a particular function within the organism. In order to do that, they specialize down the line of cellular development. Cells have a finite life though, so when they no longer work, they are replaced by releasing stress signals that activate their destruction and further mitosis in other sister cells. As long as the cell is performing its function, it's important that it does it well --cancer being an example of how bad it is that a cell stops working properly. Cancer cells get stuck in continual mitosis and just can't stop. It's their function that's essential.

Gametes, on the contrary, are some kind of "inter-phase" between one organism and the next generation. They carry random arrangements of half the genetic material  --haploid cells-- of the parent organism; and they're fundamentally like a throwing of the dice. Not a functional cell really. Not yet. So chromosomes are expendable. On the contrary, the organism cannot afford to have malfunctioning DNA in the nucleus of working cells. That's why eukaryotes have mechanisms to destroy tissue cells that are not working properly. It doesn't play around trying to fix it --replace it.

During replication cells do have an impressive proofreading mechanism, very precise --transcription and translation don't have to be that accurate--. But when DNA that's being read for transcription is just too messed up, the cell must be destroyed.

When the cell malfunctions, the DNA is replaced... by replacing the whole cell. Not taking any chances.

But a gamete turns bad? No problem for this organism.

That's more or less what I meant.

[Halc]

2 hours ago, joigus said:

Cells have a finite life though, so when they no longer work, they are replaced by releasing stress signals that activate their destruction and further mitosis in other sister cells.

But the sister cells are just as old. Excepting cells that are manufactured (*), all bad (damaged, stressed) cells are replaced by mitosis of the remaining ones, all of which are essentially as old as you are. Maybe I'm wrong about that. Hardly an expert here. The DNA is also replicated this way, not manufactured from scratch, so all the DNA is effectively as old as you are,.

Neurons can't just split since their connections cannot be reproduced, so they're intended to exist without replacement after their initial allotment is complete some months after birth.

* Blood is a nice example of manufactured cells. Red blood contains no DNA and cannot reproduce by mitosis. They're short lived (1/3 year?) and manufactured continuously by non-blood cells.

[joigus]

8 hours ago, Halc said:

But the sister cells are just as old. Excepting cells that are manufactured (*), all bad (damaged, stressed) cells are replaced by mitosis of the remaining ones, all of which are essentially as old as you are. Maybe I'm wrong about that. Hardly an expert here. The DNA is also replicated this way, not manufactured from scratch, so all the DNA is effectively as old as you are,.

Neurons can't just split since their connections cannot be reproduced, so they're intended to exist without replacement after their initial allotment is complete some months after birth.

* Blood is a nice example of manufactured cells. Red blood contains no DNA and cannot reproduce by mitosis. They're short lived (1/3 year?) and manufactured continuously by non-blood cells.

I'm no expert either, so be my guest. And of course it would be nice that some of the local experts can give us a hand. Yes, DNA does get old. That's at the basis of cellular aging, and thereby the organism's aging itself, AFAIK. The replication mechanism is some kind of bi-directional zip assembly, so it's always imprecise at the ends. In one direction the replication process is very smooth, because the initial fragment (RNA primer) and the DNA polymerase work in the 5' to 3' direction, but in the opposite strand, primer and polymerase are forced to work against the uncoiling of the double strand, so it must interrupt and restart the copying work over and over again --the so-called Okazaki fragments. That's why there's always a mismatch at the end. Eukaryotes use a meaningless[?] chunk of DNA at the end --telomere-- which is partially replenished with every replication process, to kind of delay this ongoing degrading of the information.

Also, as you point out, different cells down the line of cellular development, have different adjustments to their particular function. Red blood cells being the perfect examples of cells that will never go back to be able to produce anything in the way of stem-cells or higher-potent cells, because they've completely lost their DNA. Other extremes are neurons and cells from the digestive lining. The average life of the latter is, if I remember correctly, 48 to 72 hours. And neurons, because they never get replenished by sister cells mitotically splitting. Although new neurons do appear directly from stem cells, especially in the hippocampus*. Also, they retain some ability to reconnect, or change connections.

That's about the summary of what I know.

* Google search: "newborn neurons in hippocampus and olfactory bulb"