Oolon Colluphid

Hi folks

Over at the BBC boards, I have been using this chromosome fusion argument. There's a chap there who seems to genuinely be making an effort. He has just posted this:

Unfortunately, Oolon would not be a good candidate to explain this.... I get my genetics from books and you folks, and have a merely (or nearly) working grasp. I too am sure there's a simple answer, but... erm .... what is it?

Oolon the genetically challenged... (give me a design argument any day! )

Cheers, Oolon

Peez

A chromosome is made up of two sister chromatids only after the DNA has been replicated (in preparation for cell division). The poster seems to be confused about sister chromatids vs homologous chromosomes. Humans have 23 pairs of chromosomes (46 chromosomes), and these pairs are not identical to each other, nor are they attached together. After DNA replication (and before cell division) there are therefore 92 chromatids. The fusion of two chromosomes in a germinal cell (spermatogonium or oogonium) of the testes or ovaries that started with 48 chromosomes would result in a cell with 47 chromosomes (22 pairs and one group with two smaller chromosomes "paired" with one larger chromosome). Such a cell would function normally: essentially like one with 24 pairs of chromosomes (with two chromosomes tied together).

Of course, the fusion might happen during or after meiosis (e.g. in a sperm or egg), that would start with 24 chromosomes (not in pairs), and end up with 23 chromosomes. If the chromosomes are joined, then after DNA replication the chromatids formed would be joined as well (i.e. there would be two sister chromatids). Perhaps there were two centromeres at one time. What is the problem? What chromatid? Chromatids are just copies of the chromosome after DNA replication. Did he think that the scientists missed that? I am not familiar with the current understanding of the process involved, but it is not hard to imagine that some material was lost at the joining. Shouldn't that be "(c)"? Why?

I am sure that scigirl can address this better than I.

Peez

gallo

Close but no cigar. During mitosis (normal cell division) each chromosome reproduces itself. At that point under a microscope each chromosome will look like two chromosomes lying side by side, joined at the centromere. This is when the chromosome is made up of two identical chromatids. Later during the process of cell division, each chromatid will be pulled by the centromere toward a separate centriole at opposite poles of the cell. The cell body then divides near the middle forming two cells.

During meiosis (reduction division) the homologous chromosomes becomes associated, forming a body that looks much like the chromosomes after reproduction in the step above. However, in this case, each chromosome then reproduces itself, forming body with 4 chromatids joined at one centromere. Two divisions then take place so that 4 cells with half the number of chromosomes are formed. These are the gametes.
Apes have 48 chromosomes - 24 pairs. Humans have 46 chromosomes - 23 pairs.
Not quite sure what you are saying. Certainly, the human chromosome 2 is most likely the fusion of two chromosomes in some ancestor.
The fusion of two chromosomes would give one chromosome with two centromeres. One of those centromeres subsequently became non-functional. It's presence is known only by the remaining alphoid sequences at the appropriate spot. But in some ancestor where that fusion took place, the homologous chromosomes 2p and 2q would become associated with the fused chromosome 2. The association would be so close that it is not unlikely that they might have become fused also. Thus cell division, reduction/division would continue, and even mating would not be a problem.
No. Not quite. Both 2p and 2q have one long arm and one short arm.
During replication, chromatids don't join. Chromatids are fromed by the replication of each arm of the chromosome that remain joined at the centromere until they are separated during a later phase of division.
No. 2p and 2q are only slightly longer than chromosome 2, the difference being less than the extent of the telomers on 2p and 2q. If during meiosis they become associated with their homologue, then the telomeres of the two are in close association. So, (a) from the fusion of a 2p and 2q chromosome. The other two could remain free. (b) very little longer. Part of the telomeres was apparently lost during fusion. (c) chromosomes 2p and 2q are not telocentric. They have arms of unequal length.

I hope that helps and is clear. Perhaps scigirl can clear up any fuzzyness and make corrections.

rafe gutman

couldn't the fusion be explained by a robertsonian translocation?

(strangely, i can't find the definition of a robertsonian translocation in my genetics textbook, perhaps this graphic will explain it well enough)

gallo

That's it. Two acrocentric chromosomes fuse to form a single metacentric chromosome. Except there is no indication that the overlapping segments of the telomeres were preserved as a "new hetero-chromatic" chromosome. If it was, it has been lost. Without a centromere, I am not surprised.

Peez

Hi gallo,

I hope that you don't mind if I pick a nit or two in your very good post Actually the chromosomes "reproduce themselves" before mitosis, during the S (as in Synthesis of DNA) period of interphase in the cell cycle. Strictly speaking, mitosis is the division of the nucleus, while cytokinesis is division of the cell (the two together are called the mitotic phase, and the process is sometimes called mitotic cell division). There are cases in which mitosis is not followed by cytokinesis, resulting in a cell with two nuclei. The tetrad of two chromosomes (with four chromatids) is held together at chiasmata, but each chromosome has its own centromere. I believe that he was confused between chromatids and homologous chromosomes, and imagined that we are claiming that two chromatids (half a chromosome) fused, thereby reducing the total number of chromosomes by one half a chromosome to 23.5.

Peez

gallo

Thanks. It has been a number of years since I have been concerned with the events of cell division. I'm sure that more is known about the process now than when I was learning about it. For example, this is the first I've heard of the S period of the interphase. When I learned it, there was just the interphase and the chromosomes were duplicated during the prophase. And I forgot completely about the chiasmata. I guess I should have looked it up.