Freethought & Rationalism Archive

scigirl · 08-31-2002, 09:14 AM

Vanderzyden,

Thanks for your reply. I will try to find the time to look into the molecular and biochemical details of chromosomse fusion a bit later. Unfortunately, my textbooks are in my school locker in the moment (heh just like high school all over again!).

Let me point out a few things:
1) I do applaud you for coming here and seeking answers to your questions, and trying to understand a new field. That is a noble pursuit, and one you should be commended for. Your science questions about the fusion are awesome - and I too am excited to see what the studies are.

However, I want to ask you - do you think you are the first person to ask these questions? Do you honestly think that scientists haven't thought of this stuff before? Vander, they spend their lives not only asking questions such as "Hmm, I wonder how exactly the chromosomes fused together," but also thinking of ways to answer the question. Let me reiterate - I don't want you to accept a scientist's word just because he/she is a scientist. I just don't want you to get delusions of grandeur that you are the first enlightened human that saw the data and said, "hey, but how did it happen?"

2. Let's see if I have your position right - because scientists can't explain how exactly something happened, you think that a reasonable expalantion is "well god put the extra centromere and telomeres in there." Is that about right? You have still not clarified your position on the matter, so I am left to guess.

Ok on to your post:

Quote:

You know, I used to take it for granted. I was fed this stuff in high school and college. Unaware of the problems, I simply received what all of academia apparently accepted: evolution is fact. But, you know, it's funny: I never stopped to wonder why it was so often emphatically stated

Yes I agree - our education system is lacking in many ways, especially in science. We emphasize memorization rather than critical thinking. However, you want to place the blame for the state of education? Hmm, let's see - scientists or science-denyers? No I don't blame YECS for all the problems in the world. However, their denial of basic scence fields such as geology, paleontology, biology, etc - can't be helping the situation much.

Quote:

Yes, tragically, some of those "Christians" you know are very likely uncritical of their own beliefs.

I totally agree. I can't for the life of me understand why educated people choose to still adhere to beliefs based on a mythological ancient middle-eastern text instead of relying on their own logic and senses.

Quote:

In many discussions, it's difficult to ask questions because the defiant shouting is so loud. The name-calling and dogmatic posturing indicate clearly that beneath the surface lies a fear of facing the "tough questions". The vanity of the heart prevents many from developing a desire for the truth.

Yeah, those stupid naturalists always knocking on my door saying, "Can I talk to you about Deoxyribonucleic acid today?"

Or all those atheists on my campus around springtime shouting "NOBODY IS GOING TO HELL!"

BTW - these things have never happened to me. However, theists do stuff like this all the time, at least where I come from.

I would also say the converse - someone being a "christian" does not automatically make them a better person. Nor does being an atheist. Humans are humans. However, if a christian and atheist both want to help better humanity, but one of them is basing their beliefs about humanity on illogical and flawed conclusions, than that person is not going to be as effective. Just like a doctor must not only be compassionate, but also make the correct diagnosis.

IMHO, christians are making incorrect diagnoses about human behavior because they have chosen the wrong method as to examine it. They are trying to diagnose strep throat with an MRI or something! I think that the only way to better ourselves is through compassion, reason, tolerance, and constant re-evaluation of our beliefs and conclusions. And NOT by basing our moral systems on flawed mythological stories that are inconsistent and contrary to reality.

Sorry if that sounds harsh, but that's the way I see it.

Quote:

Who's harassing? Evolution is just one of several "junk sciences". Similar unsubstantiated hypotheses include those concerning global warming, asbestos, DDT, growth hormones, etc. Political agenda and fear-mongering dominate in these "sciences." Surely you'll agree that there has been much controversy over these subjects.

Huh? Because certain politicians or lawyers use certain scientific theories for monetary or political gain, all of a sudden the theories themselves are "junk science?" And I always thought growth hormones were real?!? Heh if they aren't, then I sure wasted my time in endocrinology lecture!

Quote:

-- Are you implying that attempts to affirm evolution are producing medical or technological benefits in a like manner to cancer research?

Nice question dodge. My point had nothing to do with the intrinsic value of each science.

What I was trying to say was: cancer research is filled with the same types of naturalistic inferences that evolutionary theory is. If evolutionary biologists are making incorrect inferrences (which you clearly believe, since you don't believe in evolution), than how is it that cancer researchers are doing it correctly? How come the cancer scientists have the ability to correctly interpret chromosomal data, but the evo scientists are incompetent?

Incidentally, a lot of evidence for both evolution and cancer has to do with the way genes are regulated and replicated. Promotor studies for example. If you think about it, cancer is very much like micro-evolution: cells with mutations that give them an edge over other cells, and outcompete thier non-mitotically active neighbors. So I would say - evolution studies are helping cure cancer. And cancer studies are helping to elucidate evolutionary mechanisms. Science fields are not discrete - they all overlap.

Please let me know if you don't understand this point, because it is an important one.

In terms of the importance of evolution, check out points I made in this thread a while back:

<a href="http://iidb.org/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=58&t=001126&p=" target="_blank">http://iidb.org/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=58&t=001126&p=</a>

Here's some excerpts:

Quote:

Scigirl:
Why do I think it's so important for people to accept evolution?

Of course as a scientist, I want people to accept it because I think it happened. And of course, I do believe that viewing humanity through the lens of evolution will help explain why we do things like fight wars and commit adultery.

Yet I think there is another reason, a much simpler one. I want people to accept evolution, because I want them to accept science.

I noticed over at the Baptist Board, back when "froggie" was still allowed to voice her opinions, that many of the Baptists rejected scientific explanations for many things besides evolution. Many of the members thought that obesity, drug addiction, and even mental illness was the result of sin. This bothered me very much as a (future) medical professional, and I tried to no avail to convince them otherwise. I would link them to NIH studies where rats display similar symptoms of withdrawal from certain drugs. Or to studies that showed a certain genetic defect would cause a mouse to gain too much weight. No matter, they still thought that humans are addicted because they are sinners, and they aren't praying enough.

When a creationist rejects his/her religious explanation of the creation of humans in favor of the scientific one, a whole new door is opened up. Instead of obese humans being guilty of gluttony, perhaps they have a genetic disorder? Instead of people with drug addictions being shunned and locked away in jails, maybe now we can think about finding cures for addictive behaviors? And mental illness. . . instead of trying to cast out demons, let's try to understand why the brain, which is another organ just like a kidney, sometimes malfunctions and what can we do about it?

I wrote earlier in another thread that I don't necessarily worry what YEC scientists are doing, but rather what conclusions/studies are they not accepting? The fact that they believe humans are a special creation, different from all animals, could be profoundly affecting their worldview in ways we haven't even considered. [/b]

Quote:

Vanderzyden:
Most critically, you are denying a prevalent understanding of what it means to be human. Proposals concerning the origin of humans necessarily must advance explanations for the non-physical attributes of humans. I am referring to consciousness and reason, primarily. Failure to admit such uniquely human qualities is to ignore the collective human experience. If you would persuade others of such a view, you must provide evidence and solid, self-critical explanation.

When did I say I don't believe in consciousness and reason? And why can't these facets have physical attributes? Have you ever taken an advanced neurobiology class? I have. Trust me, there are no doubt physical components for both of these human characteristics. Ever hear of a lobotomy?

Quote:

One more thing to ponder, scigirl. This is a question I ask myself regularly:

In everything that you do, do you choose to believe in order to affirm your presuppositions and preferences, or do you believe on the basis of a thorough, balanced search for the truth?

Are you sure you ask yourself that question every day?

Perhaps you should check out <a href="http://www.infidels.org/library/modern/theism/christianity/criticism.shtml" target="_blank">http://www.infidels.org/library/modern/theism/christianity/criticism.shtml</a>

As my favorite line from Silence of the Lambs goes, Clarice asks the cannibal psychiatrist Hannibal "How about it? Why don't you point that high-powered perception at yourself?"

One more question that you still never answered - did you reject evolution because of scientific examination, or because of the Bible?

scigirl

scigirl · 08-31-2002, 09:24 AM

One more thing I thought of:

I've heard Vanderzyden's story before, from other creationists. Basically it goes like this:

1. Said person is some type of agnostic or non-christian, or christian but not really because they chose it. Said person studies evolution in high school and learns it as "fact," just like 2 + 2 = 4.

2. Same person "gets religion" and is told by the pastor that evolution is not a fact. It is, instead, either an atheistic conspiracy, or influenced by satan, or some other thing.

3. That same person goes out to study evolution on his own. At this point one thing can happen: They discover that evolution is not indeed a fact like 2 + 2 = 4. Instead, it is a growing, enormous, complex scientific field, filled with the same caveats as every other scientific field: a few bad scientists, some fabricated data, controversies, etc. Said person then rejects evolution (and by default they reject the scientific methods used to prove the evolutionary theory, but none of them actually see it this way).

4. This reinforces the pastor's claim, and the person then critiques evolution through a variety of ways, including bible-quoting, appeal to emotion, etc. However, one of the ways they critiicze evolution is through the same science that they rejected in step 3.

Just an observation, and no this does not apply to everyone!

scigirl

Vanderzyden · 08-31-2002, 10:15 AM

Quote:

Originally posted by Zetek:
[QB]

<a href="http://www.ndsu.nodak.edu/instruct/mcclean/plsc431/eukarychrom/eukaryo3.htm" target="_blank">web page</a>

CHROMOSOMETELOMERE TELOMERECHROMOSOME

No sticking.

CHROMOSOMETELOM++LOMERECHROMOSOME

Sticking.

Any more questions?

Why yes, I do have more questions. Several, in fact.

The telomere is the TERMINAL SEGMENT (i.e. very end) found at the end of each arm of a eukaryotic chromosome. Now, when one arm of a chromsome breaks, a fragment remains. The break occurs in the middle of the arm, not in the middle of the teleomere.

In your <a href="http://www.ndsu.nodak.edu/instruct/mcclean/plsc431/eukarychrom/eukaryo3.htm" target="_blank">link</a>, in the section "Telomere Repeat Sequences", I find these two paragraphs:

Quote:

The primary difficulty with telomeres is the replication of the lagging strand. Because DNA synthesis requires a RNA template (that provides the free 3'-OH group) to prime DNA replication, and this template is eventually degraded, a short single-stranded region would be left at the end of the chromosome. This region would be susceptible to enzymes that degrade single-stranded DNA. The result would be that the length of the chromosome would be shortened after each division. But this is not seen.

The action of the telomerase enzymes ensure that the ends of the lagging strands are replicated correctly. A well-studied system involves the Tetrahymena protozoa organism. The telomeres of this organism end in the sequence 5'-TTGGGG-3'. T he telomerase adds a series of 5'-TTGGGG-3' repeats to the ends of the lagging strand. A hairpin occurs when unusual base pairs between guanine residues in the repeat form. Next the RNA primer is removed, and the 5' end of the lagging strand can be used for DNA synthesis. Ligation occurs between the finished lagging strand and the hairpin. Finally, the hairpin is removed at the 5'-TTGGGG-3' repeat. Thus the end of the chromosome is faithfully replicated. The following figure shows these steps.

What this describes is the generation of a telomere on a fragmented chromsome arm. It makes sense. However, this provides no support for translocations or fusions:

-- If the arm were to remain fragmented, it would be possible for it to "stick" to other sticky genetic material. But it won't adhere to a repellant telomere on another chromsome.

-- If the fragmented arm of another chromosome were to contact the first fragment (having no telomere), it could possibly fuse. But the resulting chromosome would have NO VESTIGAL telomeres. The ends of the fragments had no telomeres (otherwise they wouldn't stick together), and therefore there would be no "evidence" of them in the final product.

-- If the telomere is "replicated", the chromosome arm will be shorter than its original length. Supposedly, telomerase enzymes would eventually produce a telomere to "cap off" the end of the fragment. However, this new telomere would also be repellant and not a contributor to a fusion.

In addition, we would need to examine the probability of such coincidental fragmentation events:

-- What is the likelihood of two fragments occuring, and then fusing together?

-- What is the probability of four fragments occuring? This would be necessary to produce a fusion in the sense described in the William's <a href="http://www.gate.net/~rwms/EvoEvidence.html" target="_blank">article</a>. Two of the arms from the "same sides" of two chromosomes must fragment and somehow all fuse together. Is this even possible?

-- Also, how would the ends fragment at a precise location to preserve the symmetry necessary for a viable product chromosome?

Vanderzyden

beausoleil · 08-31-2002, 10:28 AM

I'm curious now. Did this chromosome fusion occur in one individual? And how did it get passed on. i.e. could they breed with another individual with 1 extra chromosome and pass it on somehow?

(I'd appreciate a pointer to a website or an entry level explanation.)

scigirl · 08-31-2002, 11:54 AM

Keep in mind Vanderzyden, the original post involved complete chromosome fusions, not fragmentation than fusion. So we have to show only one phenomenon: that chromosomes can indeed fuse (telomeres and all).

But let's examine your questions:

Quote:

Originally posted by Vanderzyden:
-- If the arm were to remain fragmented, it would be possible for it to "stick" to other sticky genetic material. But it won't adhere to a repellant telomere on another chromsome.

It won't ever adhere? How do you know? Sure usually they don't. Perhaps under cell duress, or in the presense of some type of virus or other DNA-altering molecule, it could fuse. Or maybe it was just a 'freak accident.'

Quote:

-- If the telomere is "replicated", the chromosome arm will be shorter than its original length.

No, telomere replication refers to the replication of the telomere-portion of the chromosome. Ok now we are getting into very complex genetic mechanisms.

The way a cell replicates chromosomes is rather strange: the chromosome partially unzips, and DNA polymerase enzymes come in and "copy" the sequence (like a scribe). However, these enzymes need a 'primer,' and cannot start from scratch (the can't start at the telomeres). So another special enzyme has to copy that part, then it gets stuck on to the rest of the copied chromosome.

Quote:

Supposedly, telomerase enzymes would eventually produce a telomere to "cap off" the end of the fragment. However, this new telomere would also be repellant and not a contributor to a fusion.

Huh?

Quote:

-- What is the probability of four fragments occuring? ... Two of the arms from the "same sides" of two chromosomes must fragment and somehow all fuse together. Is this even possible?

What? I don't think you understand what is going on here.

First of all, the fusion occured in a hapoid cell (sperm or egg) where only one copy of every chromosome is floating around. Two strings of DNA ended up sticking together. Here's a possible scenario:

Two chromosomes fuse (the 2p and 2q) in an egg cell. Not 4 - just 2. 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.

This egg is fertilized by a sperm. The sperm chromosomes all line up with the egg chromosomes (this what happens normally). But in this special case, because the 2p and 2q ones were fused in the egg, then the corresponding sperm chromosomes will naturally line up in that same orientation. Somehow, they also fuse (how exactly we just don't know yet).

Voila, the new fetus went from having 48 to 46 chromosomes.

You are making it way too complicated!

Quote:

-- Also, how would the ends fragment at a precise location to preserve the symmetry necessary for a viable product chromosome?

Again, they wouldn't have to. No fragmentation is necessary, I don't think anyway.

It happened in the haploid cells, not the diploid.

Here's an interesting pubmed article:

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=120528 90&dopt=Abstract" target="_blank">Chromosome instability as a result of double-strand breaks near telomeres in mouse embryonic stem cells.</a>

Quote:

Telomeres are essential for protecting the ends of chromosomes and preventing chromosome fusion. Telomere loss has been proposed to play an important role in the chromosomal rearrangements associated with tumorigenesis. To determine the relationship between telomere loss and chromosome instability in mammalian cells, we investigated the events resulting from the introduction of a double-strand break near a telomere with I-SceI endonuclease in mouse embryonic stem cells. The inactivation of a selectable marker gene adjacent to a telomere as a result of the I-SceI-induced double-strand break involved either the addition of a telomere at the site of the break or the formation of inverted repeats and large tandem duplications on the end of the chromosome. Nucleotide sequence analysis demonstrated large deletions and little or no complementarity at the recombination sites involved in the formation of the inverted repeats. The formation of inverted repeats was followed by a period of chromosome instability, characterized by amplification of the subtelomeric region, translocation of chromosomal fragments onto the end of the chromosome, and the formation of dicentric chromosomes. Despite this heterogeneity, the rearranged chromosomes eventually acquired telomeres and were stable in most of the cells in the population at the time of analysis. Our observations are consistent with a model in which broken chromosomes that do not regain a telomere undergo sister chromatid fusion involving nonhomologous end joining. Sister chromatid fusion is followed by chromosome instability resulting from breakage-fusion-bridge cycles involving the sister chromatids and rearrangements with other chromosomes. This process results in highly rearranged chromosomes that eventually become stable through the addition of a telomere onto the broken end. We have observed similar events after spontaneous telomere loss in a human tumor cell line, suggesting that chromosome instability resulting from telomere loss plays a role in chromosomal rearrangements associated with tumor cell progression.

scigirl

scigirl · 08-31-2002, 12:21 PM

Here are some more pubmed articles to sort through - I will try to summarize what I think are the important points. I found them by searching for "chromosome telomere fusion evolution"

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=114741 95&dopt=Abstract" target="_blank">Chromosomal distribution of the telomere sequence (TTAGGG)(n) in the Equidae.</a>

Quote:

Lear TL.
Telomeres are a class of repetitive DNA sequences that are located at chromosome termini and that act to stabilize the chromosome ends. The rapid karyotypic evolution of the genus Equus has given rise to ten taxa, all with different diploid chromosome numbers. Using fluorescence in situ hybridization (FISH) we localized the mammalian telomere sequence, (TTAGGG)(n), to the chromosomes of nine equid taxa. TTAGGG signal was located at chromosome termini in all species, however additional signal was seen at interstitial sites on some chromosomes in the Burchell's zebra, Equus quagga burchelli, the Hartmann's zebra, Equus zebra hartmannae, and at large heterochromatin-associated regions on the chromosomes of the donkey, Equus asinus. The interstitial signal in the zebras may be a relic of an ancient telomere-telomere fusion and mark the point at which two ancestral chromosomes may have fused. For the donkey, the heterochromatin-associated signal may represent degenerate telomere-like satellite sequences and identify a second type of satellite DNA for this taxon.

So, the first thing the researchers did was to count the chromosomes of different horse species (equine means horse). Note that they found different chromosome numbers. They then looked for evidence of fusion events, as evidenced by extra telomeres, and they did indeed find them.

How did they look for telomeres? These experiments take advantage of DNA's affinity for its exact complimentary sequence. The researchers made a probe to the known horse (equine means horse) telomeric sequence, and this sequence not only hybridized to all the telomeres, but to other parts in the chromosomes as well.

Here's another article:

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=112946 06&dopt=Abstract" target="_blank">(T2AG3)n telomeric sequence hybridization suggestive of centric fusion in karyotype marsupials evolution.</a>

Quote:

Carvalho BD, Mattevi MS.
It has been suggested that the karyotype of the marsupials derived from a low diploid number (2n = 14) which originated, through fissions of biarmed chromosomes, the karyotypes with a higher 2n. The telomeric sequence (T2AG3)n was in situ hybridized to the chromosomes of Gracilinanus microtarsus and G. emiliae, Micoureus demerarae and Marmosa murina, species with 2n = 14, in Monodelphis sp., M. domestica, M. kunsi and M. brevicaudata with 2n = 18, and in Lutreolina crassicaudata, Didelphis albiventris, Chironectes minimus, Philander opossum and P. frenata, all of them with 2n = 22. The probe hybridization occurred in the telomeric regions of both arms, short and long, of all chromosomes of the complement of all individuals of all species analysed. However, in some pairs of the karyotypes of Gracilinanus microtarsus and Micoureus demerarae (with 2n = 14), and in Monodelphis sp., M. domestica, M. kunsi and M. brevicaudata (2n = 18) ectopic signs of hybridization were detected proximal to the centromeres, suggesting the retention of this telomeric sequence in the centromeric regions of some chromosomes of these species. Based on these results, it is proposed that the karyotype of marsupials evolved from a 2n = 22 to a 2n = 14, by means of chromosomal fusions.

Once again, the differing chromosome numbers in closely related species correlates with evidence of a fusion event.

Here's an article that begins to elucidate the mechanism of the chimp chromosome fusion:

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=111961 35&dopt=Abstract" target="_blank">Comparative FISH mapping of the ancestral fusion point of human chromosome 2.</a>

Quote:

Kasai F, Takahashi E, Koyama K, Terao K, Suto Y, Tokunaga K, Nakamura Y, Hirai M.
It is known that human chromosome 2 originated from the fusion of two ancestral primate chromosomes. This has been confirmed by chromosome banding and fluorescence in-situ hybridization (FISH) with human chromosome-2-specific DNA libraries. In this study, the order of 38 cosmid clones derived from the human chromosome region 2q12-q14 was exactly determined by high-resolution FISH in human chromosome 2 and its homologous chromosomes in chimpanzees (Pan trogrodydes, 2n=48) and cynomolgus monkeys (Macacafascicularis, 2n = 42). This region includes the telomere-to-telomere fusion point of two ancestral ape-type chromosomes. As a result of comparative mapping, human chromosome region 2q12-q14 was found to correspond to the short arms of chimpanzee chromosomes 12 and 13 and cynomolgus monkey chromosomes 9 and 15. It is noted that no difference was detected in the relative order of the cosmid clones between human and chimpanzee chromosomes. This suggests that two ancestral ape-type chromosomes fused tandemly at telomeres to form human chromosome 2, and the genomic organization of this region is thought to be considerably conserved. In the cynomolgus monkey, however, the order of clones in each homologue was inverted. In addition to cosmid mapping, two chromosome-2-specific yeast artificial chromosome (YAC) clones containing the fusion point were identified by FISH.

These scientists seem to think that the chromosomes fused in tandem at the telomeres.

Here's a molecular explanation of chromosome fusion:

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=111362 54&dopt=Abstract" target="_blank">Pericentromeric organization at the fusion point of mouse Robertsonian translocation chromosomes.</a>

Quote:

Garagna S, Marziliano N, Zuccotti M, Searle JB, Capanna E, Redi CA.
In mammals, Robertsonian (Rb) translocation (the joining of two telo/acrocentric chromosomes at their centromere to form a metacentric) is the most effective process in chromosomal evolution leading to speciation; its occurrence also affects human health (through the induction of trisomies) and the fertility of farm animals. To understand the mechanism of Rb translocation, we used the house mouse as a model system and studied the organization of pericentromeric satellite DNAs (satDNA) of telocentrics and Rb chromosomes, both minor and major satDNA. The chromosome-orientation fluorescence in situ hybridization (CO-FISH) technique was used to analyze the major satDNA. To detect the very small amount of minor satDNA, a procedure was developed that combines CO-FISH with primed in situ labeling and conventional FISH and is five times more sensitive than the CO-FISH procedure alone. It was found that both the major and the minor satDNA tandem repeats are oriented head-to-tail in telocentric and Rb chromosomes, and their polarity is always the same relative to the centromere. We suggest that all tandemly repetitive satDNAs in a species probably are locked into such a symmetry constraint as a universal consequence of chromosomal evolution. Rb translocation breakpoints were found localized within the minor satDNA of telocentrics, and these sequences contributed symmetrically to the formation of the centromeric region of the Rb chromosomes. These results are important for an understanding of the geometry of Rb translocations and suggest the study of DNA orientation as a new tool for investigating these rearrangements.

Here's a nice review article - I just might have to get this one - on mechanisms of fusions.

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=960198 2&dopt=Abstract" target="_blank">Telomeres and mechanisms of Robertsonian fusion.</a>

Quote:

Slijepcevic P.
The Robertsonian (Rb) fusion, a chromosome rearrangement involving centric fusion of two acro-(telo)centric chromosomes to form a single metacentric, is one of the most frequent events in mammalian karyotype evolution. Since one of the functions of telomeres is to preserve chromosome integrity, a prerequisite for the formation of Rb fusions should be either telomere loss or telomere inactivation. Possible mechanisms underlying the formation of various types of Rb fusion are discussed here. For example, Rb fusion in wild mice involves complete loss of p-arm telomeres by chromosome breakage within minor satellite sequences. By contrast, interstitial telomeric sites are found in the pericentromeric regions of chromosomes originating from a number of vertebrate species, suggesting the occurrence of Rb-like fusion without loss of telomeres, a possibility consistent with some form of telomere inactivation. Finally, a recent study suggests that telomere shortening induced by the deletion of the telomerase RNA gene in the mouse germ-line leads to telomere loss and high frequencies of Rb fusion in mouse somatic cells. Thus, at least three mechanisms in mammalian cells lead to the formation of Rb fusions.

According to scientists who study chromosomes all day long, and know way more about them than you or I, Vander, it doesn't appear that chromosome fusion is impossible at all. In fact, there are several ways it can (and has) happened.

scigirl

pz · 08-31-2002, 12:55 PM

Quote:

Originally posted by Vanderzyden:
<strong>Let me see if I can explain this correctly in simple terms: The supposed common ancestor had two "additional" chromosomes (which, incidentally, it didn't really need? eh? ). Now each chromosome has a centromere (spindle attachment) and a telomere on each end. Now, the telomeres are "non-sticky". However somehow these telomeres contributed to a fusion event.* The evidence of this fusion is supposedly the vestigal remnants of telomeres at "precise" locations in the #2 human chromosome. This would have to be the case, since, if sequence of the original chromosomes remained intact, the telomeres would be present in the new chromosome. So, then, we come to the critical question:

How could the repellant telomeres fuse together?

Please explain how it is possible for telomeres to attach to one another and remain preserved in a fused chromosome if the telomores are non-sticky and don't attach to anything, much less other telomeres.</strong>

You are being confused by a far too literal interpretation of some of the simple words being used to try and help you understand.

Telomeres are not "non-sticky". They are made of the same stuff as the rest of the chromosome: DNA wrapped up in protein. They are made relatively inert by their structure, but they are still capable of crossing over, translocations, etc.

This is not miraculous stuff that never happens where we can see it. Out of every 100,000 live births, there are roughly 600 babies with chromosome abnormalities of various types. Of those 600, 90 are Robertsonian fusions of the type being described here.

That you personally do not understand how it happens has no impact on the demonstrable fact that it does happen. Something on the order of 200,000 people living in the US right now would show a Robertsonian fusion in their karyotype.

Vanderzyden · 08-31-2002, 10:03 PM

Scigirl,

Thanks for your detailed reply! But, no, I don't think I'm making it too complicated.

You state the following:

Quote:

First of all, the fusion occured in a hapoid cell (sperm or egg) where only one copy of every chromosome is floating around. Two strings of DNA ended up sticking together. Here's a possible scenario:

Two chromosomes fuse (the 2p and 2q) in an egg cell. Not 4 - just 2. 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.

Now, each eukaryotic chromosome has four "arms" which are comprised of two strands of DNA pinned at the centromere. Please keep in mind: NOT four chromsomes, but four arms. OK, in order for two chromosomes to fuse and become one single (whole) chromsome, four contact points must be available. This is because at least two arms from each of the two original chromosomes must somehow "connect" with two other arms. Furthermore, (ah, I just realized this!) the original centromeres must somehow "unbuckle" so that the resulting chromosome doesn't contain three spindles.

This is some major choreography, is it not? And it is further complicated by your statement that "Perhaps under cell duress, or in the presense of some type of virus or other DNA-altering molecule, it could fuse. Or maybe it was just a 'freak accident.'" Well, I find this to be a typical "just-so" or "we'll discover it in the future" story. It's really nothing more than a wild guess. Who relayed it to you, I wonder?

So, if you would, please, describe the inherently complex mechanics of how could this fusion could possibly happen, given what is known about chromosomal genetics.

I will look at the articles when I find time. You should know that I have several other methodological questions in the queue--ones that are "bigger" picture than the genetical mechanics we are now discussing. But I don't want to get too far ahead just yet. Perhaps we are only getting started in this discussion!

Again, thanks for the opportunity to think this through. I can also say that I'm learning quite a bit!

Vanderzyden

Jesus Tap-Dancin' Christ · 08-31-2002, 10:05 PM

This reminds me of playing Galactic Battlegrounds as the Trade Federation with heavy destroyer droids being sent after the Gungan camps in an all-consuming wave.

Or a chicken getting its head cut off and then running around for a while, because it hasn't quite figured out its beaten.

Either way, VZ is losing. Badly.

Dan828 · 09-01-2002, 02:08 AM

Yikes Vanderzyden, that last post showed a complete misunderstanding not only of genetics in general, but of mitosis and meiosis as a well. This far in to the debate, I would have expected you to have looked up some basic biology by now.

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08-31-2002, 09:24 AM	#122
scigirl Veteran Member Join Date: Jul 2001 Location: Seattle Posts: 4,261	One more thing I thought of: I've heard Vanderzyden's story before, from other creationists. Basically it goes like this: 1. Said person is some type of agnostic or non-christian, or christian but not really because they chose it. Said person studies evolution in high school and learns it as "fact," just like 2 + 2 = 4. 2. Same person "gets religion" and is told by the pastor that evolution is not a fact. It is, instead, either an atheistic conspiracy, or influenced by satan, or some other thing. 3. That same person goes out to study evolution on his own. At this point one thing can happen: They discover that evolution is not indeed a fact like 2 + 2 = 4. Instead, it is a growing, enormous, complex scientific field, filled with the same caveats as every other scientific field: a few bad scientists, some fabricated data, controversies, etc. Said person then rejects evolution (and by default they reject the scientific methods used to prove the evolutionary theory, but none of them actually see it this way). 4. This reinforces the pastor's claim, and the person then critiques evolution through a variety of ways, including bible-quoting, appeal to emotion, etc. However, one of the ways they critiicze evolution is through the same science that they rejected in step 3. Just an observation, and no this does not apply to everyone! scigirl

08-31-2002, 10:28 AM	#124
beausoleil Senior Member Join Date: May 2002 Location: US and UK Posts: 846	I'm curious now. Did this chromosome fusion occur in one individual? And how did it get passed on. i.e. could they breed with another individual with 1 extra chromosome and pass it on somehow? (I'd appreciate a pointer to a website or an entry level explanation.)

08-31-2002, 10:05 PM	#129
Jesus Tap-Dancin' Christ Veteran Member Join Date: Apr 2001 Location: Heaven Posts: 6,980	This reminds me of playing Galactic Battlegrounds as the Trade Federation with heavy destroyer droids being sent after the Gungan camps in an all-consuming wave. Or a chicken getting its head cut off and then running around for a while, because it hasn't quite figured out its beaten. Either way, VZ is losing. Badly.

09-01-2002, 02:08 AM	#130
Dan828 Junior Member Join Date: Jul 2002 Location: Davis, CA USA Posts: 83	Yikes Vanderzyden, that last post showed a complete misunderstanding not only of genetics in general, but of mitosis and meiosis as a well. This far in to the debate, I would have expected you to have looked up some basic biology by now.

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