pz

What a mish-mash. You clearly don't know what you are talking about.

You are just trying to invent complications at this point. All this requires is that one strand of DNA get linked to another strand of DNA; that chromosomes are found as tetrads is completely irrelevant to the issue.

The simple answer is that there are many very, very long threads of DNA present in the dividing cell. There are mechanisms that are supposed to align these threads, but sometimes it can go wrong. There are mechanisms that are supposed to swap pieces of DNA between the threads, but only between homologous strands; and sometimes they go wrong, too.

As I've already pointed out to you, this is not a hypothetical or a guess. It happens every day. It is routinely observed -- undergraduate genetics labs do experiments all the time to see the results of errors like this one. What scigirl was telling you in that list of possibilities was not that we don't know what causes it and are speculating, but that we do know mechanisms that induce chromosomal errors, and there are lots of them.
I've got half a dozen current genetics textbooks on the shelf in front of me, and they all describe in rigorous detail exactly how this common occurrence happens. I suggest that you make the small effort of going to your local library and reading one. The bottom line is that your incredulity is misplaced -- this is basic stuff that is seen all the time.

scigirl

No, you are still mistaken. In the haploid cell (sperm and egg), chromosomes no longer exist as pairs. They must be divided in half, so that there is not a duplication of our genome every time fertilization occurs.

Each chromosome has only two arms. The arm of one chromosome got stuck to the arm of another chromosome, resulting in our fused chromosome 2.



Look at the cells on the right. Now the pic is simplified - our hapoloid cells have 23 single chromosomes floating around, not just 2. Note - the "pairs of arms" you are thinking about only occur in diploid cells (cells before the haploid division).

You may want to peruse this <a href="http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookTOC.html" target="_blank">on-line biology book</a>.

scigirl

scigirl

I also want to add. . .

How do we know that the evolutionary fusion event happened in the haploid cell?

Because that's where all the action of evolution takes place!

If my arm cell sustains a fusion or a genetic mutation, then perhaps my arm will grow a tumor or deformity. This will in no way affect evolution.

However, if a mutation occurs in my egg cell, and this egg cell goes on to be fertilized, then and only then can evolution occur.

scigirl

Vanderzyden

A clarification:

Since it's necessary to examine the details here,
I would rather not make flippant use of common, abstract terminology, such meiosis and mitosis. Let's talk about what happens "underneath the hood".

From what I've learned, a "fusion" wouldn't happen as part of meiosis or mitosis. In the prophases, the chromosomes "combine", but this isn't a fusion--at least not in the sense described by scigirl. The chromosome pairs cross and intertwine at many points, exchanging information. Here is a brief description taken from the last link below:

Here are some online resources:

Don't worry, they're all pro-Darwin! Please inform me if any of them are in error, and if you know of better ones.

Scigirl, I would like someone to explain how two separate chromosomes could connect--end to end--and fuse in the fashion that you are advocating.

Show me the way,

Vanderzyden

Skeptical

Vander,

In case you haven't seen it, I've started a new thread with a specific question for you <a href="http://iidb.org/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=58&t=001333" target="_blank">here</a>.

If you get the time, I'd be very interested in hearing your thoughts on my question.

Vanderzyden

Oh, I see, the fusion supposedly occured in one of the gametes--in the ovum or the sperm. Actually, in your subsequent post, you explicitly indicate that it occurs in the ovum.

Permit me to make a simple illustration. Here we have two of the chromosomes in the "common ancestor". These are two (2p and 2q) out of 24. Let's call them the "parents":


T---------C----------T T---------C----------T


Now, you are guessing that, somehow, these two "just stuck together":


T---------C--------x---T
T---------C--------x---T


The supposed end product is one single "fused" chromosome that is approximately the combined length of the two "parents". :


T------------------C-------------------T


Actually, I should be more specific. What is claimed by the <a href="http://www.gate.net/~rwms/EvoEvidence.html" target="_blank">William's article</a> is that vestigal components are present in the resulting chromosome. According to that hypothesis, we have the following:


T-----------C-----t-f-t---------c---------T


Where t represents vestigal telomores. As for C and c, Williams states that "The normal centromere found on human chromosome 2 lines up with the 2p chimp chromosome, and the remnants of the 2q chromosome is found at the expected location based upon the banding pattern". Supposedly, then C is the existing centromere on human chromosome #2, and c is the vestigal centromere.

However, is this what we should expect? Consider the "parents" again (2p and 2q):


T---------C----------T T---------C----------T


Now, Williams and others are guessing that, somehow, these two "just stuck together":


T---------C--------x---T
T---------C----x-------T


Remember the telomeres won't stick to each other, so the "fusing" would occur somewhere in the middle. That would produce this:


T---------C--------f----C---------T


Where f marks the fusion location. Note that one telomere from each of the parents would have broken off, removing any telomeric material that would be the source for vestigal remnants. The result is surely a defective chromsome, having two centromeres and components from two completely distinct chromosomes. We have to wonder if it is even possible for this to occur.

You also ask:

Well, you don't KNOW that. It's a guess. That is the subject of dispute here. Correct? I'm not sure if you meant something else by asking this question. But you should realize that it is this type of apparent posturing and overconfidence that serves as the greatest barrier to productive dialogue between IDs and naturalists.

Therefore, although you say the fusion occured in a haploid cell, the same critique applies to a "four-arm" or "two-arm" scenario. Either one doesn't support the fusion argument. I still don't see how it came about.

Perhaps I'm still missing something. Let me know.


Vanderzyden

[ September 01, 2002: Message edited by: Vanderzyden ]</p>

Blinn

Wow, I think I'm just going to sit back and watch. Vanderzyden, it's great that you're making an attempt to understand this material, but it appears you're only getting yourself more confused. Maybe I didn't help by oversimplfying some things myself (I'm re-learning a few things as I follow this thread). Instead of diving right in you really should get yourself a biology text and start reading from Chapter 1.

The_Gorgonzola

Nevermind. It's just not worth getting upset over.

Gorgo

[ September 01, 2002: Message edited by: The_Gorgonzola ]</p>

scigirl

I wasn't aware that those terms were abstract. They are discrete processes that go on in cells.

The entire reason that I was forced to even bring up "meiosis" and show you a picture was to prove to you that we were talking about 2 chromosomes, and not 4 (i.e. 2 pairs) of chromosomes.

Well, it would occur during meoisis, since this is the only time that chromosomes are condensed and 'lined up.' If you look at those pictures I provided you, note that the chromosomes are arranged in a pattern along the middle of the cell. Thus, the ends are in phyiscal contact with each other.

All other times, the chromosomes are scattered all over the place like spagetti on a plate, and it's highly unlikely that the ends could even find each other, much less fuse.

My guess is that fusion would occur sometime between metaphase II and anaphase II of meiosis in either a sperm or an egg cell.

Yes you are talking about homologous recombination. Incidently, this is another huge source for evolution, namele gene duplications.

What we are talking about is not recombination, but fusion, although both could occur in given chromosomes.

scigirl

scigirl

Explicitly indicate? What I said was, "I see this as a pretty good possibility, knowing what I know about oogenesis. I have, however, no probabilities to give you at this time." Please do not put words into my mouth, ok?? And I'll try to be more clear this time.
Partially correct. I think you are still hung up on that one telomere breaking study. In this particular case, no telomeres were broken off. They are still there. Also, there's another important part of the chromosome to consider here, and that's the pre-telomeric region.

I really liked your idea of drawing out the chromosomes (after all, a biology lesson is worthless without diagrams!)

Here's a diagram of the 2p and 2q chromosome (I don't know which one is actually longer, but this is an overall schematic): I will try to illustrate the pre-telomeric sequences as well with a (&lt;--p), which since they are directional will have an arrow.

Chimp 2p: T--&lt;--p---c---p--&gt;--T

Chimp 2q: T--&lt;--p------c------p--&gt;--T

Now let's look at what a chromosome would look like if these two simply fused together (no breaking, just plain old fusion) :

T--&lt;--p---c---p--&gt;--TT--&lt;--p------c------p--&gt;--T

If indeed 2q and 2p fused like this, you would have the following features in the new chromosome:
1) Two centromeres (if the chromosome were to become functional, then only one of the centromeres would be used in meiosis and mitosis).

2) Two "real" telomeres - one at each end - together with their pre-telomeric sequences pointing in the correct orientation (in this case, towards the telomere).

3) Two extra telomeres at the fusion point (i.e. in the middle of the new chromosome). Also, you would find two extra pre-telomeric sequences but in this case, they would point away from the "real" telomeres or ends.

4) Oh and of course, the new chromosome length would equal the length of 2p + 2q.

Ok that's the fusion prediction. What does the human chromosome #2 look like? From <a href="http://www.gate.net/~rwms/EvoEvidence.html" target="_blank">http://www.gate.net/~rwms/EvoEvidence.html</a>
Now, this description fits my nice drawing. I do notice that Williams states, "The first prediction (evidence of a telomere at the fusion point) is shown to be true in reference 3." Since my predicted picture would have in fact 2, not 1, extra telomeres, I read the reference 3 closer. Here's what it said:
So, Williams probably should have put an "s" on that sentence.

Our chromosome 2 indeed looks just like what you would predict if a simple fusion occured (although I hesitate to use the word 'simple' in regards to any molecular process!)

Please make note of the new corrected drawing, and use that one in subsequent posts.

How did you come to this conclusion?

Vanderzyden, whether you believe that God gave our chromosome an extra centromere, or that evolution did, the fact that this particular chromosome has two sequences characteristic of a centromere is NOT in dispute. Is it? Are you denying the actual sequence data now??

And since our chromosome 2 works just fine, obviously having two centromere-like sequences is not a drastic problem.

However, you are correct in stating that chromosome fusions can be a problem in some cases. Or maybe there are subtle problems. Perhaps this chromosome experiences more non-dysjunctions (when the pairs don't segregate properly, like in Down's syndrome)? Does anyone have data on whether non-dysjunctions occur more frequently in one chromosome over another, and if this coulde be related to having an extra centromere? Perhaps evolutionary theories such as this one will help biologists understand and cure non-dysjunction and other chromosomal abnormalities, and thus we will have answers to the common question "what has the theory of evolution ever done for medicine?"

Furthermore, even if most, or even if nearly all, chromosomal aberrations are harmful, evolution has had millions of years to wait around for the one aberration that wasn't.

Here's a study that I already linked to earlier (not sure if you read it) about chromosome stability: <a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=120528 90&dopt=Abstract" target="_blank">http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=120528 90&dopt= Abstract</a>
So, given enough time and patience and chromosomal alterations, it does seem possible that one would arise and contribute to the formation of a new species, if we can even observe stability occuring in a cell line!
Obviously you didn't understand my point. IF you start with the assumption that fusions and other such things occur (IF - ok IF - just bear with me), then these events absolutely have to occur in the germline cells in order to be passed down, and thus acted upon by natural selection.

I was making a logical statement - it has nothing to do with arrogance or over-confidence. Here is another logical statement in regards to evolution, that may help you understand what I mean:

From River Out of Eden:
Does that clear up what I mean about all evolutionary changes occuring in germline (haploid) cells?

Let me reiterate - changes to my arm, pinkie, or even brain cannot be passed down to my offspring. ONLY changes in the DNA content of my egg cells can be passed down to my offspring and thus contribute to natural selection, and thus participate in evolution.

Now, these changes in DNA content may affect the arm, pinkie, or brain of my child. But the changes to the DNA themselves had to occur in the egg (or if I was a man, the sperm).

Does that clear it up?

Well we still don't understand the specifics of gravity, but I still accept the tenet of physics that says gm1*m2/r2 = F!

I believe I already linked you to papers that begin to address how the fusion occured. They are at the top of page 6 (and pz also elaborated on it).

scigirl

[ September 01, 2002: Message edited by: scigirl ]</p>