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Author Topic:   Questions on "Random" Mutations
taylor_31
Member (Idle past 4035 days)
Posts: 86
From: Oklahoma!
Joined: 05-14-2007


Message 1 of 80 (409665)
07-10-2007 6:09 PM


As I finished up a book about evolution today, I learned a surprising fact. Until today, I had assumed that mutations were totally random and there was no way to predict what would "show up" in any given mutation. I learned, however, that the processes by which mutations arise are actually nonrandom; rather, they are affected by many other factors. The only thing that's random is the effect that the mutation has on the organism's fitness.

In addition, the book indicated that the primary causes of mutation are physical events like X-rays, chemicals, and other genes. I had always thought that mutations occured by a random "shuffling" of the parents' DNA; instead, it appears to be caused by nonrandom, physical causes. When do these physical causes actually happen? While the embryo is developing or something?

Another way that mutations are nonrandom is that it can only effect the existing processes of embryonic development. Doesn't that limit the potential effects of mutations? Does each organism have a certain number of potential mutations, and no more? If this is true, then as a population evolves more mutation "possibilities" should become available. Is that correct?

And, are some evolutionary "pathways" so ridiculous that they'll never be "walked upon"? For example, and I feel stupid typing this, but we don't see any fire-breathing dragons. And yet we see plenty of amazing features in the animal kingdoms which have staggering complexity. With slight, successive mutations and natural selection, why can't an animal breath fire? (God, I feel stupid!)

Thanks for putting up with my questions! And for reference, the book I read was The Blind Watchmaker by Richard Dawkins, and the chapter that inspired these questions is called "Doomed Rivals" (specifically pgs. 434-445).


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AdminModulous
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Posts: 897
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Message 2 of 80 (409759)
07-11-2007 5:58 AM


Thread moved here from the Proposed New Topics forum.
    
mick
Member (Idle past 3098 days)
Posts: 913
Joined: 02-17-2005


Message 3 of 80 (409768)
07-11-2007 7:05 AM
Reply to: Message 1 by taylor_31
07-10-2007 6:09 PM


Hi taylor,

taylor writes:

Until today, I had assumed that mutations were totally random and there was no way to predict what would "show up" in any given mutation.

Random things are not necessarily unpredictable. For example a coin toss might give a random result, but we are able to predict, with narrow confidence intervals, the number of heads obtained after a few thousand tosses. Similarly, completely nonrandom processes are not necessarily predictable, chaotic systems being a good example.

taylor writes:

I learned, however, that the processes by which mutations arise are actually nonrandom; rather, they are affected by many other factors. The only thing that's random is the effect that the mutation has on the organism's fitness.

One often hears the phrase "random with respect to fitness".

taylor writes:

In addition, the book indicated that the primary causes of mutation are physical events like X-rays, chemicals, and other genes. I had always thought that mutations occured by a random "shuffling" of the parents' DNA; instead, it appears to be caused by nonrandom, physical causes. When do these physical causes actually happen? While the embryo is developing or something?

The random shuffling that you are thinking of is probably recombination, which is not a form of mutation but generates genetic diversity by its effects on redistributing diploid alleles within two haploid genomes. Remember you have a maternal and paternal copy of each chromosome. Imagine there are two genes on the maternal chromosome, called A and B. On the paternal chromosome you have a and b (lower case reflecting slightly different versions or alleles of the genetic sequence). When the paternal and maternal chromosomes align with each other during meiosis, some crossing over can occur. One of your sperm or eggs might end up with the combination A-b while another might end up with a-B. Neither of these combinations was present in either of your parents. Thus, recombination can increase genetic diversity by giving rise to new allele combinations in the offspring. Obviously recombination generally occurs during the production of sper m or eggs.

As for physical causes of mutations, you are correct that factors such as radiation (including sunshine), toxins, etc. are involved. Obviously, if a mutation is going to be passed on to your offspring it must occur in the germ line (that is, in the sperm or eggs). The vast majority of mutations occur in our somatic cells (simply because we have more of them) and whether these are beneficial, neutral or cause diseases such as cancer, out children will not benefit or suffer from them. For this reason it is often believed that most mutations of evolutionary importance occur in the cell cycles involved in gamete production, before the embyo even exists.

In principle, however, mutations occurring in the developing embryo might also be passed into the germ line. For example, when the embryo has only single cell, any mutation occurring in its genome will inevitably end up in the sex cells (which must develop from that single cell at some point). In a two-cell embryo there is a 50% chance of the mutation ending up in the sex cells. In a four-cell embryo the chance is 25%. Since the growth of cell number in an embryo increases at an exponential rate, I would expect that the chances of a somatic mutation ending up in the sex cells becomes vanishingly small quite quickly. After that, only mutations occurring directly in the sex cells will be relevant to evolution.

An important form of nonrandomness in mutations is the result of the chemistry of nucleotides. Apparently it is energetically more feasible for mutations to convert from purines->purines or pyrimidines->pyrimidines than it is for mutations to convert from purines->pyrimidines or vice versa. For this reason, mutations like C->T or A->G tend to be several times more common than mutations like A->T. The mutation C->T is by far the most common single point mutation in the human genome. But these mutations are still "random" in the sense that the number of mutations from each class that occur over some period of time is a random variable drawn from some sort of probability distribution. The only thing to note is that each class of mutation has a probability distribution with its own characteristic mean rate.

Another important kind of non-randomness results from the existence of "mutation hot spots" on each chromosome. Apparently some chromosomal regions accumulate mutations more rapidly than others. Regions with high levels of G and C nucleotides also tend to mutate less often.

taylor writes:

Another way that mutations are nonrandom is that it can only effect the existing processes of embryonic development. Doesn't that limit the potential effects of mutations? Does each organism have a certain number of potential mutations, and no more? If this is true, then as a population evolves more mutation "possibilities" should become available. Is that correct?

The number of potential mutations is, I suppose, limited by the size of your genome. But in practice mutation events are so rare that they are not constrained in this way. Developmental constraints, as you suggest are more important. You can see a list of developmental genetic diseases (and some photos) at this link. As you might imagine, mutations that badly disrupt the developmental system of an organism will likely compromise future reproductive output severely. However developmental processes are quite plastic - they can often adapt to minor and even major disruptions of the developmental program. In her book "Developmental plasticity and evolution" Mary-Jane West-Eberhard describes a bipedal goat whose developmental system was able to accommodate a genetic or environmental disruption to its front leg formation, resulting in upright walking! So it's probably more correct to say that the number of functional mutations is limited by the extent to which the developmental program of the organism can accommodate peturbations. I'll see if I can find the photo of the goat and post it here - it's quite cool.

As to your last point, I don't see why we would expect "more evolved" animals to have more plastic developmental programs. In fact we might as well expect the opposite, with an animal becoming more and more adapted to some specific niche from which it cannot depart without catastrophic consequences.

taylor writes:

And, are some evolutionary "pathways" so ridiculous that they'll never be "walked upon"? For example, and I feel stupid typing this, but we don't see any fire-breathing dragons. And yet we see plenty of amazing features in the animal kingdoms which have staggering complexity. With slight, successive mutations and natural selection, why can't an animal breath fire? (God, I feel stupid!)

Nature doesn't care about being ridiculous. We don't have fire-breathing lizards but we do have glow-in-the-dark jellyfish and chameleons whose skin can change color to match the background. There is a jumping millipede, and a kind of frog that curls into a ball and rolls away to avoid predators (though that may have been from Labyrinth, I tend to get reality and Jim Henson mixed up). What more do you want?

On dragons, there is a quirky book by Peter Dickinson called "The Flight of Dragons" in which the biology necessary for the fire breathing and flight of large reptiles is exhaustively described. I don't remember the details, but he made it sound quite straightforward, as long as breathing fire and being filled with hydrogen is not an evolutionary liability. The book was somehow turned into a cartoon.

Mick


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 Message 1 by taylor_31, posted 07-10-2007 6:09 PM taylor_31 has responded

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Dr Adequate
Member
Posts: 16094
Joined: 07-20-2006
Member Rating: 9.0


Message 4 of 80 (409783)
07-11-2007 8:30 AM
Reply to: Message 1 by taylor_31
07-10-2007 6:09 PM


As I finished up a book about evolution today, I learned a surprising fact. Until today, I had assumed that mutations were totally random and there was no way to predict what would "show up" in any given mutation. I learned, however, that the processes by which mutations arise are actually nonrandom; rather, they are affected by many other factors. The only thing that's random is the effect that the mutation has on the organism's fitness.

I think you're confusing "random" with "uncaused".

The mutations are random in two closely related senses: first, as you say, with respect to the fitness of the organisms, and secondly, because we can't predict which mutations will occur.

By analogy, the fall of dice in a casino has underlying physical causes, but they do not favor any particular gambler, nor can we predict how they're going to come down.


This message is a reply to:
 Message 1 by taylor_31, posted 07-10-2007 6:09 PM taylor_31 has responded

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 Message 6 by taylor_31, posted 07-11-2007 8:05 PM Dr Adequate has responded

  
taylor_31
Member (Idle past 4035 days)
Posts: 86
From: Oklahoma!
Joined: 05-14-2007


Message 5 of 80 (409863)
07-11-2007 7:57 PM
Reply to: Message 3 by mick
07-11-2007 7:05 AM


Thanks for your post, it was very interesting and helpful!

Random things are not necessarily unpredictable.

Okay, but mutations are not statistically random because some mutations are more like to occur than others. But in terms of aim, reason, or pattern, then mutations are indeed random, because they cannot possibly aim towards anything.

I hope this doesn't get semantic, but I was just curious.

One of your sperm or eggs might end up with the combination A-b while another might end up with a-B. Neither of these combinations was present in either of your parents.

So over great periods of time, the composition of genes, through the process of these recombinations, can change drastically. I thought, however, that any change from the parent to the offspring could be considered a mutation. Why can't recombination be considered a mutation?

For this reason it is often believed that most mutations of evolutionary importance occur in the cell cycles involved in gamete production, before the embyo even exists.

What exactly are these mutations, and when do they occur? Is it when a cell copies itself, and the DNA is slightly changed?

In principle, however, mutations occurring in the developing embryo might also be passed into the germ line.

But this mutation would probably not be caused by radiation or anything else "external" of the body, right? I'm sure that I'd have to understand the process that gives rise to an embryo before I understand how the mutation got there, and that's probably a whole other topic. ;)

However developmental processes are quite plastic - they can often adapt to minor and even major disruptions of the developmental program.

I'm sorry, but I don't understand what you mean by "plastic". Do you mean that an organism can work hard to get around the mutation and survive, like your goat example? (Btw, a picture would be cool!)

As to your last point, I don't see why we would expect "more evolved" animals to have more plastic developmental programs.

Sorry, I probably wasn't very clear. What I meant was, are the potential mutation possibilities (meaning type of mutation, potential number of mutations, etc.) the same between, say, an ant and a human? Or am I misunderstanding the principles of mutation?

We don't have fire-breathing lizards but we do have glow-in-the-dark jellyfish and chameleons whose skin can change color to match the background.

Again, I probably wasn't very clear. In my book, Professor Dawkins seems to imply that a fire-breathing dragon was incredibly unlikely or something. When I read that I thought, well, look at nature! A great deal of it already is incredibly unlikely!

But does Prof. Dawkins have a point? Surely there are some limits to what mutations can produce, right?

Thanks for your help!


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taylor_31
Member (Idle past 4035 days)
Posts: 86
From: Oklahoma!
Joined: 05-14-2007


Message 6 of 80 (409865)
07-11-2007 8:05 PM
Reply to: Message 4 by Dr Adequate
07-11-2007 8:30 AM


The mutations are random in two closely related senses: first, as you say, with respect to the fitness of the organisms, and secondly, because we can't predict which mutations will occur.

But I thought that the probability rates of different mutations happening are different. If that's true, then why can't you predict which mutation would occur?

If the probability rates of all the mutations were the same, then it would be statistically random.


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crashfrog
Inactive Member


Message 7 of 80 (409870)
07-11-2007 9:11 PM
Reply to: Message 5 by taylor_31
07-11-2007 7:57 PM


But this mutation would probably not be caused by radiation or anything else "external" of the body, right? I'm sure that I'd have to understand the process that gives rise to an embryo before I understand how the mutation got there, and that's probably a whole other topic.

Imagine your whole life run in reverse.

1) You as you are now; an adult human being composed of billions of cells that, individually, contain your entire genome. As your cells age and are exposed to elements in your environment that damage DNA (which we call mutagens), some cells accrue changes to that copy of your genome that makes them different from other cells. Some of these differences can cause diseases like cancer. Some cells become so damaged that your body sends them a chemical "self-destruct" signal, and they die. As your cells die, they are replaced by new cells formed from old ones by mitosis.

2) All your body's cells - including the gametes produced by your genitals - are descended through mitosis from the original single cell that, at one point, implanted itself in your mother's uterus and began to gestate.

3) That cell is the result of a fusion of two cells, one donated by each of your parents. Each cell contributes one half each of 23 pairs of homologous chromosomes. The 23rd chromosome donated by your father determined your sex. The cell donated by your mother contributes a small city of cellular organelles, including mitochondria, which have their own "private" genetics.

4) While the physical organs that produce gametes differ between the sexes, the process of gamete formation is similar. A special kind of stem cell undergoes mitosis, then undergoes a special form of cell division where half of the chromosomes go one way and the other half go the other way. (This is called meiosis.) These produce gametes with one each of 23 pairs of your parent's chromosomes, assorted randomly.

For your mother, this process occurred about 500 times in her embryonic ovaries, before she was even born. For your father, this process happened roughly 1.5 million times every day.

5) and so on for your parents, and their parents, etc.

The long and the short of this is that there's really two different kinds of mutations. The mutations that happen during Part 4 are passed onto you if that's one of the cells that combines to form you, and then they're replicated into every cell in your body, including the gametes that you produce (and thus, stand a chance of being passed on to your offspring.) We call these germline mutations.

Mutations that accrue after your fertilization, when your zygotic self began to produce the rest of your body's cells through mitosis, aren't shared by your entire body. Mutations that happen when you're an adult (usually somewhere near the surface of your skin, as a result of radiation) are limited to that cell, typically. Since your gametes don't share the same mutation, you can't pass these mutations on to your offspring. We call these somatic mutations.

Mutations can occur in your gamete-producing cells. Like somatic mutations they're not shared by the rest of your cells but they can be passed on to your offspring. These are somatic mutations to you and germline mutations to your offspring.

Since your germline mutations are shared by all your body's cells, these mutations can result in characteristics we can observe at the macro level. For instance, a mutation that prevents your body from producing a certain kind of pigment results in albinism.

Somatic mutations typically don't result in macro-observable changes, but occasionally they cause localized problems, like tumors.

I guess the take-home message here is that, in the same way you have a family tree, your body's cells have their own family tree; and like the way all species have a single common ancestor, your body's cells have a single common ancestor, as well.


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taylor_31
Member (Idle past 4035 days)
Posts: 86
From: Oklahoma!
Joined: 05-14-2007


Message 8 of 80 (409885)
07-12-2007 12:07 AM
Reply to: Message 7 by crashfrog
07-11-2007 9:11 PM


That cell is the result of a fusion of two cells, one donated by each of your parents.

Those "cells" are called gametes, I think. And they are haploid, meaning they only contain half of the 23 pairs of chromosomes.

Just clarifying for my own sake.

The cell donated by your mother contributes a small city of cellular organelles, including mitochondria, which have their own "private" genetics.

So the father only contributes 23 chromosomes, and the mother provides the rest of the cell. Quick question: when does a zygote (I think that's the right term) become an embryo?

A special kind of stem cell undergoes mitosis, then undergoes a special form of cell division where half of the chromosomes go one way and the other half go the other way.

What's the difference between a stem cell and an ordinary cell? This has always made me curious, especially in light of the debate over stem cell research.

(You don't have to answer that if you don't want to; it's probably off-topic anyway. I might start a separate thread on it eventually.)

These produce gametes with one each of 23 pairs of your parent's chromosomes, assorted randomly.

For your mother, this process occurred about 500 times in her embryonic ovaries, before she was even born. For your father, this process happened roughly 1.5 million times every day.

Okay, so both of my parents have 46 chromosomes in each of their cells.

These gametes, however, only have 23 chromosomes.

So using probability, the odds of any single gamete forming through meiosis is 1/46!, or 1/5.5026221598120889498503054288003e+57. Add to that the odds of hooking up to the opposite gamete, and the probability is effectively squared, and my computer calculator won't let me type in the whole number.

That remarkable. We really are fortunate to be here, aren't we?

The mutations that happen during Part 4 are passed onto you if that's one of the cells that combines to form you, and then they're replicated into every cell in your body, including the gametes that you produce (and thus, stand a chance of being passed on to your offspring.)

Since a gamete is haploid (I'm finally getting use to this terminology!), then the chances of one particular chromosome getting passed on to my offspring is one-half, right?

And, how do most mutations actually affect the genome? Do they only affect one chromosome, or more than one? Do they "shuffle" the order of the chromosomes in the genome? I suppose that all of the above can be true.

We call these germline mutations.

I'm guessing that germline mutations are much less frequent than somatic mutations, if only because the number of genes involved in germline mutations is much less. So what causes germline mutations?

I don't think it can be "external" factors, like the sun's rays, so I was wondering what other factors can cause mutations.

Somatic mutations typically don't result in macro-observable changes, but occasionally they cause localized problems, like tumors.

So the mutations that play an important role in evolution are germline mutations, not somatic mutations. And these germline mutations are less frequent, and I'm not sure what causes them.

Sorry if I'm playing the role of an annoying, bright-eyed student; feel free not to answer any of these questions, especially if you consider them off-topic.

Thanks for your patient explanations!


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 Message 7 by crashfrog, posted 07-11-2007 9:11 PM crashfrog has responded

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Wounded King
Member (Idle past 2206 days)
Posts: 4149
From: Edinburgh, Scotland
Joined: 04-09-2003


Message 9 of 80 (409900)
07-12-2007 5:08 AM
Reply to: Message 8 by taylor_31
07-12-2007 12:07 AM


I'm guessing that germline mutations are much less frequent than somatic mutations, if only because the number of genes involved in germline mutations is much less. So what causes germline mutations?

Exactly the same things that cause somatic mutations. I'm not sure there is any reason to assume that Germline mutations are less frequent. You will probably have less new mutations in your Germline cells in total than in your somatic cells because there are less of them and they have less DNA, but I don't see why the frequency, in terms of the number of mutations per DNA stretch of a particular length, should be any different.

I would say that the impact of any particular mutational source may vary between the somatic and germline cells, but then this would be true of somatic cells is different tissues too.

TTFN,

WK


This message is a reply to:
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crashfrog
Inactive Member


Message 10 of 80 (409946)
07-12-2007 11:21 AM
Reply to: Message 8 by taylor_31
07-12-2007 12:07 AM


Quick question: when does a zygote (I think that's the right term) become an embryo?

In humans, embryo stage is considered to begin around the zygote implants in the uterine lining, at which point the original zygotic cell has undergone enough cell division to form an inner and outer cell mass; the inner mass develops into the embryo and the outer mass forms such auxiliary structures as the placenta.

What's the difference between a stem cell and an ordinary cell?

Differentiation. Cells in the body perform a wide variety of different functions, depending on how they've differentiated. But ultimately all the body's cells are daughters of a "stock" of relatively-undifferentiated cells called "stem" cells.

If you imagine a "family tree of cells", these progenitor cells are the root, or the stem, hence the name.

That remarkable. We really are fortunate to be here, aren't we?

Indeed. It's even less likely than your scenario would indicate, because there's a "crossing-over" effect that sometimes happens in meiosis that I omitted. The long and the short of it is that homologous chromosomes occasionally exchange sequences, and that produces even further genetic variation in offspring.

Since a gamete is haploid (I'm finally getting use to this terminology!), then the chances of one particular chromosome getting passed on to my offspring is one-half, right?

About that, yes. (As is usually the case with biology the "real" story is a little more complicated than the simple models we talk about, but let's save that for later.) If you consider the X/Y chromosomes, you can see this principle in action - half of your offspring should get your Y chromosome and be male, and half should get your X chromosome and be female. (The mother, of course, always passes on one of her two X chromosomes.) And it's well-known that the human race breaks down roughly half male and half female.

I'm guessing that germline mutations are much less frequent than somatic mutations, if only because the number of genes involved in germline mutations is much less. So what causes germline mutations?

If we were to add up all the mutations present in your current adult body, from every cell, we would indeed find that you have a lot more somatic mutations than germline mutations, but that's not because your gametes have less genes; it's because your body has a lot less gametes than somatic cells.

If you can imagine an ultraviolet ray from the sun heading towards you, it could theoretically shoot right through the nucleus of any one of your body's cells, but most likely, it's going to hit the nucleus of a cell in your skin - because that's what's closest to the sun, obviously. That ray makes a chemical change to your DNA that may or may not be repaired.

The odds that its going to hit a cell in your testes is fairly low, and the odds that the cell in your testes is a sperm cell is even lower.

But not non-zero. And the result could be a genetic change that you pass on to your offspring. Consider how many ultraviolet photons are streaming out of the sun per second, and you start to get an idea of what kind of assault the genetics of your body are under, every moment of your life.

I don't think it can be "external" factors, like the sun's rays, so I was wondering what other factors can cause mutations.

Certain chemicals, certain products of your own metabolism, denatured forms of certain enzymes, errors that occur during DNA replication, etc. DNA is a fairly fragile molecule. Most of the time it's stored in a highly coiled, stable state within your nuclei - packed away like a wedding dress - but it has to be "unpacked" to be read (for replication or transcription) and at that time, it's more vulnerable to damage.

And these germline mutations are less frequent, and I'm not sure what causes them.

Factors that cause mutations, we call "mutagens."

http://en.wikipedia.org/wiki/Mutagens

I don't think there's a comprehensive list here, but this will give you some idea of what kind of mutagens exist. There's also a certain level of "background" mutation that exists because the mechanism of DNA replication that occurs during mitosis is not perfect, and it occasionally introduces errors into the daughter DNA.


This message is a reply to:
 Message 8 by taylor_31, posted 07-12-2007 12:07 AM taylor_31 has responded

Replies to this message:
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New Cat's Eye
Inactive Member


Message 11 of 80 (409952)
07-12-2007 11:37 AM
Reply to: Message 10 by crashfrog
07-12-2007 11:21 AM


typo?
The odds that its going to hit a cell in your testes is fairly low, and the odds that the cell in your testes is a sperm cell is even lower.

But not non-zero.

Huh!? Is that a typo? Not non-zero is zero.


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crashfrog
Inactive Member


Message 12 of 80 (409958)
07-12-2007 12:12 PM
Reply to: Message 11 by New Cat's Eye
07-12-2007 11:37 AM


Re: typo?
Oops, yeah, typo. It actually is non-zero.
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taylor_31
Member (Idle past 4035 days)
Posts: 86
From: Oklahoma!
Joined: 05-14-2007


Message 13 of 80 (410043)
07-12-2007 11:08 PM
Reply to: Message 9 by Wounded King
07-12-2007 5:08 AM


You will probably have less new mutations in your Germline cells in total than in your somatic cells because there are less of them and they have less DNA, but I don't see why the frequency, in terms of the number of mutations per DNA stretch of a particular length, should be any different.

I'm sorry, I misstated what I was trying to say. I wrote that the number of genes involved in germline mutations is much less, but what I meant was the number of gametes that could potentially be affected is much less.

So, the odds of a reproductive cell being mutated are less than an "ordinary" cell. Right?


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taylor_31
Member (Idle past 4035 days)
Posts: 86
From: Oklahoma!
Joined: 05-14-2007


Message 14 of 80 (410046)
07-12-2007 11:40 PM
Reply to: Message 10 by crashfrog
07-12-2007 11:21 AM


But ultimately all the body's cells are daughters of a "stock" of relatively-undifferentiated cells called "stem" cells.

After a zygote is formed, then when does it proceed to form stem cells? Or is every cell that originates from the zygote a stem cell until the embryo takes shape and the cells begin to differentiate?

The long and the short of it is that homologous chromosomes occasionally exchange sequences, and that produces even further genetic variation in offspring.

Sorry, but I don't understand. How could a chromosome changing sequences increase the odds of our existence? I mean, there's only a certain pool of chromosomes to choose from, and only a certain number of "slots" for them to fit into. I think I'm misunderstanding something.

If you consider the X/Y chromosomes, you can see this principle in action - half of your offspring should get your Y chromosome and be male, and half should get your X chromosome and be female.

So the male determines the sex of the offspring. The sex of my children depends on whether or not they inherit my X chromosome (which has to be my mother's chromosome) or my Y chromosome (which is my father's).

If I had a daughter, then, would she more closely resemble my mother than my father?

And are these X/Y chromosomes at the "end" of the chain of the 23 pairs of chromosomes?

-

I'm still confused as to how mutations account for the diversity of life on Earth. I loath creationist websites, but I must admit that I don't understand what they're talking about. Here's a website I found that confused me, and maybe this forum can discuss it:

http://www.carm.org/evolution/evodds.htm

Maybe tomorrow I'll go through it and ask specific questions, but right now I'm tired of thinking.


This message is a reply to:
 Message 10 by crashfrog, posted 07-12-2007 11:21 AM crashfrog has responded

Replies to this message:
 Message 15 by crashfrog, posted 07-13-2007 12:05 AM taylor_31 has responded

  
crashfrog
Inactive Member


Message 15 of 80 (410051)
07-13-2007 12:05 AM
Reply to: Message 14 by taylor_31
07-12-2007 11:40 PM


After a zygote is formed, then when does it proceed to form stem cells?

Oh, gosh, let me look it up...

Yeah, that's what I thought. Blastocyst stage, about 4-5 days post-fertilization.

How could a chromosome changing sequences increase the odds of our existence?

I guess I was unclear. I meant this as an example of it decreasing the odds of your existence, as defined by your genome - since chromosomal crossing increases the number of potentially different gametes.

If I had a daughter, then, would she more closely resemble my mother than my father?

Well, obviously she's going to resemble your mother more than you in the sense that she's a woman and you're not.

But really it depends on whether or not a characteristic in question - like, say, whether she has "your eyes" or your mother's - inhabits the X chromosome. The X doesn't actually have a whole lot of genes, it actually has the least genetic density (that is, number of genes compared to total length) of any chromosome.

A couple of genetic diseases are linked to the X chromosome. Without getting too off-topic, the most noteworthy is probably color-blindness, a nominally recessive trait. Unfortunately, because a man who inherits the recessive trait on his X chromosome only has the one chromosome, he is invariably color-blind. Male-pattern baldness follows the same pattern of inheritance, which is why you'll sometimes hear people tell you to look to your mother's father to see if you'll inherit it. (Your grandfather passed his single X to your mother with baldness "on it", and she stands a 50% chance of having passed it onto you.)

And are these X/Y chromosomes at the "end" of the chain of the 23 pairs of chromosomes?

They don't chain up. They exist in a normal cell as 46 individual lengths of DNA, all tangled up with each other like a ball of spaghetti. Typically they're the last chromosome pair listed in a karyotype:

But that's just a convention that cytologists use, not a representation of biological reality.

I'm still confused as to how mutations account for the diversity of life on Earth.

The long and the short of it is this - if not for mutations, all organisms would be clones of each other, genetically identical. Mutation is the sole original source of all genetic diversity - remember, diversity just means "individuals being different than each other." Mutation is ultimately responsible for all variation between individuals. The reason you are not my identical twin is because of mutations - not just your own, but the scores of mutations you inherited, all the way up your direct lineage.

Here's a website I found that confused me, and maybe this forum can discuss it:

If Ambrose has some proof of it taking "five mutations per new feature", it's not presented in the article. Regardless, new function from single mutations is not unheard of. And if you're not used to thinking in terms of exponential population growth, this:

quote:
Since only one mutation per 1,000 is non-harmful (Davis, 66), there would be only one non-harmful mutation in a population of 10,000 such cells. The odds that this one non-harmful mutation would affect a particular gene, however, is 1 in 10,000 (since there are 10,000 genes). Therefore, one would need a population of 100,000,000 cells before one of them would be expected to possess a non-harmful mutation of a specific gene.

might sound like a steep hill to climb, until you realize that a population of E. coli doubling in size every half-hour hits well over 1 billion individuals in about fifteen hours.

You, yourself, carry about 300 germline mutations that are specific to you, plus who knows how many mutations from each of your parents (and a countless number of somatic mutations, mostly in your skin. Use sunscreen.) So clearly there's no shortage of mutational variety in our genomes. Over several billion years of living things that's an incomprehensible number of genetic variations.


This message is a reply to:
 Message 14 by taylor_31, posted 07-12-2007 11:40 PM taylor_31 has responded

Replies to this message:
 Message 19 by taylor_31, posted 07-13-2007 1:11 PM crashfrog has responded

  
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