Re: Plant and animal cell replacement question

On 1 Dec 2003 07:14:25 -0800, [EMAIL PROTECTED] (David Oderberg)
wrote:

>r norman <[EMAIL PROTECTED]> wrote in message news:<[EMAIL PROTECTED]>...
>> On 28 Nov 2003 05:57:52 -0800, [EMAIL PROTECTED] (David Oderberg)
>> wrote:
>> >r norman <[EMAIL PROTECTED]> wrote in message news:<[EMAIL PROTECTED]>...
>> >> On 26 Nov 2003 06:27:28 -0800, [EMAIL PROTECTED] (David Oderberg)
>> >> wrote:
>> >> 
>> >> >Is it the case that all plants and animals, including humans, turn
>> >> >over every single one of their cells during the course of their mature
>> >> >life? One hears stories to the effect that human cells change in their
>> >> >totality every 7 years, or (on the contrary) that certain neurons do
>> >> >in fact remain in the human being until the person's death, etc. But
>> >> >I've never seen an authoritative statement of the facts.
>> >> >
>> >> Nerve cells, in particular, are not ordinarily replaced.  Until
>> >> recently, it was believed that mammalian neurons were not produced in
>> >> adults at all, ever.  Now, it is known that there are least a few
>> >> brain centers where at least a few new neurons are generated.  Still,
>> >> the large bulk of the billions of neurons in our brain do not get
>> >> replaced.
>> >> 
>> >> Many invertebrates have identfiable and named neurons, each with a
>> >> specific function.  Some vertebrates have some similar cells --
>> >> Mauthner cells in fish and amphibians, for example.  The lobster
>> >> cardiac ganglion has nine neurons.  These things are constant
>> >> throughout life -- the cells don't change.
>> >> 
>> >> Some invertebrates have absolutely fixed cell number.  The roundworm
>> >> Caenorhabditis elegans, for example, has 556 cells in the newly
>> >> hatched larval stage and 959 somatic cells in the adult.  These are
>> >> not replaced during the lifetime of the animal.
>> >> 
>> >> It is quite likely, though, that te cells themselves completely turn
>> >> over during their life so that a cell that remains is made of
>> >> completely new proteins and organelles.  The DNA does not turn over,
>> >> though.
>>  
>> >Thank you for this. Do I understand you correctly in saying that the
>> >proteins and organelles would change even in those animals that
>> >retained exactly the same cells during their adult life? Would this be
>> >a general truth?
>> >
>> 
>> The is a constant synthesis of new proteins and breakdown of old
>> proteins in all cells -- a process called protein turnover.  In fact,
>> there is a specific mechansm to degrade damaged or unused proteins
>> involving a special protein called ubiquitin (because it is so
>> widespread) that "marks" other proteins for degradation and a special
>> cell structure called a proteosome to do the dirty work.  The amino
>> acids are recycled to rebuild proteins or else they are metabolized.
>> Free radicals in a cell are a major source of damage which is why
>> antioxidants are so important to us. The lifetime of various proteins
>> may be hours or days to weeks and months and years.
>> 
>> Lipids are also subject to turnover, as is even the mineral in bone.
>> DNA is never replaced, but there are DNA repair enzymes that work to
>> fix problems.   I don't really know if EVERYthing in the body (except
>> DNA) is replaced, but certainly a lot of things are. There are
>> organisms (like Mayflies) that have such short short lifetimes that
>> probably the molecules in those cells aren't around long enough to be
>> replaced.
>> 
>> The question remains -- what do you mean by the identity of an object?
>> If I replace every molecule in your body with another identical one,
>> are you the same person?  There is an old joke about a museum with
>> George Washington's original hatchet -- the one that (according to an
>> almost certainly false legend) he used to chop down the cherry tree.
>> "Of course, the handle has been replaced three times and the head
>> twice, but it is still the same hatchet!"   I have heard that Chinese
>> view ancient buildings with great respect, still they perform routine
>> maintenance to keep them in good working order.  So you rebuild the
>> damaged roof and the walls and repair the floors and replace the
>> broken furniture so the building can remain functional.  An ancient
>> building may have no part left that is ancient.  Still, the "building
>> itself" is still there.
>
>Indeed. As a philosopher, I research exactly this topic! But re. the
>question of DNA not being replaced, do you mean that the very matter
>of the DNA does not turn over, or only that the code remains the same?
>Would I be right in saying that the matter constituting the DNA also
>turns over, whilst preserving the code?
>
The DNA is a rather simple and special case. The DNA molecule itself
doesn't change (unless there is repair to be done).  So once an atom
of carbon gets put into a sugar in one of the bases in the DNA
backbone, it sits there until the cell is dead.  There are not a lot
of molecules in the cell where that happens. I am not sure of all the
biochemical details, perhaps DNA is the only one.

Of course, the physicists claim that there is no way to tell the
difference between two elementary particles -- if you swap one
electron for another wth the same spin (and other quantum numbers)
there is no detectable difference. If you replace one deoxyribose for
another or one adenosine for another, there would be no way of knowing
that a change was made.