Perplexed about DNA - Help please?

[Filip Sandor]

I have some questions about DNA and was wondering if someone could help me.

Apparently DNA is a highly comlex, ordered structure composed of several compounds. Does anyone know approximately how many lines or 'segments' of code exist for something like the most basic, single celled organisms which first inhabited our planet? Also, in conjunction with the previous question, does anyone know approximately how much alteration such a strand of DNA requires before the organism would most likely not live or be able to reproduce. In other words, how much damage would you have to do to the DNA to essentially destroy the organism?

 

[Dymanic]

Does anyone know approximately how many lines or 'segments' of code exist for something like the most basic, single celled organisms which first inhabited our planet?

As far as the very first organisms are concerned, no one really does. As for modern organisms, a theoretical lower limit to genome size has been proposed at around half a million base pairs (genome size isn't measured in "segments", and certainly not in "lines"). Have a look at this:

http://www.genomenewsnetwork.org/articles/05_02/undersea_creature.shtml

 

[espritch]

This [url="http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/G/GenomeSizes.html" ]link[/url] gives genome sizes for some current living things. The smallest genome for a free living organism listed has 1,308,759 base pairs.

Of course this would probably be far more than in the first living cellular organism since all living organisms are the product of billions of years of evolution. And the first cellular organism would have been a lot more complex than the first self replicating molecules from which it evolved. Scientist have developed self replicating RNA sequences in the lab and think RNA sequences with as few as 150 base pairs could replicate.

As far as how much genetic damage would kill the most simple organisms, I would guess it wouldn't take much. Of course if they reproduced quickly, this wouldn't be a big issue since if one dies there are plenty more around to preserve the genome.

 

[Kevin_Lowe]

As far as how much genetic damage would kill the most simple organisms, I would guess it wouldn't take much. Of course if they reproduced quickly, this wouldn't be a big issue since if one dies there are plenty more around to preserve the genome.

The short answer is that much like damage to a human body, damage to a genome depends on where you hit it. Some chunks of DNA are obsolete remnants, and as far as we know (yet) they do not do anything at all. Other genes are necessary for life and if the body could not produce them then the organism would not live.

So how did this life gig get started, I suspect you are going to ask? Nobody knows.

Theists like to leap to the conclusion that God Did It, on the basis that a primitive organism just happening to develop somehow out of non-living but self-replicating chemical building blocks is less likely than a spooky God-thingy just happening to pop up out of nowhere. Needless to say I don't think their proposed theory has any advantages over the theory that we don't know but it probably involved perfectly ordinary chemical processes.

 

[epepke]

I have some questions about DNA and was wondering if someone could help me.

Apparently DNA is a highly comlex, ordered structure composed of several compounds. Does anyone know approximately how many lines or 'segments' of code exist for something like the most basic, single celled organisms which first inhabited our planet? Also, in conjunction with the previous question, does anyone know approximately how much alteration such a strand of DNA requires before the organism would most likely not live or be able to reproduce. In other words, how much damage would you have to do to the DNA to essentially destroy the organism?

The big problem here is that the most basic organisms (they weren't even cells) probably didn't use DNA at all but used RNA instead.

As far as those, it's probably on the order of 100 base pairs. Maybe fewer.

 

[espritch]

The short answer is that much like damage to a human body, damage to a genome depends on where you hit it. Some chunks of DNA are obsolete remnants, and as far as we know (yet) they do not do anything at all. Other genes are necessary for life and if the body could not produce them then the organism would not live.

I was basing my remark on the assumption that the earliest cellular organisms probably didn't have much junk DNA. If you are talking about an organism representing the bare minimum genetic code needed for basic life function and you assume it has not had time to accumulate much junk, then any genetic damge could mess up something vital and be fatal.

Of course I have no idea how long it would take to accumulate a significant amount of junk DNA.

 

[Eos of the Eons]

It takes very little change to cause cancer (out of control growth). Various cancers are caused by "dna damage" (the sun's radiation will cause skin cancer). The surrounding tissues are overtaken, function ceases, and you die. Depending on the type of cancer, you can die quickly or it may take a while. Skin cancer will kill you, but not as quicky as tumours in your heart or brain.

http://www.infoaging.org/b-dna-5-cancer.html

 

[clarsct]

Well, actually, it is unlikely the Sun's rays will affect the DNA molecule directly. Your body is some 70% water, or so, IIRC, and it is MUCH more likely that the radiation will rip an electron off a water molecule, creating a peroxide or superoxide. These will, of course, react with the first thing they touch, be it a protein, RNA, DNA, etc. This is how most cancers are formed when caused by radiation.

Just a little Radiation Biology coming back to me....

 

[Asolepius]

I have some questions about DNA and was wondering if someone could help me.

Apparently DNA is a highly comlex, ordered structure composed of several compounds. Does anyone know approximately how many lines or 'segments' of code exist for something like the most basic, single celled organisms which first inhabited our planet? Also, in conjunction with the previous question, does anyone know approximately how much alteration such a strand of DNA requires before the organism would most likely not live or be able to reproduce. In other words, how much damage would you have to do to the DNA to essentially destroy the organism?

There is a wealth of discussion on this topic in Mendel's Demon by Mark Ridley. I am half way through the book so far, and can recommend it highly.

 

[Soapy Sam]

Eos- bear in mind that most cancers seem to simply involve the failure of a "stop" switch. So it may take only one cosmic ray in the wrong place, or just one copying error in cell division.
It's all the complex cell reproduction equipment that does the real work in producing the cancer.
Extremely simple organisms of the type Filip asks about would not be subject to runaway sub systems, because they basically don't have any subsystems to begin with. Cancer is a curse of complexity. (It's like what happens to Windows after about a year. Hoist with it's own petard, kind of thingy...)

Filip- Being psychic, I know that nobody here knows the answer to your next question.
Nobody else does , either.

But for some people who have ideas- here is one place to start looking.

http://originoflife.net/cairns_smith/

 

[DrPangloss]

Dna

The question as to the amount of damage DNA can handle is really complicated because most organisms have evolved error-checking repair mechanisms.
DNA polymerase, an enzyme that is a major player in the copying of DNA strands during cellular division has error-checking properties.

 

[Asolepius]

The question as to the amount of damage DNA can handle is really complicated because most organisms have evolved error-checking repair mechanisms.
DNA polymerase, an enzyme that is a major player in the copying of DNA strands during cellular division has error-checking properties.

All covered by Mark Ridley in his book.

 

[Eos of the Eons]

Well, actually, it is unlikely the Sun's rays will affect the DNA molecule directly. Your body is some 70% water, or so, IIRC, and it is MUCH more likely that the radiation will rip an electron off a water molecule, creating a peroxide or superoxide. These will, of course, react with the first thing they touch, be it a protein, RNA, DNA, etc. This is how most cancers are formed when caused by radiation.

Just a little Radiation Biology coming back to me....

Unlikely my dear? More like rare...not unlikely. Unlikely points to not possible, but even you acknowledged how it can happen. A sunburn is a clear warning you've put yourself at increased risk, indicating damage HAS been done.

UV radiation can also damage the DNA in the gene that codes for p53. As a result, the p53 protein doesn’t repair mutated cells as it should, and they end up dividing and propagating. A tumour is born.

http://www.ahfmr.ab.ca/publications/newsletter/Summer00/sum00/inside/skin.cancer.feat.htm

The originally question:

does anyone know approximately how much alteration such a strand of DNA requires before the organism would most likely not live

The answer in layman's terms is "not very much".

 

[Capsid]

The answer in layman's terms is "not very much".

Cystic fibrosis is caused by a deletion of only 3 base pairs.

 

[Paul C. Anagnostopoulos]

And Down Syndrome is caused by an extra chromosome 21.

It's a marvelously complex game, this evolution thing.

~~ Paul

 

[Eos of the Eons]

More excellent examples

I would call it a "genetic thing" over an "evolution thing" though. These dna anomalies will hardly help humans to evolve into a different species

 

[Paul C. Anagnostopoulos]

No, but sickle cell helps with malaria. How about a genetic evolution thing?

~~ Paul

 

[Eos of the Eons]

Interesting

 

[Filip Sandor]

Wow, thanks for all the information and thank you for the links you guys.

One person said it may take about 100 base pairs of RNA compounds (sorry if my terminology is wrong here) - basically, 100 'instructions' to build the most basic organism. Since I have not read or heard of anything like this in genetics I take it with a grain of salt; however, I have seen a lot evidence that it takes far more than that to form a living cell. The half-million guestimate sounds more in line with what I've heard on nature shows and read in science mags. Still, I appreciate everybody's input.

Now here is where it gets foggy for me, when we introduce the theory of evolution via random mutation. If it is indeed true that the first organisms had very little or no junk DNA and that only miniscule amounts of damage to the strand would cause the organism to be destroyed then the idea of random mutation seems to break down. By definition, random means there is a 50/50 chance of a positive mutation occuring that will be beneficial to the overall structure of the DNA or the organism and 50/50 chance of a negative mutation happening.

Practically speaking, it is impossible to have a half-million positive mutations occur consecutively, even if we give room for say 1000 negative mutations that won't damage the DNA. This is basically your 1,000,000 monkey/ typewriter for 1,000,000 years idea. Realistically speaking, you would probably need several trillion monkeys and typewriters typing over eons to have this happen and even then it seems very bleak unless the monkeys are somehow more inclined to press certain keys more than others at different times, based on a set of ordered influences.

What do you guys think about that?

 

[Eos of the Eons]

I think you have limited evolution to simply be caused by mutation? Doesn't happen that way. Instead, some traits are built on in a way that may or may not be described as "mutation". I don't like to think of it that way, but a beneficial mutation can be building block rather than just a "mutation". Technically it is a mutation though.

You already can see that a tiny change can have huge consequences. One tiny change can also provide a HUGE BENEFIT. This benefit is passed on because the organism is better able to survive than its competitors by getting resources better or invading a niche the competitors cannot follow it into.

Once you've built upon a huge number of traits, then mutation becomes a non-topic. Instead selection takes over. There are a number of factors when it comes to selection. You no longer need a single mutation to get a whole new species.

You've alreadyd answered your own question. It doesn't take much to render something unable to live. It also doesn't take much to change it..

Do you realize how much mutation or wrong sequencing is going on? Babies are born without brains quite often enough. Some are born missing heart valves. Just as many beneficial mutations happen, but if they are not selected for, then we may never realize they ever happened.

Some people have different T cells from the rest of us, and cannot be infected with HIV. If HIV never came around, then someone like me would never have even known these differences exist from human to human.

You cannot realize the scope of this unless you are actually well educated in this area.

Selective pressure almost become mandatory in complex organisms like humans. Not a good selective pressure, but one requiring surving an extreme change in environment. I've tried to picture such a scenerio.

Some humans get stranded on a planet with differences in atmospheric pressure, slight different mixture of atmospheric gasses, colder climate, and let's say food. Water may also not be so plentiful. We already have some humans born with hair that grows all over the body, and some that don't perspire. Let's say the people that don't perspire survive better, along with the folks with increased hair growth that survive better in the cold. They get very tall as well, as tall folks are deemed more attractive and their bodies fare better in the different gravity. Let's throw in some useful mutation. What would be useful? Something with the ears...they end up closer to the top of the head and smaller for whatever beneficial reason (hear better through all the hair if they are up there, and catch predatory noises...whatever). All the people with "normal ears" get eaten by some predator, leaving the weird eared people around as now normal.

Soon enough you would have people that are no longer homo sapiens. Humans still exist on earth, but these people have taken over the new planet. They survive better there than we would.

The history of earth tells this story over and over again. Selective pressure cause change. Most species are extinct because of evolving animals. Others survive alongside new species if they can compete.