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Author Topic:   Random mutations
Sylas
Member (Idle past 5260 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 16 of 35 (209556)
05-19-2005 1:33 AM
Reply to: Message 15 by Limbo
05-19-2005 1:22 AM


Not stopping, but slowing.
Limbo writes:
Are mutations as random as we thought? Maybe mutations are controlled by pre-programming. If so, how could we have evolved the program in the first place?
This is quite counter to the actual ideas explored in the cited article. They are proposing that there is a protein which helps modulate the rate at which mutation occurs. In conditions of stress, the mutation rate can increase.
There is no suggestion that the mutations are controlled in the sense of affecting the result of mutations. Only the rate is affected. The mutations remain as random as ever.
Hence it is not about "stopping" evolution. It is about "slowing" evolution. There will always be a background mutation rate. If there is an accelerator protein, however, then perhaps its effect can be blocked so that bacteria are not quite as "mutable", and will take rather longer to become resistant to antibiotics.
Cheers -- Sylas
This message has been edited by Sylas, 05-19-2005 01:38 AM

This message is a reply to:
 Message 15 by Limbo, posted 05-19-2005 1:22 AM Limbo has replied

Replies to this message:
 Message 17 by Limbo, posted 05-19-2005 1:45 AM Sylas has replied

  
Sylas
Member (Idle past 5260 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 20 of 35 (209566)
05-19-2005 2:23 AM
Reply to: Message 17 by Limbo
05-19-2005 1:45 AM


Re: Not stopping, but slowing.
Limbo writes:
I mean, life would have to arise with the slowest possible mutation rate as a default rate, right?
No, in fact.
Generally speaking, mutation is a bad thing. But chemistry being what it is, replication is never perfect, and this is why there is always a small background mutation rate.
If, perchance, something changes to make the replication a bit more stable, that will tend to persist. So over time, the fidelity or accuracy of mutation has tended to increase. In Eukaryotic organisms, for example, some quite complicated cellular machinery has developed to repair errors that creep into DNA replication. The accuracy achieved by DNA replication is quite astounding. It seems that only about one error for every 100000000 bases copied is the going rate. Without the evolved repair mechanisms, there would be more errors.
Since a human genome has about 6000000000 bases, we have roughly 60 or so mutations occurring with each replication of cell. These numbers are a bit off and not known to full accuracy, but basically we all carry a significant number of new mutation introduced since our parents. The situation is complicated by the fact that there are hot spots where mutation is more common, and cold spots where it is less common. Repair slows replication, which carries a cost. So repair tends to be focused on important parts of the genome.... I think.
In any case, the very first replicators probably had high mutation rates, and over time evolution helped bring these down. A major problem for origin of life ideas is how to get an earlier replicator of sufficient accuracy that it is not overwhelmed by mutation loads.
But under stress... that is, an organism finding itself in an environment for which it is not well adapted... there may be a benefit to having a slight increase in mutation rates.
Cheers -- Sylas

This message is a reply to:
 Message 17 by Limbo, posted 05-19-2005 1:45 AM Limbo has replied

Replies to this message:
 Message 22 by Limbo, posted 05-19-2005 3:06 AM Sylas has replied

  
Sylas
Member (Idle past 5260 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 26 of 35 (210757)
05-23-2005 10:14 PM
Reply to: Message 22 by Limbo
05-19-2005 3:06 AM


Re: Not stopping, but slowing.
Is this where natural selection kicks in? The organism which can speed up mutation survives to reproduce?
Yes.
There is a rather amusing analogy here with how some bacteria can swim towards light. Some bacteria have flagella that can beat in two different modes. In one mode, the bacterium tends to move in a straight line. In the other mode, the bacterium tends to "tumble" in place.
The bacterium alternates between these two modes, tumbling, and then moving in some direction, then tumbling again, and so on. The direction of movement after tumbling appears to be completely random. How then do they manage to swim towards the light?
Basically, the length of time between tumbles increases when the light level is low.
This means that in low light levels, each straightline movement is longer, and moves it further. In high light levels, the straightline movements are quite short. In either case, the movement is a random walk, but when it is dark the walk tends to cover longer distances, which increases the chance of moving out from the dark regions. When it is in the light, it tends to remain in that vicinity. If it happens to move deeper into darkness; tough. They played the odds and lost.
In an analogous fashion, a bacterium in a stressful environment may replicate with less accuracy, leading to larger "steps" within the genome space. Essentially, it is not where it wants to be, and plays the odds in the "hope" that its daughter bacteria end up into a better position.
None of this is thought out by a bacterium. The chemistry in both cases can be studied. It is complex organic chemistry, but no additional control is applied other than normal chemistry. The selection for changes in the chemistry controlling replication, or flagellum beating, will tend to mean that the survivors in each generation are those that took larger steps in bad circumstances.
Cheers -- Sylas

This message is a reply to:
 Message 22 by Limbo, posted 05-19-2005 3:06 AM Limbo has not replied

  
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