Naked Science Forum
Life Sciences => Cells, Microbes & Viruses => Topic started by: SymeAaro on 30/06/2017 22:02:38
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Sorry, this may seem amateur, but how do bacteria evolve? I thought they were all clones? Also, how does this work in animals, if all the DNA is passed on from parents? How do variations rise up?
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Bacteria may not reproduce sexually, but they certainly exchange genetic information. Look up "lateral gene transfer" or "horizontal gene transfer"
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Sorry, this may see, amateur, but how do bacteria evolve? I thought they were all clones? Also, how does this work in animals, if all the DNA is passed on from parents? How do variations rise up?
Cloning is never perfect; copying DNA sometimes produces errors, so changes do occur occasionally.
Nuclear radiation and the products of metabolism also cause mutations.
There was a long-running experiment where bacteria were provided with 2 nutrients:
- a large amount of one nutrient that they normally only use under anaerobic conditions
- a small amount of one that they normally used under aerobic conditions
Over thousands of generations the scientists could see changes in the genetic mix, to optimise frugal living on their normal nutrient. Eventually a mix of mutations arose that allowed the bacteria to use the more generous nutrient under aerobic conditions.
Horizontal gene transfer allows large packages of genetic changes between different types of bacteria.
It is suspected that viruses may also sometimes transfer genetic material from one bacteria to another.
In animals using sexual reproduction, the set of genes is continually being shuffled in children. Crossover mutations sometimes allow good genes from both parents to appear on a single chromosome.
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Evolution is the adaptation of a species to its environment. This is achieved through "natural selection" whereby an organism with the combination of genes that make it fittest within its habitat will enjoy greater reproductive success. This will result in the progressive enrichment of those genes within the population as a whole.
So where do these different genes come from in the first place? The answer is that there are many different variations of the same gene in a population. So every human has about 20,000 genes, each one doing a specific job to make a cell work, but there are variants of each of those 20,000 genes. This occurs because the sequence of DNA letters that make up the gene can differ slightly between individuals, and the differences can alter the function of the gene, in some cases by a trivial amount but in other cases quite dramatically.
The next question is from where and how did these gene variants arise in the first place? The answer is through mutation. When DNA code is copied in a cell, either to produce new cells from stem cells to renew or grow tissues, or to produce sperm and eggs for the purposes of reproduction, there is a low-level error rate whereby genetic spelling mistakes are made occasionally. In humans the error rate is about 1 in 100 million (1 in 108) genetic letters copied. So, in a genome containing 3 billion genetic letters, that's about 100 new mutations in every sperm or egg that you make.
These mutations are passed on to your children. If those changes are beneficial, they are more likely to be maintained in the population subsequently.
Bacteria are not much different. Every time they copy their 5 million or so genetic letters they make the odd mistake. These mistakes change the way some of their genes work, and if those changes affect processes that are targeted by antibiotics, then bugs carrying those mutations will have a better chance of survival than bacteria unendowed with the gene. And because bacteria grow at a ferocious rate to achieve very high population densities very rapidly, the chances of helpful mutations being disclosed rapidly are very high.
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Bacteria are not much different. Every time they copy their 5 million or so genetic letters they make the odd mistake.
I understand that humans have a number of layers of profreading every time a cell divides. Human cells err on the side of killing the cell if there is an uncorrectable mutation. If humans didn't do this, we would be riddled with cancer before we reached reproductive age.
Peto's paradox (https://en.wikipedia.org/wiki/Peto%27s_paradox) points out that this is even more crucial in larger species like elephants and whales. Genetic sequencing in the past few years shows that elephants have more copies of these proofreading/apoptosis genes than humans.
Bacteria, with their smaller number of DNA bases have a lower chance of mutation. The fact that they do not rely on careful coordination of multiple cells and organs to survive to reproductive age means that a mutated cell is left to limp on as best it can, instead of being actively killed, as in human cells.
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Bacteria are not much different. Every time they copy their 5 million or so genetic letters they make the odd mistake.
I understand that humans have a number of layers of profreading every time a cell divides. Human cells err on the side of killing the cell if there is an uncorrectable mutation. If humans didn't do this, we would be riddled with cancer before we reached reproductive age.
Peto's paradox (https://en.wikipedia.org/wiki/Peto%27s_paradox) points out that this is even more crucial in larger species like elephants and whales. Genetic sequencing in the past few years shows that elephants have more copies of these proofreading/apoptosis genes than humans.
Bacteria, with their smaller number of DNA bases have a lower chance of mutation. The fact that they do not rely on careful coordination of multiple cells and organs to survive to reproductive age means that a mutated cell is left to limp on as best it can, instead of being actively killed, as in human cells.
Humans pose a problem to evolution in that negative genetic mutations can now be compensated for with medicines. This allows humans to by-pass natural selection and pass on negative genes to the next generation. The next generation may then also require medicines to compensate. I often wondered what the long term implication of artificial selection, through prosthesis, will be on the human race. I suppose this would be a good business model in terms of driving the demand for evolving support technologies. It would allow the industry to expand due to compounding problems.
Picture every compounding adverse genetic mutation, having a medical prosthesis, such that what nature would not have selected becomes artificially selected within the breeding population. This could place humans in a precarious position if there was ever a disruption of culture from human or natural disaster. In a few hundred years everyone could be on some medication.
Bacteria compensate for genes with lateral genetic transfer, from other bacteria. This natural schema model would be essentially be equivalent to an apex human technology, where we insert genes for genetic corrections geared toward natural selection, thereby eliminating the need for external prosthesis.
Once this is developed the current business model will decline. There may be a push back against any such apex technology, that could cut into an industry wide bottom line. There is a possibility that first world cultures of the future may be the most at risk, if we ever did have a global type disaster. Poor countries who make more use of natural selection by default, due to lack of available prosthesis will have a higher survival rate.
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Through enviromental pressure and random mutations which help the better individuals stay alive and bring their offspring. (Eg develop Beta Lactamases)